Update

 
I apologize for not writing much lately. I've spent the past 2 weeks traveling to/from and attending a volcano conference and acoustics workshop. I have some great pictures with some great science, so hopefully I'll be able to find the time to start posting more regularly again. Thanks for reading!

How Do Animals Dry Themselves by Shaking?

** Apologies on not posting for a while. I hadn't noticed how long it's been! **

This is not a photo (!) but a video that I came across a couple years ago. The authors from Georgia Institute of Technology applied some basic physics to determine how quickly an animal has to shake in order to dry itself. Here's their website where you can find a more detailed description of their study, including the resulting published paper. Being a fan of applied physics, I really enjoyed this, especially because it is simple enough for an introductory physics student to understand.

The basic idea is to look at the forces acting on the water droplets in an animal's fur. The force exerted by air pressure and water tension work to hold the droplets in the fur. By shaking, the animal produces a force that acts to move the water droplets outward. This is called the centripetal force. When you are next in a car, notice how you move when the car makes a turn, especially if it's a quick turn. You should feel your body moving away from the pivot point. In other words, a right turn will cause you to move toward the left side of the car. This is also the centripetal force at work, and it occurs whenever a body is rotating.

The centripetal force produced is proportional to the angular frequency, or speed, of the rotation as well as the distance from the pivot point to the object that is rotating. For our rotating mammals, the pivot point is the center of the body and the object, the water/fur. Therefore, the distance is just the radius (or half the thickness) of the mammal's body. The conclusion that smaller animals must shake faster makes sense because of this proportionality.

Let's introduce the actual equation for rotational motion and centripetal force: Fc = m*r*w^2, where Fc is the force, m is the mass of the water droplet, r is the radius of the animal, and w is the angular frequency with which it shakes. Assume we want to keep Fc constant - that water droplets always require the same amount of force to remove them. Since a smaller animal has a smaller radius r, it will require a larger rotating speed w to compensate for the smaller r. Larger animals with larger radii, can shake more slowly to produce the same centripetal force as their smaller companions.

Here's a challenge: Next time you're at the beach or get out of the shower, try to shake yourself dry. Can you do it? If so, what do you think your shaking speed (oscillations per second) is? If not, what makes you different from the mammals that do dry by shaking?

References

Wet mammals shake at tuned frequencies to dry by Andrew Dickerson, Georgia Tech

Why Does the Moon Have Phases?

** My apologies on the lateness - it's been a busy month. **

 

Full moon. Crescent moon. New moon. There's no doubt you've previously heard these or even talked about them. Perhaps you even know the less common or more specific phases of the moon. Yet, there still seem to be misconceptions about the moon's appearance. I've already covered why the full moon can seem so large and why the moon can appear orange when it's on the horizon. Here's some more information about the moon's appearance:

The Moon's Orbit & Apparent Size

Let's start with some important background knowledge. The moon orbits the Earth on a nearly, but not perfectly, circular path. As a consequence, there are two points where the moon will be at its closest (perigee) and farthest (apogee) distance from the Earth with every other point somewhere in between. The difference in distance between the two extremes is enough to be noticeable, at least with pictures. It might be a bit difficult to see the difference by just going outside and taking a look. Another neat thing about the moon is that it's tidally locked in its orbit. For every one orbit around the Earth, it will rotate exactly one time. The result is that someone on the Earth can only ever see one side of the moon. If you compare the two pictures above, you'll notice that the dark lunar mare near the bottom right corner of the crescent moon is the same as that on the right side of the gibbous moon. On the flip side, there is one side of the moon that we can never see from the Earth. This is where the "dark side" of the moon comes from. The dark side of the moon isn't dark in the true sense of the word. There are times when the sun shines on it. However, it's considered "dark" because people were never able to see it until lunar orbiter spacecraft were built and sent into space.

Moon Phases

Unlike the sun, the moon does not emit light on its own - we only see it because of the sunlight that it reflects. Like the Earth, the moon will only have sunlight illuminating half of it at a time. The half that is illuminated is not always that half of the moon that faces the Earth, which leads to the phases. I'll be a bit more specific with a few examples to help clear up any confusion. When the moon is full, the half that is illuminated is also the half that faces the Earth. The moon is on the opposite side of the Earth than the sun, so the sunlight illuminates the half that faces us. The opposite of this is the new moon. In this case, the moon and sun are on the same side of the Earth, so the half that is illuminated is the "dark" side of the moon. 

This case also brings up another interesting point: the new moon should be located close to the sun in the sky. In other words, we see the new moon during the day rather than at night. It doesn't disappear from the sky or become dark. Instead, it is just visible at a different time of day. 

