When light strikes an object it can either be absorbed, transmitted, or reflected. Opaque objects absorb or reflect light. Transparent and translucent materials transmit light, but can also reflect it. The energy carried by the light must be conserved. The sum of the energy reflected, transmitted, and absorbed must equal the energy that strikes the material. Light energy that is absorbed increases the thermal energy of the material so the material becomes warmer.

Reflection occurs when light “bounces” off of a surface, such as a mirror or a sheet of paper. A smooth surface, like that of a mirror, reflects light according to the rule that the angle of incidence equals the angle of reflection. Light that strikes the surface at, for example, 30° above the surface is reflected at 30° above the surface. You can test this for yourself with a small flashlight, a mirror, a sheet of paper, and a helper. Hold the piece of paper. Have the helper hold the flashlight, aim it at the mirror, and move it around until the reflected light ray hits your piece of paper. Change the angle with which the light from the flashlight strikes the mirror and see that you have to move the paper closer to the mirror to have the light hit it.

What happens when light hits a sheet of paper? Replace the mirror with a sheet of paper. Darken the room. Aim the flashlight at the paper and use a second sheet to catch the reflection. You’ll see that the second sheet is illuminated in many different locations. The light is reflected from the paper, but into many directions. This kind of reflection is called diffuse reflection.

Polished, smooth surfaces that do not absorb light are the best reflectors; examples of reflective materials are shiny metals, whereas non-reflective materials are dull metals, wood, and stone.