Waves - Reflection

When light waves strike a plane or curved mirror, they bounce off the shiny surface. The key point from the law of reflection is that the angle of reflection equals the angle of incidence. So if you want to turn a wave through 90°, the wave must hit the surface at 45°.

To measure the angles, you need a reference point, this is called the normal. The normal is a construction line that is drawn at 90° to the surface of the mirror (or whatever the reflecting surface is) at the point of incidence of the ray of light being studied. Angles are measured from the normal and not the surface of the mirror. Mirrors are used in telescopes (unsurprisingly called reflecting telescopes) for astronomical use, examples of these telescopes are Newtonian reflectors and the more complicated Cassegrain reflectors.

There are two types of reflection - regular and irregular (sometimes called diffuse). Regular reflection produces a clear image such as the image seen in a mirror. It is produced by reflection off smooth, polished surfaces. This type of image is a virtual image. It is as far behind the mirror as the object is in front, the right way up and same size as the object but it is laterally inverted. Laterally inverted means that it is swapped round horizontally. If you look at your reflection in a mirror and raise your right hand, your reflection will appear to raise its left hand. Writing appears to be reversed, which is why some ambulances have the word 'ambulance' written in 'mirror writing' on their bonnet.

Irregular reflection occurs when a wave is reflected by a rough surface. This produces no image because the individual rays of the wave coming into your eye from the object are all reflected through different angles and are coming from different parts of the reflecting surface. Although some surfaces can feel quite smooth, on a microscopic scale they are quite rough. All you see is the reflecting surface itself, no image.

Waves can become trapped inside objects because of reflection. This is a special type of reflection called total internal reflection. It happens where a wave enters a long thin object through one of the ends. The usual example of this at senior high school is in glass fibers. The fibers have a small diameter compared with their length so any light entering through the end of the fiber will hit the outside surface at a low angle and be reflected instead of escaping from the fiber. Total internal reflection is very efficient and light can be transmitted through glass fibers over long distances.