Let's leave this little detour and get back to the phases. The last example that I'll give is the half moon. To see a half moon in the sky, the moon would be 90 degrees from the sun. If you have trouble visualizing that, picture it this way: if the Earth is a clock, the sun will appear near the 12 and the moon will appear near the 3. For this phase, half of the "dark" side is illuminated and half of the side that we see is illuminated. To get the rest of the phases, we can just rotate the moon around the Earth. The farther it gets from the sun, the closer it will get to a full moon. As it moves from full to new, a smaller and smaller section (visible to us) will be illuminated. If you need some extra visualization, you check out this chart or try the activity listed below.

Eclipses

I'm sure most people are well aware of solar eclipses, but did you know that the moon can be eclipsed, too? A lunar eclipse happens when all of the moon, Earth, and sun are aligned with the Earth in between the other two. As the moon passes behind the Earth, it moves into the Earth's shadow. Since the moon appears lit due to reflected sunlight, being in a shadow will make the moon appear darker. At the start of the lunar eclipse, a growing fraction of the moon will appear blacked out. As more and more of the moon moves into the Earth's shadow, it starts to turn a dark orange color. The next total lunar eclipse visible for the Americas will be on October 8, 2014, so get your binoculars ready.

Try It at Home

You can see for yourself how this process works with just a (solid) ball and a flashlight. Begin by placing the ball on a table with the flashlight pointed at it from a couple or so feet away. Walk around the table with the surface at eye level and watch the appearance of the ball. When you're opposite the flashlight (sun), the ball (moon) appears as a dark, "new moon" phase. Go around to the same side as the flashlight, and you'll see a full moon (or rather full ball?). As you walk around, you'll see the phases of the ball changing just like the moon. However, the moon orbits the Earth, not the other way around. For a more realistic demo, place a larger ball (or globe if you have one on hand) a few feet from the flashlight. You can simulate the moon and its phases by moving the small ball around the large one. Place a "person" on the large ball. How does the small ball's phase change from your person's point of view as it moves around in its orbit?

Keep in mind that the moon's orbit is slightly tilted. This means that the moon and Earth rarely appear directly between the sun and the other. By aligning all three, you are creating a solar (moon in middle) or lunar (Earth in middle) eclipse.

References & Further Reading

Lunar Eclipse Page - by NASA

Volcanism on the Moon - by Robert Wickman

The Lunar Orbiter Program - by Lunar and Planetary Institute (NASA)

Earth's Moon Phases, Monthly Lunar Cycles (Infographic) by Karl Tate

Water Waves

Chances are you've been to a beach before, and it's quite likely that there were waves whether you were at the ocean, a lake, or some other sizeable body of water. If you've ever watched the waves for a while, you might have noticed that they tend to arrive parallel, or nearly so, to the shoreline. Even if they are at angle when they are far away, they still reach the shoreline nearly parallel. Why? Another thing noticeable to anyone who has watches waves, and is visible in the above photo, is the crashing and "breaking" of waves as them come towards the shoreline. How does that happen?

The first question can be answered by fluid dynamics. In shallow water, that is at a depth which is smaller than the wavelength (measure of distance between wave crests), the speed of the waves depends on the water depth and gravity only. Specifically, the speed is equal to the square root of gravity times depth. This tells us that waves will move slowest in the shallowest water. Let's assume that there is a nice downward slope as you move away from the shoreline. Start by imagining a wave that begins parallel to the shore. As it moves closer, the shallower water will cause it to slow down. Now, what happens if the wave is rotated so that it starts at an angle to the shoreline?

If you think of the wave crest as a straight line, one end will be closer to shore than the other if it's at an angle. The far end is in deeper water and will travel faster than the shallow end. This causes the wave crest to turn until both ends are at the same depth or it hits the shore (whichever comes first). If you're having trouble imagining this scenario, try thinking about an axle with a wheel on each end. If you move both wheels at the same speed, the axle will move in a straight line. What happens if you hold one wheel stationary (more or less) and move the other? The moving wheel will rotate around the other one. Now, allow both wheels to turn but move one a little faster than the other. The axle will move forward while also gradually turning. This is analogous to our wave scenario.

That's one question answered, so on to the next one: what causes waves topple over? It may seem strange but this answer relies on the same principle as the first one. In this case, we can no longer treat the wave crest as just a line; it does have a finite width, after all. Since the back of the wave is in deeper water than the front, the back will travel faster and eventually catch the front. The water particles will accumulate behind the crest until it reaches a point when the added water forces the wave to topple over. If the wave is large enough, you may notice how the top rolls over the bottom. As water piles up behind the wave, the water becomes even deeper than the water in front. Consequently, the water in back moves even faster, particularly that on the surface. This helps the top of the wave crest to roll as the wave topples over. Keep in mind, however, that this is only valid for waves in shallow water. It's not often that you see breaking waves in the middle of a lake, partly for this reason. When you do see them, the wind is generally very strong and helps force them to topple over.

Fortunately, these are easy things to go see for yourself, so get out and enjoy the coming summer. Maybe the next time you find yourself at a beach, you'll pay a little more attention to the waves.


