Young’s double-slit interference

In Young’s double-slit experiment, light reaches two closely spaced slits. Light spreads out from each slit, and the two waves overlap on a screen. At each point where they overlap, the waves combine.

The slits act as coherent sources: the waves they produce maintain a constant phase difference. At a fixed point on the screen, the waves therefore combine in a consistent way, producing a stable pattern.

The waves travel different distances from the two slits to most points on the screen. This affects their phase difference when they arrive:

• Where the waves arrive in phase, their peaks and troughs line up. They reinforce each other by constructive interference, producing a bright fringe.
• Where the waves arrive in antiphase, a peak from one wave lines up with a trough from the other. They cancel by destructive interference, producing a dark fringe.

Different points on the screen have different path differences, so bright and dark fringes alternate. This interference pattern demonstrates the wave nature of light.

Acceptance of the wave theory of light

Huygens proposed that light behaves as a wave, but his theory was not immediately accepted. Newton’s influential view that light consisted of particles helped a competing explanation remain prominent.

Young’s double-slit evidence

Young passed light through two narrow slits. Where the light from the slits overlapped, it produced a pattern of alternating bright and dark bands, called interference fringes.

The wave theory explained this pattern: where the light waves reinforced each other, a bright fringe formed; where they cancelled each other, a dark fringe formed. The pattern was therefore evidence that light could behave as a wave.

Young’s experiment did not immediately persuade everyone. The wave interpretation gained wider support later, as further work on interference and diffraction strengthened the evidence for it.