7.3: Interference and Diffraction

Interference is a characteristic phenomenon exhibited by waves. When two electromagnetic waves interact with their peaks and troughs coinciding, a resulting wave with enhanced amplitude is produced. This is known as constructive interference. In this case, the two waves interacting are in phase with each other.

Alternatively, if the two waves coincide and interact in a way such that the trough of one wave coincides with the peak of the other (in an out of phase manner), the resultant wave will display a much lower amplitude. This is known as destructive interference.

Waves also show a characteristic behavior called diffraction. When a beam of light passes through a slit with a size comparable to the wavelength of the incident beam － the beam bends (or diffracts) around the slit. On the contrary, when a stream of particles is passed through a slit, the particles simply come out through the aperture.

Further, when a beam of light passes through a pair of closely spaced slits separated by a distance comparable to the wavelength of light, circular waves are produced at each slit by the process of diffraction. These two waves interfere with each other such that an interference pattern with alternate dark and bright lines is obtained on a screen placed at a short distance behind the slits.

The bright-line is produced at the center of the screen as the two waves travel an equal distance to reach this point and interfere constructively. When the two waves travel the small distance away from the center in either direction, they travel slightly different distances. They are out of phase. When the difference in their traveled distances is exactly one half of the wavelength, they meet by producing destructive interference. The dark regions correspond to regions where the peaks for the wave from one slit happen to coincide with the troughs for the wave from the other slit (destructive interference), while the brightest regions correspond to the regions where the peaks for the two waves (or their two troughs) happen to coincide (constructive interference). The diffraction pattern is an inherent property of waves and presents compelling evidence for the wave nature of light.

This text is adapted from Openstax, Chemistry 2e, Section 6.1: Electromagnetic Energy.

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.