Light is a wave-like phenomenon that consists of alternating electric and magnetic fields. The speed at which light travels is a constant, but the wavelength - the distance over which the alternating electromagnetic fields make one full cycle - varies from one color of light to another. The electric portion of light can alter the positions of molecules (called 'rhodopsins') in our eyes, which enable us to see it. Different kinds of rhodopsins require more energetic, shorter wavelength (more blue) light to affect, which is how we distinguish different colors. There is no limit to how powerful or how weak light can be, however. Light that is too energetic (ultraviolet and up) will simply destroy the rhodopsins, while light that is not energetic enough (infrared and down) are just not strong enough to affect any of our rhodopsins, and as a result, we can't see them.

We can, of course, build machines that can detect colors of light that we can't, and many colors of light we can detect in ways other than vision. All colors of light still carry energy, so if you are having sufficiently bright light shining on you, you will feel it as heat, even if it's of a color you can't see. To prove it, you can try the following: hold your arm out in the sun, and note how warm it feels. Now hold your arm in the shade, and note that it is cooler. Last, place your arm so that it is in the sun, but behind a window. Your arm will feel in-between the full sun and the full shade. This is because the glass is stopping the infrared and ultraviolet light, so your arm is only being hit by a fraction of the sun's light.

You can also build an antenna that will collect light of a color with a wavelength equal to the length of the antenna. That's how radios work.

When scientists, and especially physicists, talk about light, they're referring to any wavelength of radiation. There are certain wavelengths of radiation that we can see (visible light) but there are other wavelengths that we cannot see. Some ranges of those radiation wavelengths that you may have heard of before include ultraviolet and microwave radiation. You probably know that ultraviolet radiation can cause sunburn, even though you cannot see it. You probably also know that microwaves can cook food, even though you cannot see that light either. If you want to look at radiation in a little more depth, here's a neat website I found:

Our eyes are only able to see certain colors of light. Mostly, we have evolved to see the light that is emitted from the sun. This is the visible light range which is around 400-700 nm. Light out side this range would be invisible to us. Some animals such as snakes can see light that we can't, like near infra red light, or infrared light from 800-1100 nm. This allows them to see even in the dark, when there is no visible light. Night vision goggles use this same idea, allowing us to see NIR light.

Light occurs in a wide range of wavelengths. We only see in a very small section of the entire wavelength (visible light). Invisible light is only the wavelengths that are outside the ability of our eyes to see. On either side of visible light is infrared light and ultraviolet light. Photography can, using special film, capture these lights in ways that we can see.Other animals, such as some insects see in ultraviolet light. Certain flowers have ultraviolet markers in them to guide insects to where the pollen is.