When you spray water from a garden hose into the air, with the sun behind you, you see a rainbow which appears to stretch right across the sky, in the same way that rainbows form by normal rain appear. In the garden hose case, the water droplets are only a few feet in front of the observer. Is the image of a normal rainbow also only a few feet away or is it formed by droplets within the total volume of the rain shower? If this latter case is true, does the rainbow in fact form a complete circle that is cut off by the horizon? — RP, Solihull, England

A rainbow isn’t an image that originates at a specific distance away from your eyes. It consists of rays of colored light that travel at particular angles away from the water droplets that produce them. You see red light coming toward you from a certain angle because at that angle, the water droplets are all sending red light toward you. In the garden hose case, the water droplets are so densely arranged that they are able to create a brilliant rainbow in only a few meters of thickness. In a typical rainstorm, sunlight must travel through hundreds or thousands of meters of raindrops to produce an intense rainbow. When you look up toward the red arc of the normal rainbow, you are seeing light directed toward your eyes by millions of water droplets, some close and others distant, that are all sending a part of the red portion of the sunlight striking them toward you and the other wavelengths of sunlight elsewhere.

You are correct that a normal rainbow is cut off abruptly by the horizon and that it would continue down below to form a full circle if the ground weren’t in the way. People in airplanes sometimes see full 360° rainbows.

When you hold a flashlight to your hand, some of the light comes through. What light frequencies shine through people? Is it possible to see inside people? — PC

Biological tissues themselves are relatively transparent. They’re not good conductors of electricity and electric insulators are typically transparent (quartz, diamond, sapphire, salt, sugar). But we also contain some pigment molecules that are highly absorbing of certain wavelengths of light. For example, the hemoglobin molecules in blood absorb green and blue light quite strongly, so that they appear red. When you look at a flashlight through your hand, the light appears red because of this absorption of green and blue light by hemoglobin. If you use a bright enough red light source and are willing to look very carefully, probably with sophisticated light sensing devices, you can probably see a little light coming through a person’s body. But that light will probably have bounced several times during its passage, so that you won’t be able to learn anything about what the person’s internal organs look like. To get a better view of what a person’s insides look like, you need light that penetrates more effectively and that doesn’t bounce very often. Moreover, you must employ techniques to that block this bouncing light as much as possible so that you only see light that travels straight through the person. The light that does this isn’t visible light—it’s X-rays. X-rays are very high frequency, very short wavelength “light” (or rather electromagnetic waves). Tissue doesn’t absorb these X-rays much at all and they can go through people to form images.

What would you hypothesize the effects of black light bulbs to be on the tanning of human skin?

I would expect that certain black light sources would cause tanning with only modest burning while other black light sources would cause burning with only modest tanning. Black light—also known as ultraviolet light—consists of very energetic light particles. The particles or photons of ultraviolet light contain enough energy to break chemical bonds and rearrange molecules. When you’re exposed to such energetic light, it causes damage to molecules in your skin cells and your skin may respond by darkening in the process we call “tanning.” But ultraviolet light is a general term that covers a broad range of wavelengths and photon energies. Long wavelength/low energy ultraviolet light tends to cause tanning while short wavelength/high energy ultraviolet light tends to cause burning—it directly kills cells. But these differences aren’t sharp and any ultraviolet light will cause some amount of skin damage.