Let There Be Light!

[ Note: I use "light" and "electromagnetic radiation" interchangeably. So when I say light, I don't just mean visible light, unless I say I do. =P ]

Light comes in one form, but it interacts with matter in such complex ways that it appears there are different forms. By one form, I mean that light is a self-sustaining fluctuation, or disturbance, in the electric and magnetic fields, together called the electromagnetic field, that is created by a moving charge, like an electron. The disturbance, called an electromagnetic wave, arises when the charge changes either how fast it's moving and/or in what direction it's moving, both cases qualifying as an acceleration. (You might picture an electron accelerating upwards within a metal antenna. Such an act would cause both the electric and magnetic fields surrounding the antenna to fluctuate, and this fluctuation or disturbance flows outwards sort of like a water wave expanding out from the point at which a stone is dropped into a pond.) Once the fluctuation starts, it won't stop until it can act on matter, i.e. matter can absorb its energy. It very well could travel through the vacuum of outer space for a billion years. (Remember, it's self-sustaining.) The disturbance, which in some cases appears wavelike and at other times acts as a particle, flies through empty space at a constant speed. In 1983, the meter was redefined such that light travels (in vacuum) exactly 299,792,458 meters in one second. Using more familiar units, this is about 186,000 miles per second. This value - more so, this concept - is special in that it is believed to be constant regardless of place, time, or the motion of the observer. Contrast this with a car traveling a steady 50 mph (as indicated on its speedometer) on the highway. Placed in the car's passenger seat is a suitcase. Relative to a stationary observer on the side of the road, the suitcase is moving 50 mph down the road, but the driver of the car observes the suitcase as stationary. We conclude that the speed of the suitcase (as well as the car) is not constant; i.e. it can vary depending on your frame of reference. Not so with light, which appears to be going the same speed no matter how fast you move or what you're doing or where you are.

When people say that the speed of light is constant, they mean it's constant in vacuum (empty space). Light actually travels at speeds different from 299,792,458 meters per second (represented by the letter c, as in E = mc²) in different materials. For example, visible light slows down when it travels through water or glass. We assign a number to each material which indicates by how much light is slowed when it travels through the material, relative to how fast it travels in vacuum (i.e. relative to c). The number, called the index of refraction (and labeled n), is the ratio of c to the speed v in the material. Glass has an index of refraction of about 1.5, which means that light travels at only 2/3rds of c (i.e. about 124,000 miles per second) within glass. (In special cases, light, though not visible light, can travel faster than c in a material!)

These fluctuations in the electromagnetic field, which make up a pulse of light traveling through space, can vary in some ways. One of these ways is in how fast the fluctuations fluctuate. Not how fast the whole "packet" is moving, which is c in vacuum, but how fast the electric and magnetic fields grow and shrink as the packet moves along. If they cycle 384,000,000,000,000 times per second, then the light looks red to us. That is, the light interacts with the eye in a way that the brain interprets as "red." If they cycle 520,000,000,000,000 times per second, then the light looks green. If they cycle 10,000,000,000 times per second, then you can't see the light but it can cook your food, as these are microwaves and they are just the right frequency required to energize the water molecules in your food, which is how food is heated in a microwave oven. Radio waves oscillate some 1,000,000 times per second (and at other rates, or frequencies, a bit above and below this number). Other types of light, or electromagnetic radiation, are gamma rays, X rays, ultraviolet light, and infrared light. Visible light falls between ultraviolet and infrared, when the categories are ordered according to frequency, as they are in the preceding list (with gamma rays having the highest frequencies). Microwaves and radio waves follow infrared in this list. When you tune your car's radio to FM 94.7, this means the electromagnetic waves delivering your music are fluctuating at 94,700,000 times per second. (Compared to the frequency of, say, red light, this isn't that fast.) These categories are arbitrary, though, and don't correspond to any natural breakpoints in what is a continuous range of frequencies from the very tiny to the enormous.

When light travels from one transparent material into a different transparent material (with a different index of refraction), it either slows down or speeds up. We already saw that light slowed down when traveling from air into glass. At the interface between the two materials, light also changes direction. This is called refraction. This is apparent when you place a drinking straw in a glass of water. The portion of the straw beneath the surface of the water does not appear to be aligned with the portion above the surface of the water, when the glass is viewed from certain angles. When you look straight down into a body of water, any object in the water appears at only 3/4ths of it true depth. This, too, is due to refraction.

As mentioned before, light sometimes acts as a collection of particles, called photons. Each photon carries an amount of energy equal to its frequency times a constant called Planck's constant, so green light is more energetic than red light because it has a higher frequency. Blue light is more energetic than red or green light, or orange or yellow, for that matter, because it has a higher frequency than any of these other colors. This plays a role in why the sky is blue. First, remember that sunlight is composed of all different colors of light. (Visible light, together with ultraviolet (UV) and infrared, make up 99% of sunlight.) The molecules of nitrogen and oxygen, etc. that make up the upper atmosphere of the Earth find it much easier to absorb the blue light component of the sunlight at its high frequency (and high energy level) than the other colors of light. (They actually prefer ultraviolet and violet light, but there's not a lot of violet or UV in sunlight. Fortunately for us, the sunlight that makes it to Earth is only about 6% UV, and ozone absorbs most of it. Some of the UV that does make it down to ground level can cause sunburn and skin cancer.) When one of these air molecules has absorbed a photon of blue light, it then immediately emits it in a random direction. So blue light is sucked up by countless air molecules as it streams in from the sun, and then it's spit out in all directions, illuminating the sky. This happens to a lesser degree with the other colors of light, which for the most part pass through the atmosphere unscattered.