References
Waves in Fluids (video) by National Committee for Fluid Mechanics Films (1960s; more here)

Sonoluminescence

I believe the credit for this one goes to my lab partner.

At first sight, those three little pin-pricks of light may not seem very spectacular. However, the phenomenon that creates them (sonoluminescence) is really quite interesting. Some scientists once believed that the light was created by the ever elusive nuclear fusion, although this idea has since been almost completely rejected. We can get to the basic idea by breaking apart the name, sonoluminescence. The "sono" refers to sound, while "luminescence" is the glow. In essence, the lights are created by sound.

In the experiment above, we start with a rectangle container filled with purified water. The metal cylinder at the top that sticks into the water is a transducer; in other words, it will create the sound for us. Sound is a pressure wave; when it travels through the water (in this case), it causes the water molecules to oscillate back-and-forth causing regions of compression, where molecules are pushed together, and rarefaction, where particles are pulled away. The sound that we used was a higher pitch than humans can hear, known as ultrasound. If the frequency of the sound (that is, how many times it oscillates per second) is chosen correctly, a standing wave will form in the water. If you and a friend each hold one side of a rope and shake it in sync with one another, a standing wave will form. You'll know when one forms because there will be at least one point on the rope that doesn't seem to move. The stationary point is called a node. Between each node is an anti-node, the point that has the most motion. The anti-node is what we're interested in.

The next step is to make a very small bubble very near the anti-node. It is a difficult task, but if done correctly, the bubble move into the anti-node and get trapped there. Because the anti-node is where the wave motion varies the most, this point will rapidly oscillate between a high pressure state (compression) and a low pressure state (rarefaction). During the high pressure state, the air in the bubble will be squeezed into a smaller volume. The opposite happens during the low pressure state; that is, bubble's volume grows and the air inside can spread out more. This should help to explain why it is so difficult to trap a bubble. If you squeeze or stretch it just a little too much, the bubble will pop forcing you to start over.

So, we have a bubble in water that is changing volume due to being trapped in an anti-node of a sound wave. Now that the experiment is set-up, we can move to our initial question: why does the bubble light up? This is a difficult question to answer and, frankly, the exact answer remains a mystery of physics. However, we can make measurements that give us some insights into the process and allow us to speculate on what it could be. The light is emitted when the bubble is squeezed. At this point, the volume that air molecules have is very small. When the bubble is squeezed, the air molecules heat up. In essence, this heat causes the molecules to emit light. What isn't known is the process through which the light is emitted. The most accepted idea is that the heat energy pulls the molecules apart. When the molecules re-form, they emit light. However, this does not explain the spectrum (or variety) of light emitted. As the experiments continue, scientists hope to one day find the answer.

While this is a very neat experiment, it might not have much practical use. However, sonoluminescence does occur in nature. Have you ever heard of the beautiful mantis shrimp? Perhaps you've seen the comic about it that's been getting passed around lately. The mantis shrimp can close its claw so quickly that it creates a high enough pressure wave to form a cavitation bubble that can glow by sonoluminescence. Just be careful if you go looking for one. Not only do they create a large enough pressure wave to kill their prey, but it is even strong enough to break through the glass of an aquarium.


References and Further Reading
The Sonoluminescence Process by the American Institute of Physics
Sound is a Pressure Wave by The Physics Classroom
Sonoluminescence: Sound into Light (Scientific American article) by Seth Putterman
Mantis Shrimp by Chesapeake Bay Program
Why the Mantis Shrimp is my new favorite animal by The Oatmeal

Schrodinger's Cat

Up to this point, I've covered mostly topics involving geology and light. To shake things up a bit, this post will cover something a bit different - quantum mechanics. Generally speaking, quantum things are too small to photograph without a powerful microscope. However, there is a good macroscopic analogy for one of the most important quantum processes. The Schrodinger's cat paradox was originally used by Erwin Schrodinger to show how silly quantum mechanics seemed when applied to the macroscopic world in which we live. In contrast, his cat paradox has turned into one of the most widely used analogies for explaining the quantum phenomenon of superposition.

If you're not familiar with the Schrodinger's cat paradox, here's how it goes:
(Note: Do NOT try this at home. This is a thought experiment only, not something actually carried out. Please do not needlessly harm animals.)

Start with a solid box. Inside the box is a vial of poison attached to a special device that contains an unstable atom. This unstable atom has a 50% chance of decaying within an hour. If the atom decays, the device will trigger a hammer to smash the vial of poison. Let's now place a cat inside the box and then close the box. If we wait an hour, there is a 50% chance that the device was trigger and the vial smashed. If that happened, we would expect to see a dead cat upon opening the box. However, there is also a 50% chance that nothing happened and the cat is still alive. So, without opening the box, is the cat dead or alive after an hour of waiting?

