Interference occurs when multiple waves interact with each other and is a change in amplitude caused by several waves meeting. In Young’s Double-slit experiment, a coherent light source goes through 2 small slits to create a dark and white pattern on a wall beyond the slits.

Unlike solid objects, two waves can share a point in space. In physics, interference is a phenomenon in which two waves (passing through the same point) superimpose to form a resultant wave of higher or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other (i.e., “interfere” with each other), either because they come from the same source or because they have the same or nearly the same frequency.

Interference can be constructive or destructive. In constructive interference, the two amplitudes of the waves add together and result in a higher displacement than would have been the case if there were only one wave.

Destructive interference is when two waves add together, and the result is a smaller displacement than would have been the case. When the waves have opposite amplitudes at the point, they meet they can destructively interfere, resulting in no amplitude at that point.

Young’s double slit experiment

The double-slit experiment, also called Young’s experiment, shows that matter and energy can display both wave and particle characteristics.

The double-slit experiment, also called Young’s experiment, shows that matter and energy can display both wave and particle characteristics. In his experiment, he sent light through two closely spaced vertical slits and observed the resulting pattern on the wall behind them. The wave characteristics of light cause the light to pass through the slits and interfere with itself, producing the light and dark areas on the wall behind the slits. The light that appears on the wall behind the slits is scattered and absorbed by the wall, which is a characteristic of a particle.

The black-and-white pattern proves that light behaves like a wave. Destructive and constructive interference occurs when the waves collide. Constructive interference: when the light waves are in sync, this creates bright spots on the wall. Destructive interference: creates dark spots when the light waves are out of sync and cancel out one another.

Since light behaves as a wave, it has a wavelength (λ) and a frequency (f).

The pattern on the wall depends on these factors:

– a: The aperture of each slit. Aperture must be much larger than the wavelength.
– λ, f: Wavelength and frequency. This defines the interference pattern.
– d: Distance between the 2 slits.
– Medium of the distance D between the slits and the wall affects the pattern. D must be much greater than d and D.

Note that the intensity of the light source does not affect the pattern of interference. Interference normally refers to the phenomenal with 2 slits. Interference can occur with only 1 slit, but this is known as diffraction instead.

Practice Questions 

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Key Points

• Interference is a phenomenon of wave interactions. When two waves meet at a point, they interfere with each other.

• There are two types of interference, constructive and destructive.

• In constructive interference, the amplitudes of the two waves add together resulting in a higher wave at the point they meet.

• In destructive interference, the two waves cancel out, resulting in a lower amplitude at the point they meet.

• When waves collide, destructive (out-of-sync) and constructive (in sync) interference occurs, create a black-and-white pattern.

• Light behaves as a wave, with wavelength (λ) and a frequency (f).

• The interference pattern depends on a, λ, f, d, and the medium between D.

• The wavelength (λ) must be much smaller than the aperture (a) for interference to happen.

Key Terms

interference: (of waves) to be correlated with each other when overlapped or superposed.

diffraction: collision of light waves through 1 small slit

displacement: a vector quantity that denotes distance with a directional component

amplitude: the maximum absolute value of some quantity that varies

coherent: of waves having the same direction, wavelength and phase, as light in a laser