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

The Big Full Moon

When I was a kid, I had the pleasure of seeing a huge, bright orange moon low on the horizon one autumn evening.  We were heading back after a day spent out on the boat and right in front of us was this huge gorgeous moon.  Only time will tell if I see another like it.

I would imagine that most people have seen similar large moons at some point in their lives.  But why does the moon sometimes look so much larger?

One's first thought might be that the moon is closer when it appears larger.  While this is true, it does not have much of an effect on the above phenomenon.  The moon's orbit, like those of the planets, is slightly elliptical rather than perfectly circular.  At the moon's minimum distance to Earth on its orbit (perigee), it is 26,465 miles (10.5%) closer than at its maximum distance (apogee).  When photos are taken comparing the size of the moon at both perigee and apogee side-by-side, the difference in size is noticeable.  However, it is too small of an effect for you to notice it by just looking up at the moon on any given night.

So if the moon's size isn't changing much, why can it appear so much larger at times?

Think about where the moon was in the sky the last time it noticed it being larger than usual.  Was it close to the horizon?  Were there any objects (buildings, trees, etc) nearby?  You most likely will have answered yes to one, if not both, of those questions.  This is the key to an explanation.

The large moon effect is simply an optical illusion.  When the moon is high above the horizon, nothing appears to be close to it.  The area surrounding it is vast and open making it appear smaller.  In contrast, the moon looks much bigger when it is seen next to a building or tree.  You know how large those objects are, even if they are distant.  Your brain can compare the moon's size with that of the objects near it, making it seem much larger than normal.  In the picture above, I managed to catch a plane and its contrails near a gibbous moon.  This gives you something with which to compare its size.

You can test this out quite easily.  On a night when the moon is full (or close to it), it will be low on the horizon just after sunset.  Go outside and compare it to the size of your thumb held up at arm's length.  A few hours later on the same night, go outside again and repeat the test.  You should find that the moon is still the same size compared to your thumb.

The other effect described at the beginning is the moon being bright orange in color rather than white.  This will be discussed in the next blog.

Reference
Earth's Moon: Facts & Figures by NASA