A Physics Digest—Part 6: Light

Last time I described how the electromagnetic spectrum spans a huge frequency range—from radio waves to gamma rays, a factor of a trillion or so. Only the tiniest part (one billionth) of that range, in the very middle, is the light visible to our eyes. Everything we see is confined to that incredibly narrow strip—it’s our constricted view of life.

One property of visible light that’s crucial to our sense of sight is reflection. If ordinary objects didn’t reflect the sun’s light, we’d not see them (sort of like the Klingon cloaking device on Star Trek). Ordinary things don’t emit light on their own, so we need reflections from the sun or other light source to know they’re there.

While light waves reflect from opaque objects, they penetrate materials like glass and water—wherein they move more slowly. When a light wave is forced to slow down upon entering a transparent medium, it will bend or refract. If you look at a spoon sitting in a glass of water, it appears bent. Refraction is also used in eyeglasses and telescopes, to focus rays of light.

In air or in a vacuum all the colors (all frequencies) of light move at the same speed, but in glass the low frequencies (reds) move faster than high frequencies (blues). Thus light can get separated into its constituent colors upon entering a glass prism and becoming refracted. The same process creates a rainbow; each raindrop acts like a tiny prism.

Sunlight appears white because it contains all colors mixed together. (Actually, it’s a little yellowish, since the sun emits a little more light energy at yellow wavelengths.) So how does a red balloon appear red, when only “white” sunlight strikes it? The balloon absorbs all light frequencies other than red; only the red wavelengths are reflected. If all the colors get absorbed, it’s a black balloon.

Why is the sky blue? White sunlight enters our atmosphere and some of it gets scattered by particles in the air. Blue (high) frequencies get scattered more than any other color, so the sky gets filled with “blue rays” of light. Why is the sunset red? When overhead, the sun is whitish-yellow, but as it sets (or rises) its rays must then travel a longer path through the atmosphere to reach our eyes. The lowest frequencies (reds) are the least scattered color. So red rays penetrate the thick atmosphere best and are the ones we see.

Our eyes’ retinas contain light sensing rods and cones. Cones (at the center) respond to color and also give us acute central vision. Rods (surrounding the center) do not sense color and give us less detail in peripheral vision. They can “see” at light levels a hundredth of what cones need, however, so in dim light we can still see, but with fewer details and no color.

Two or more overlapping electromagnetic light waves can either reinforce or interfere with each other. Scientists have learned much about light—its speed, its color composition, and the nature of the media it’s traversing—by studying the reinforcement and interference patterns that the waves make.

Although we see most objects by the sunlight they reflect, a few things emit light themselves. Fireflies do it beautifully. An incandescent light does it by getting heated up so much that it glows. When some types of gas are zapped by electrical energy, the electrons in their atoms are bumped up to a higher energy state. These excited electrons quickly drop back to their usual lower energy level, and emit photons of light. This is how neon and fluorescent lamps glow.

The electrons in some elements don’t immediately jump back down to a lower energy state right away, but hold on for a while. This is phosphorescence. And when the emitted light waves are all in step with each other (reinforcing each other) we get the intense beam of a laser.

Every element emits its own unique signature of photons (i.e., color of light) when excited electrically. Astronomers use these signatures to determine what elements are present in various stars, so we don’t have to travel there to sample them directly.