• Hot objects give off light. The sun, incandescent light bulbs, and a red-hot poker are examples. The color of the emitted light depends on the temperature -- objects that are merely warm only emit infrared light, but with increasing temperature the object glows red, then orange, then yellow, and so on (the spectrum is a mixture of many wavelengths, so that the sun's light is white, rather than blue). This is how astronomers determine the temperature of stars; some are so hot that they are brightest in the blue and appear blue-white. Incandescent light bulbs are not as hot as the sun -- all materials melt well below this temperature -- and are actually much redder. Most of the power that goes into an incandescent light bulb comes out in the form of invisible infrared light. This is not very efficient, and this is why incandescent light bulbs are increasingly being replaced by other kinds.
• Excited atoms give off light. When we run electricity through a gas, electrons get knocked loose (gaining energy from the electrical current); when they rejoin the atom, the energy is released as light. This is what happens in neon and mercury lamps.
  Light made this way tends to have a very complicated spectrum, because the electrons can only have certain energies in the atom. The pattern is different for each element. The colors of fireworks are determined by which elements are present for this same reason.
  Incandescent objects give off much of their energy in the form of invisible infra-red light, while excited atoms may convert most of the electrical energy into visible light. This is why street lights tend to be sources with funny colors and complicated spectra: we get more visible light for the electricity we pay for.
• Some materials can absorb light energy and then emit light, changing the color in the process. This is used in fluorescent lights to turn ultraviolet light made by exciting mercury vapor into visible white light. Without the coating on the inside walls of the fluorescent tube, the mercury vapor light would not be very useful.
• Some chemical reactions result in the emission of light. Fireflies make their glow this way.

The brightness of a beam of light is the total amount of energy that is delivered in a second, divided by the cross-sectional area of the beam. A lens focusses light to a tiny spot that is very bright, because all the energy that hit the lens is brought together into a very small area.

The ability of light to cause chemical reactions depends on its color. It is rather generally true that light from the blue end of the spectrum is more effective than light from the red end in making things happen. Light actually delivers its energy in little packages (called quanta), and blue light has more energy per package than does red light, so that it "hits harder." As a consequence, photocopiers and black-and-white film do not "see" red light (the necessary reaction does not occur); ultraviolet light fades the drapes and gives us a sunburn.

Here is how the luminescent paper works: (A) when you shine short wavelength (blue) light on it, some electrons are knocked loose. We could think of these electrons as if they were golf balls, initially at the bottoms of wells: it takes some energy to lift them out, and the light has to deliver enough energy to make this happen. Red light and green light don't have enough energy, apparently, but blue light does. (B) Now the electrons run around for a while, looking for a well to fall into. They may run a little faster if the material is warmer. (C) When they fall back down the well, the energy they give up is emitted as visible light. However, this is generally less energy than it took to get them out of the well, and so the light is longer wavelength (green instead of blue).
Radio waves are a kind of light, too; they belong very far beyond red light in the electromagnetic spectrum. Television and FM radio stations cannot be received well unless the antenna can "see" the broadcast tower, because this kind of light travels in straight lines, too.

Microwaves are a kind of light. Most things are fairly transparent to microwaves, but materials containing water absorb them to a certain extent. This still allows microwaves to penetrate to the center of a potato, so that it is warmed up uniformly throughout the interior, rather then making the skin crispy and leaving the center raw.

Because microwaves are a very very red kind of light, they can agitate molecules (warm them) but cannot cause chemical reactions. For this reason, physicists are sceptical of the claims that power lines, microwaves, or cell phones can have any health effect: sunlight is surely more dangerous.
Conservation of energy