As long as we don't observe the cat by opening the bow and looking at it, we can say that it is in a state of being both dead and alive. Now, this doesn't make much sense. Obviously, an animal can only be dead or alive, not both. This is why it is a paradox and why Mr. Schrodinger used this as an example of how ridiculous the quantum theory of superposition seemed. Now, what exactly is superposition and why is it important?

From the cat paradox, you may have figured out what superposition is. In simple terms, superposition means that a quantum particle, such as an electron, can be in multiple states at the same time. There is one catch though - if you try to measure or observe the particle, it will assume only one of the states. Going back to the cat paradox, once you open the box and check to see what happened, the cat will be either alive or dead. You can no longer say that it exists in both states.

One consequence of this is that the act of measuring a system can affect how the system is behaving. This is something that rarely happens in the macroscopic world. If you use a ruler to measure the length of a pencil, the act of measuring will not change the length of the pencil. Its length will be exactly the same before and after you measure it. In the quantum world, however, things are different. Because particles are so small, even light can easily affect the properties of a particle. For example, when light collides with an electron, it can pass momentum to the electron and change its speed and/or path of motion. This consequence becomes significant when trying to do things on a quantum level, such as building a quantum computer. These computers use the fact that an atom can simultaneously be in multiple states to speed computations by doing more than one at the same time. However, to get the computation results you would need to interact with the atoms. This causes them to lose all but one of their current states and breaks the multiple-state basis of the computer.

There are experiments that modern day scientists can use to test the theory of superposition. Even though it was questioned back in the 1930s when the cat paradox was first introduced, it is now a well accepted theory. That said, from Mr. Schrodinger's days until now, one thing remains the same - the quantum world remains a very strange one that few people can even begin to fully understand.


Further Reading
Schrodinger's Cat Comes into View - By physicsworld.com
No. 347: Schrodinger's Cat - By John Lienhard at the University of Houston
Another step toward quantum computers: Using photons for memory - By Eric Gershon, phys.org
What Is Quantum Superposition? - By the Science Channel

The Life of a Star

(Apologies for the blurriness - long exposure photos require a tri-pod, which I didn't have on hand that night.)

Orion, the warrior. This is probably one of the most well known constellations, particularly Orion's belt. If you're not familiar with constellations, here's how to pick out Orion. The three stars that make a vertical line at center are Orion's belt. Just to the right of Orion's belt, there is a near horizontal chain of stars. This is Orion's sheathed sword hanging from his belt. The four brightest stars that form a box around the belt and sword are Orion's feet and shoulders. You may also notice a sub-horizontal arc of bright stars between the top of the frame and Orion's (highest) shoulder. This is the bow that he is using to hunt Taurus the bull, or so the legend goes.

If you look closely, you might notice that the lower of Orion's shoulders appears to have a reddish color as opposed to the more usual blueish white color of stars. This star is called Betelgeuse (generally pronounced "Beetle-juice") and is a type of star known as a red supergiant. Similarly to people, stars are born, change as they age, and eventually die. The very largest of the stars die young, sometimes after only a few tens of thousand years. The smaller stars, however, can look forward to a long life, often reaching the ripe old age of billions (our sun) or possibly even trillions of years. Astronomers use the Hertzsprung-Russell (or H-R) diagram to help find out what stage of life a star is in.

Stars spend most of their lifetimes as part of the main sequence. Our sun is roughly an average main sequence star - not a big, bright blue giant nor a small, dim red dwarf. The hotter the star is, the more blue it appears. Therefore, the red Betelgeuse is now cooler than it was as a youngster. As stars age, most move off of the main sequence and into the realms of the red giants and supergiants. (The smallest stars may never quite reach the "giant" phase.) When stars become red giants or supergiants, it is a sign that they are nearing the end of their lifetime. The smaller red giant stars, such as what our sun will become, release their outer atmospheres when they die. The "stardust" expands into space over time, forming a planetary nebula and leaving just the star's core, now called a white dwarf, at the center to slowly cool and fade away. The biggest giants and supergiants, however, meet a much more exciting end. They die in massive stellar explosions, supernovae, which send much of the star's matter flying through space. The remaining core cannot support its own weight anymore and collapses into a very dense neutron star or, in the case of the biggest of the big stars, a black hole.

For stars to continue living, they require energy just as we do. People and animals get their energy from breaking down the food that they eat. Stars, on the other, are more like plants, in the sense that they both make their own energy. However, stars do not require energy from an outside source like plants do when they take in sunlight to help produce their energy. Stars are actually made of their food - in a sense, they live by eating themselves. The most common element in the universe, by far, is hydrogen followed by helium, lithium, and so on down the periodic table. Stars get energy by fusing these small "light" elements into big "heavy" ones. Inside the core of a star, temperatures and pressures are so extreme that the electrons are ripped off the atoms to form a plasma of electrons and ions, the atomic nuclei that is made of protons and neutrons. This hot, dense plasma is the perfect environment for fusion to occur.

