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