Interference

The net effect of two (or more waves) meeting whilst travelling in the same medium is called wave interference.

When two waves arrive in step they reinforce and when two waves arrive out of step they cancel out. This can be shown by considering two waves travelling in opposite directions in a rope.

Interference of Light

In order to explain the interference of light let us assume light from two sources with the same wavelength and amplitude is projected onto a screen. The light from the two sources are also vibrating in phase with each other i.e. the crest of one wave is produced at exactly the same time as the crest from the second wave.

The light from two such sources will produce a pattern of the screen of alternating bright and dark bands. This is a result of the interference of light between the two sources and the pattern produced is called an interference pattern.

Where the crest from one wave source meets the crest from the second wave source they reinforce to produce a wave of double the amplitude and a higher brightness, visible by the bright bands in the interference pattern. This also happens at the points where two troughs meet. This type of interference where the crests and troughs from two wave sources arrive at the same time and reinforce is called constructive interference.

Where the crest from one wave source meets the trough from the second wave source the resultant amplitude is zero so they cancel each other out and these points are shown by the dark bands in the interference pattern. This type of interference where the crest from one wave source arrives at the same time as the trough from a second wave source and they cancel each other out is called destructive interference.