Interference and Diffraction

A unique property of waves is that when they come in contact with each other, they interact in such a way that the resultant wave has either an enhanced or diminished amplitude. This phenomenon is called interference. 

When the crests of two waves are aligned, they are ‘in phase’. As these waves interact, their amplitudes add up and a wave with increased amplitude is produced. This is constructive interference. 

If the two waves do not align — so that their crests are in opposite directions  — they are ‘out of phase’. In this case, the resultant wave has a lower amplitude. This is destructive interference. When the trough of one wave completely overlaps with the crest of the other, the waves cancel each other.

Diffraction is another characteristic behavior of waves. Take a top view perspective of a wave — so that the crests are represented by vertical lines — and imagine that it is traveling towards a barrier with a narrow aperture. The opening in the barrier is similar in size to the wavelength. 

As the wave passes through, it bends around the corners of the opening and spreads out in different directions. 

But how do waves behave when there are two or more apertures in the barrier? This can be observed using a light source. When light is emitted, the wave passes through the pair of closely spaced slits, and two diffracted waves emerge. These waves interfere with each other to create a pattern.

When the two waves travel the same distance, they interfere constructively. This is observed by the appearance of a bright line. 

A small distance away from the center, in either direction, one wave travels a slightly longer distance than the other wave and they become ‘out of phase’. The destructive interference creates a dark region. 

Depending on the distances traveled by the waves, an interference pattern of alternating bright and dark lines is formed. This double-slit experiment demonstrates the wave nature of light.