Wavefronts and rays: the law of reflection

1 A useful representation of em waves: wavefronts and rays

Light waves, or in general em waves produced by a certain source, propagate in the surrounding space with crests and valley (points where electric field and magnetic field are stronger or weaker) as water waves do. A useful representation of waves is therefore by sketching the wavefronts, that are circular lines representing the points of the wave having a distance equal to a wavelength l from one another. The behaviour of the wave can be described by these wavefronts and by using rays perpendicular to these wavefronts. [see the figure below]

2 Law of reflection

When light waves strike a smooth reflecting surface like a mirror, they are reflected away with the same speed and forming the same angle to the surface they had before striking it. By tracing rays and the surface normal (a line perpendicular to the surface) on a drawing , the phenomenon becomes clear:

when rays of light are reflected from a reflecting surface, the angle q_r the reflected rays make with the surface normal is equal to the angle q_i the incident rays make with the surface normal. Shortly, the angle of incidence is equal to the angle of reflection:

q_i=q_r

The law of reflection explains for instance how an image is formed in a plane mirror.

Looking at the figure above, few rays are traced from one point of the nose to show their reflection from the mirror. they diverge after reflection as though they were coming from a virtual point behind the mirror. Hence when we look at the mirror we see our virtual image as if it were behind the mirror at a distance equal to our distance from the front of the mirror.

File translated from T_EX by T_TH, version 3.22.
On 05 Mar 2003, 00:07.