Interference of Waves

In contrast to refraction, interference can only be naturally explained by thinking of light as waves. Consider two waves meeting as shown in the Figure below:

 

Figure 10.15: Interference of waves

For constructive interference, the waves meet in phase, i.e. so that the crests of each wave coincide. In destructive interference, the waves meet out of phase, so that the crest of one wave coincides with a trough of the other wave, and they cancel each other out. This readily explains the double slit interference pattern of Fig.10.1.6. The two light rays start off in phase since they come from the same source. By the time they reach the screen after having been diffracted through the slits, they have travelled different distances, so that the crests of the ray that travels further (the bottom ray in Fig.10.1.6 lag a little behind those of the top ray. If they lag behind by a half a wave length, the crest of one ray will meet the trough of the other at the screen, and destructive interference will naturally result. On the other hand if one ray lags behind by a full wavelength (or two or three full wavelengths, etc.) a crest will still meet a crest at the screen, and a bright spot will appear due to constructive interference.