Objects are visible because they give off or receive light. Objects that give off light are called luminescent, and those that receive light are said to be illuminated. Some illuminated objects transmit light, and nearly all absorb and reflect light. Transparent objects transmit most of the light that reaches them, thus making it possible to see objects through them. Clear glass is an example. Translucent substances, such as frosted glass and waxed paper, are partially transparent; they diffuse light, and thus objects cannot be seen through them. Opaque substances transmit no light.

The colors of objects depend on how they reflect and absorb light. White light is made up of violet, blue, green, yellow, orange, and red light. When passed through a prism, it separates into these colors. A substance that reflects all the colors of white light is white. A substance that absorbs all these colors is black. Most objects absorb some colors and reflect others. They are called by the colors they reflect. Thus an object that absorbs all colors except blue is called blue.

Vision depends on how light affects the eye, and this depends on the condition of the eye itself.