Within the hot, charged plasma soup at the star's center, collisions between nuclei are unavoidable. If the collision has enough energy involved, the two nuclei will fuse together into a single larger one. For example, two hydrogen nuclei can combine to create a helium nuclei. This is the process which builds about half of the elements on the periodic table. However, fusion will only release energy when small nuclei are involved. The larger the nuclei, the more energy it takes to fuse them and the less energy is released in the process. As the star turns more and more of its light nuclei into heavier ones, it becomes harder and harder for the star to continue fusing nuclei to get the energy it needs to survive. This is the stage of life that Betelgeuse and other red giant stars have reached. After reaching the red giant stage, the star will continue fusing nuclei until it reaches the point when fusion requires more energy than it produces. For smaller stars like the sun, this limit is reached when carbon is produced. The giant stars, however, can continue fusing nuclei until the entire core has been changed into iron.

Iron is the most stable of all elements. The elements that come before it on the periodic table can produce energy when they are fused together, and those after iron produce energy when they are split (fission). Fission is the process in nuclear bombs that use uranium or plutonium. Because a star cannot create nuclei heavier than iron with fusion, these nuclei can only form when there is enough energy for fusion without needing any energy to be produced - during the star's death. The extreme forces involved in supernovae are capable of creating the heavy nuclei. Since supernovae are basically giant explosions, both the heavy nuclei and the lighter nuclei that were never fused are thrown out into space and end up as part of the next generation of stars and planets. Eventually this stardust, created in extreme environments with fusion and scattered throughout the galaxy by explosions, is brought together in the perfect way to create beings capable of understanding this process - us. As it turns out, we all have an explosive past.


References
Orion - by the Peoria Astronomical Society
The Hertzsprung Russell Diagram - by Richard Powell
Supernovae - by C.R. Nave at Georgia State University
Periodic Table of the Elements - by Mark Winter at The University of Sheffield
Basics: What are Plasmas? - by Dr. Timothy Eastman at Plasmas International
Fission and Fusion - by Yoseph Murtanu on the ChemWiki by University of California, Davis

Geologic Faults

By far the most famous fault, in the US at least, is the San Andreas Fault in California. As a kid on the other side of the country, I always pictured it as one giant chasm that went on forever - similar to what you might see in a movie. Of course, I later learned what the San Andreas Fault really is and also that faults are not really giant gaps in the ground as the movies might have you believe. In fact, most faults, such as the one pictured above, are much smaller, sometimes even microscopic. Nor is California the only state with faults and earthquakes. Over half of the states in the US have at least a moderate earthquake risk, which can be seen on earthquake hazard maps published by the USGS.

Simply put, a fault is a crack in rock on which slip can occur. The rocks on top can either slip downward (normal fault), upward (reverse fault), or move side-to-side (transform fault). Can you figure out what type of fault is pictured? Hint: try to match the strata (or rock layers) on each side to see which way the rocks on top moved. This fault is far from California; it's located at the Seneca Stone quarry in the Finger Lakes region of New York. There is a big difference between this fault and the San Andreas.

Faults form for different reasons. The San Andreas Fault is a major fault because it lies at a plate boundary. The Earth's crust is made of multiple large pieces called tectonic plates. These plates "float" on the mantle and are slowly moving (a few inches per year). At plate boundaries, three main things can happen: one plate can move underneath the other, the plates can move apart, or they can move past each other sideways. Of course, they aren't moving continuously. Friction can cause the plates to get stuck on each other at the boundary. However, stress builds up if the plates are stuck. When the stress becomes large enough to overcome friction, the plates will slip and release the energy that had been stored. This energy is mostly released as waves in the Earth that we call earthquakes.

However, many faults don't occur on plate boundaries. Central New York, for one, is far from a plate boundary. Volcanic processes, such as magma intrusion and gas release, can create faults or enlarge cracks. Colliding plates create stress and strain throughout the plate, not just at the boundary. This can cause intra-plate faulting and create fold mountains. Old, brittle rocks tend to break more easily and will be more likely to have faults and cracks. For our New York fault, we can look at the geologic history of the region. The Finger Lakes were created by glaciers. When glaciers form, their weight causes the crust to sink - similar to what happens when you sit on a mattress. Eventually, the glaciers receded which allowed the crust to rise back up. The stress and strain associated with the crust's bending can create cracks in the rock. This would be a good guess to the origin of the fault in the picture.

Many faults are hidden beneath the surface. We only find them when they become active (i.e. move and create earthquakes) or when we dig into the ground for things like rock quarries, mining, and roads. At this moment, you could be sitting right above a fault and not even know it. Don't worry too much though. Few faults are large enough or have enough energy built up to create a major earthquake. The areas that do have to worry are well known and generally well prepared. Make sure to check out the earthquake hazard map to see how likely an earthquake is to occur near your house.



