Fig.4.1. Law of reflection.

When a narrow beam of light strikes a flat surface (Fig.4.1) we definethe angle of incidence, to be the angle an incident ray makes with the normal to the surface ("normal" means perpendicular) and the angle of reflection, , to be the angle the reflected ray makes with the normal For flat surfaces, it is found that the incident and reflected rays lie in the same plane with the normal to the surface, and that the angle of incidence equals the angle of reflection: .

This is thelaw of reflection and is indicated in Fig.4.1 ( which is a side view). It was known to the ancient Greeks, and you can confirm it yourself by shining a narrow flashlight beam at a mirror in a darkened room.

When light is incident upon a rough surface, even microscopically rough such as this page, it is reflected in many directions. This is called diffuse reflection. The law of reflection still holds, however, at each small section of the surface. Because of diffuse reflection in all directions, an ordinary object can be seen from many different angles. When you move your head to the side, different reflected rays reach your eye from each point on the object. Let us compare diffuse reflection to reflection from a mirror, which is known as specular reflection ("speculum" is Latin for mirror). When a narrow beam of light is shone on a mirror, the light will not reach your eye unless it is placed at just the right place where the law of reflection is satisfied. This is what gives rise to the unusual properties of mirrors. (Galileo, using similar arguments, showed that the Moon must have a rough surface rather than a highly polished surface like a mirror, as some people thought.)

When you look straight in a mirror, you see what appears to be yourself as well as various objects around and behind you. Your face and the other objects look as if they are in front of you, beyond the mirror; but, of course, they are not. What you see in the mirror is animage of the objects.

Date: 2015-12-11; view: 921