When light falls on a surface, one of four things can happen – it can be reflected, refracted, scattered or absorbed.
Reflection occurs when light bounces back from a surface. A skinny beam of light (such as a laser) still bounces back as a skinny beam. Reflection occurs from smooth surfaces. We usually think of reflection from shiny surfaces, such as a mirror. A smooth, transparent glass surface, such as a window pane, can also reflect light, but it will reflect less efficiently than a shiny surface. The surface need not be flat, since even curved mirrors reflect light. Reflection of light is governed by a set of rules, which we will learn about in the section on reflection.
Refraction occurs when light travels from one material to another. The speed of light in different materials is different – light is fastest in vacuum, and slower in all other materials. This change in speed also causes light rays to bend. We will learn more in the section on refraction.
Scattering is sometimes called diffuse reflection.Scattering usually occurs when the surface is not smooth –and by that, we mean smooth on the level of the wavelength of light (about 0.0000005 m).A sheet of paper is smooth, but not on that level. When light bounces off paper, each incoming ray breaks up into several rays that bounce off in different directions. Scattering is why a whole room appears lit by a couple of light bulbs, and why the sky looks bright during daytime (and also why it looks blue).
Absorption occurs when a material absorbs some of the light falling on it. When the molecules of the material are able to absorb some of the light, then the amount of light the material reflects, transmits and scatters is less than the amount of light that falls on it. For example, when you look through sunglasses, the scenery looks dimmer than without sunglasses. That’s because the sunglasses absorbed some of the light. Some materials may absorb certain colors but not others. For example, a yellow-colored sheet of cellophane absorbs a lot of blue and green light, so the light coming through it appears yellow.
Notice that absorption deals with the amount of light, rather than the direction of the light rays, as reflection, refraction and scattering do.