References and Further Information
San Andreas Fault - by David Lynch
Hazard Mapping Images and Data - by the US Geological Survey (USGS)
United States Geological Survey
Fault Motion - by the Incorporated Research Institutions for Seismology (IRIS)
Plate Boundaries - by Maggi Glasscoe at NASA's JPL
Fold Mountain - by National Geographic
Earthquake Facts & Earthquake Fantasy - by USGS

Rainbows of Light

  

Our world is illuminated by light. Sometimes, we are stunned by the colors that we see: sunsets and rainbows, flowers and birds. I have previously written about how the sky changes colors, from blue in the day to the reds of sunsets and sunrises. In that post, I also discussed how objects that don't emit light, such as flowers, get their colors. But what about atmospheric phenomena like rainbows?

Most people have seen a rainbow, whether it was a true one created on a sunny, yet rainy day or one that appeared in the mist from a hose or waterfall (as seen above) on a bright day. What is the common theme here? All of these rainbows were formed with water droplets and sunlight. Now, the question is how do the water droplets interact with the sunlight to produce such a beautiful spread of colors?

When we discussed sunsets, we found that they were mainly created by scattered light. However, if the water droplets are just scattering light, then we would expect to see rainbows whenever there are both sunlight and water droplets. But rainbows don't take up the whole sky and only form on some occasions, so this is likely not what's happening here. In that case, what else do we know about light?

Light is what physicists call an electromagnetic (EM) wave. There are many different types of these waves, most of which can't be seen by people. When all types of EM waves are grouped together, we call it the electromagnetic spectrum. The part we see is the visible light, or more commonly just called "light". Other types of EM waves include infrared, which is used to locate warm bodies in dark places, and microwaves, which are used to heat up your lunch. The visible light part of the spectrum contains all of the colors of the rainbow. When these colors are combined together, we see the light as being white. Now, this should give us a clue. The white color of the sunlight shines on water droplets to produce a rainbow with an array of colors. Maybe the water droplets can separate the white sunlight into its component colors? But, how could it be done?

You've probably heard about the speed of light, the ultimate speed limit of the universe, and may even know its value. However, the speed of light value is only valid for light passing through completely empty space, aka vacuums. When light passes through an object, such as a window, the atoms that make up the object slow the light down to a lower speed. The ratio of the speed of light in a vacuum to that in an object is called the index of refraction. The value of this ratio depends on the properties of the material from which the object is made. Another special property of this ratio is that it is affected by the frequency, or color, of the light. Now, you might be thinking, "Hold on. What is this 'refraction' that the index is referring to?" Well, that's the last piece of the puzzle.

In the sunset post, we discussed how light rays can be reflected and scattered. However, the rays can also be refracted. In general, light rays travel in a straight line until they hit an object. Upon hitting an object, they will usually be absorbed or reflected. This is how the object gets its color. But if an object is not opaque, some of the light travels through it. Remember how the speed of light depends on the material that the light is traveling through? When a light ray passes from the air into, say, a window, its speed is lowered. The change of speed causes the light ray to bend. This bending at the boundary between two different materials is known as refraction. As soon as the light leaves the window and re-enters the air, its speed increases and causes the ray to refract a second time. If you know the index of refraction for the two materials and the angle with which it hits the boundary between them, you can actually calculate the angle at which the light will bend. Since water droplets are not opaque, light can travel through them. Because water slows down the speed of the light, the rays will bend when they enter and exit the droplet. So how does this process result in a rainbow of color?

If you look two paragraphs up, you'll recall that I said the speed of light through a material is dependent on its color (or frequency). Since red light has a lower frequency than blue light, it will travel faster through the droplet and bend less. This effect can also be seen with prisms, which are constructed to disperse light. In other words, they are made to break light into an array of its component colors - just like a rainbow.

Now, rainbows are a bit more complicated than a simple prism. After the light ray enters the water droplet and refracts, it then must reflect off of the opposite side of the droplet before exiting and refracting again in order to form a rainbow. Secondary rainbows can appear if some of the light reflects twice before exiting. For this entire process to happen, there is a limited range of angles at which the light rays must enter the droplets. Therefore, the sun must be in the right position relative to the water droplets for it all to work. This is why we don't see rainbows every time it rains on a sunny day.


References
Hyper Physics: Electromagnetic Spectrum by C.R. Nave at Georgia State University
Index of Refraction and Snell's Law by Eric Weisstein and Wolfram Research
Rays Through a Large Raindrop by Les Cowley

Impact Craters

My first thought upon seeing Meteor Crater? That's one BIG hole.

On Earth, it is somewhat rare to come across craters caused by impacts, especially ones that are recognizable as such. So why is Meteor Crater (in Arizona) so well preserved?

