Diffraction

Diffraction is also readily explained in terms of light waves. It is will known that when waves can come in two basic shapes. Plane waves are waves in which the crests are essentially straight lines that follow one another like lines of soldiers walking in formation. Circular waves consist of crests that move out from a point source in circles of ever-increasing radii. Circular waves are the easiest to observe: one need only throw a pebble into the middle of a still lake. Plane waves are formed by the wind in lakes and oceans, and in the case of extreme weather, can be very dangerous. Both plane and circular waves are illustrated in Fig.10.2.2 below. The lines represent the crests of the waves in each case. When plane waves come to a barrier, such as a wall, they are stopped. But if the wall has a narrow opening some of the wave gets through. If the opening is sufficiently narrow, specifically it must be of the same size as the wavelength of the wave, then the part of the wave that gets through is a circular wave, that looks like it is coming from a point source (i.e. the opening). In this way the wave appears to bend around the corners of the opening. This explains the diffraction phenomenon illustrated in Fig.10.1.5. Now we can understand the semi-circular lines on the right of the slit as representing the crests of the circular waves that get through, whereas the ``rays'' represent the direction in which the crests are moving.

  

Figure 10.16: Plane and circular waves.