Two important points are made in the unit on light energy: Light carries energy, and bluer light is better able to cause chemical and physical reactions than is redder light.

Light carries energy

No doubt you already knew that sunlight brings energy, and having all the lights on costs you on the electrical bill. However, there is also a lot of energy carried about by invisible infrared light. You can experience this for yourself in early evening as you walk by a wall that has been in direct sunlight all afternoon. The presence of a warm object is sensed immediately, because it is glowing in the infrared.

The glass container in a thermos bottle is silvered, because this prevents transfer of energy by light (it is reflected, instead of being absorbed). The double-wall construction of the thermos prevents energy transport by conduction; but emission and absorption of light would still allow your coffee to cool.

Conservation of energy dictates what kind of display you can have on your digital wristwatch or pocket calculator. Displays that actually emit light are emitting energy, and run the batteries down a lot faster than the black-on-silver kind, which are reflecting the room light (or not) to make the pattern that you see.

The first pocket calculators used light emitting diodes, giving a bright red display. (Light emitting diodes are the little red lights used in the brake light display panels in the rear windows of cars). These are very efficient -- nearly all of the electrical energy is turned into light energy, because the light is made one quantum at a time. But even then the battery on a pocket calculator was only good for an hour or so.

Bluer light is better able to cause reactions than redder light.

When the laundry soap people claim their product makes shirts "whiter than white" they are not kidding: they include a dye that absorbs the small amount of ultraviolet light that fluorescent bulbs emit and converts this into visible light. Some kinds of brightly colored markers and paints do this, too. This trick will not work with incandescent light (or candles) because they don't emit ultraviolet light.

Incandescent lights do make a lot of infrared light. It would be great if there was a dye that would turn this light into visible light, too. But this is much harder: the dye molecule will have to absorb one infrared quantum and store this energy until another quantum is absorbed, to gain enough energy to make a visible quantum. There are such materials, but they only work when the light is very bright (which means lots and lots of energy is arriving) -- that way, the second quantum is more likely to arrive before the first one escapes.

The reason we can't see infrared light is that the quanta are not energetic enough to start the chemical reactions that make vision work. The reason we can't see ultraviolet light is that it gets absorbed in the interior of the eye before it gets to the retina (this is probably to protect this delicate and valuable organ -- you would not want a sunburned retina! ).
The unit on light energy