The impact that formed Meteor Crater was fortunate enough to happen in the middle of a desert. Here there are fewer of the weathering and erosion processes, such as rain and frost, that would wear away the features of the crater. You can find out more about the crater here. The meteorite which created this 1-mile wide crater was thought to have been only 150 ft (46 m) across. So, how does such a small object create such a large hole?

Try throwing a rock into dry, loose sand. What does the mini-crater it makes look like? What happens why you throw it harder? By throwing the rock harder, you are giving it more energy (which is why it moves faster). Upon colliding with a much larger object (the Earth), the rock stops moving. Because energy can't be created or destroyed, the energy that the rock had while moving must go somewhere. Some of this energy goes directly into the Earth and will cause the sand directly under the collision to become compressed. The rock's collision will also create a mini-shock wave in both the ground and the air. A shock wave in air is similar to wind and will blow the loose sand particles away from the collision site, causing a crater to form that is larger than the rock. The faster the rock is moving, the more energy it has and the larger the crater it will create. This same general process is what formed Meteor Crater and all other impact craters - the only difference is the scale.

While impact craters may be hard to come by on the Earth, they are very easily found all around the solar system. In fact, we see impact craters on nearly every rocky body in the solar systems from planets to asteroids. The easiest place to view impact craters is the moon - all you need is a clear night and a decent pair of binoculars. However, most of the craters on the moon can't be seen from Earth. They are located on the "dark side" of the moon that always faces away from Earth. If craters are so easy to find, why aren't there many on Earth?

Most impact craters were formed billions to millions of years ago when the solar system was still very young. There were more asteroids around that crossed paths with the larger objects. As these smaller objects continued to collide with the larger ones, the solar system was cleaned up and fewer small bodies remained in the paths of the planets and moons. However, collisions do still happen such as when a comet hit Jupiter in 1994. While many planets and moons still bear the visible scars from these violent times, the Earth seems to stand out. You might be thinking of reasons why the Earth avoided these collisions. But, it didn't. When scientists started to wonder about this, they also started looking for evidence of impact craters on Earth. Unsurprisingly, they found them. The Earth is more geologically active than many solar system bodies. Craters get eroded and weathered away, covered up by ocean, filled in as lakes, or recycled into the Earth through plate tectonics. Our planet also has something else that's special: life. It is difficult to recognize a crater from the ground or from space if it is covered by trees and plants. There is also a theory that the Earth was the victim of a giant collision while it was still forming. This collision, it is thought, is what created our unusually large moon.

There is another solar system object that stands out even more than the Earth when it comes to impact craters. Jupiter's volcanic moon Io has few, if any, impact craters on it. Not only is Io geologically active, it is also the most volcanically active body in the solar system. The volcanoes here erupt so often, that it is entirely re-surfaced on roughly a daily basis. This means that any impact craters would be filled in with lava and disappear relatively quickly.

Have you ever seen a meteor shower? None of the objects will create a crater like the one in Arizona. The meteors that we see today often start off as solar system dust particles, not even big enough to be considered a rock. Astronomers have done a very good job of keeping track of all of the known large asteroids that come near the Earth. However, asteroids are very hard to see. This means that astronomers are still discovering new ones quite often. That said, we will likely have at least a few months warning before any large collision!


References and Further Reading
Weathering by Pamela Gore at Georgia Perimeter College
Meteor Crater - Fun Science
The Explorer's Guide to Impact Craters by the Planetary Science Institute
Comet Shoemaker-Levy Collision with Jupiter by NASA's Jet Propulsion Laboratory
Moon Formation Theory by NASA
Solar System Exploration: Io, Overview by NASA

What I Study, Using Only the Thousand Most Common English Words

This post is a bit different. A popular science & technology comic (XKCD) recently described NASA's Saturn V rocket using only the thousand most popular English words. A parasitologist named Theo Sanderson wrote an application that will let you do this too. It is named "The Up-Goer Five Editor" after the name given to the Saturn V rocket in XKCD's comic.

This is my attempt at writing a little about what I study using only the thousand most commonly used English words (and a photo to go along, of course!).

(From Humphreys Peak: highest point in Arizona and on the rim of a former volcano's caldera)

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Even though the ground under your feet feels very still, it is actually moving really, really slowly. But sometimes, the ground moves so quickly that it feels like it is shaking. When the ground-shakes are big and strong, they cause houses to fall into pieces and make people get hurt and die. Big ground-shakes can make us very afraid. We still don't know how to tell when they will happen, so they usually hurt a lot of people. But not all ground-shakes are bad! There are small ground-shakes that you can't feel. We can use these small ground-shakes to learn more about the inside of our world.

While you can't feel the small ground-shakes, we have ground-shake-computers that can feel them. Usually the ground is still, so the ground-shake-computers just draw a straight line. But when a ground-shake happens, the straight line starts to move all over the paper. We can use these not-straight lines from many ground-shake-computers to find out when the ground-shake happened and where it started from. We can then use these facts to tell us how fast the ground-shake moved from one place to another. Because the ground-shakes move through the inside of our world, knowing how fast they move can tell us about the things that are under the ground you stand on.

In some places, there are really huge groups of rocks that are so tall that they touch the cold white water in the sky. Most of the time, these groups of rocks are safe. People like to climb them for fun. But sometimes, many fire rocks are thrown from the tops of these climbing rocks. The fire rocks are very hot and hurt people and houses. The good news is that we can use the small ground-shakes to find out where these fire rocks come from, which climbing rocks might throw fire rocks, and when the fire rocks will be thrown out.

The fire rocks start out deep inside our world and slowly move toward the top of the climbing rock. When they are high enough, the fire rocks move the smaller rocks that make up the climbing rock. This causes small ground-shakes that we can feel with the ground-shake-computers. If the ground-shake-computers feel enough small and big ground-shakes, we know that the climbing rock might start throwing out fire rocks soon. This gives us enough time to tell people to move so that they can be safe.

So remember: even if ground-shakes sometimes hurt people, we can also use them to keep people safe from fire rocks.

Colors of the Sky

The oft admired beauty of sunsets is a common theme in photography, movies, and visual arts in general.  Their vibrant colors lend inspiration to people around the world.  However, not all sunsets are so colorful.  How often do you see a picture-worthy sunset?  Usually you won't see more than the sun disappearing below the horizon taking the daylight along with it.  So what is the reason behind the magnificent sunsets?

Let's start with a seemingly more simple question: is the sun actually yellow? (Warning: Never look directly at the sun!)  The answer to that is no.  Even though the sun appears to be yellow, it is actually pinkish-white in color.  It may seem odd at first, but the explanation for how we see the sun is tied to why the sky is blue.  Yes, the answer to the question that every kid asks is also responsible for the yellow daytime sun.

People can only see the light that enters their eye.  Light can get to your eye in one of two ways.  The first way applies to objects that are light sources.  These objects emit light that can travel directly to your eye.  Fires and the sun both fit into this category, but most things that you come across do not.  However, a light emitter is needed in order to see them, which should be obvious to anyone who has tried to walk through a dark room.  Let's say you want to walk through your dining room at night.  You first turn on the lamp.  The light-emitting lamp sends light rays around the room.  Some of these light rays will be absorbed.  The ones that are not absorbed bounce off and travel through the air to your eyes, allowing you to see the object.  An object appears white if all visible colors of light are reflected and very little is absorbed.  When the opposite happens and only a little light is reflected, the object will appear black.  The color of an object depends on the colors of light rays that it reflects, since those are what you see.  The reflected colors are determined by properties of the molecules which make up the object.

Back to the sky: The Earth's atmosphere is made of many types of molecules.  Nitrogen is by far the most common gas in the atmosphere.  If you were to take a sample of 1000 air molecules from a desert (dry air - no water), about 780 of them would be nitrogen gas.  Oxygen would come in second with about 210 molecules.  Third place would fall to argon with a measly 9 molecules.  That adds up to 999 molecules.  The last molecule in your sample would likely be carbon dioxide or possibly another low-quantity molecule, such as helium.  Can you guess which color of light nitrogen reflects?  Because it is the most numerous molecule type in the atmosphere, nitrogen has the largest effect on the color of the sky.  Since the sky is blue, you should have concluded that nitrogen reflects blue light.

Now that we've solved why the sky is blue, we need to figure out the connected problem which is the color of the sun.  After the sunlight leaves the sun, it must first travel through space (which is mostly empty) and then the Earth's atmosphere.  As the blue sunlight is scattered (or reflected) by the air molecules, most of the light left to continue on the path to our eyes is red and yellow  Thus, the sun appears to be yellow (the remaining blues balance the reds).

When the sun is low in the sky at dawn and dusk, the sunlight has to travel through the atmosphere at an angle.  This makes the path through the air molecules longer.  Since the light is moving past more molecules on a longer path, the chances of its being reflected increases.  This results in a larger amount of blue light being reflected.  By the time the light reaches your eyes, all of the blues will have scattered and only the reds and yellows remain. 

Dust particles, smog, and clouds can enhance the color of sunsets.  Dust particles and smog tend to filter out the blues and greens creating a more reddened sunset.  Clouds reflect all colors of light, which is why they generally appear white.  Therefore, they will reflect whichever colors of sunlight reach them and can add to the colorful effect.  They can appear dark in the foreground and add texture and variety, as seen in the photo above.

The above explanation is also why full moons can appear bright orange when seen at dusk.  The moon is visible to us because it reflects sunlight.  The initial moonlight contains many of the same colors as sunlight for this reason.  As with sunsets, the blue light is scattered while traveling a longer path through the atmosphere leaving only the red colors for us to see. 

References
Earth Fact Sheet by NASA
What colour is the Sun? by Prof Hamilton, Univ of Colorado