What causes interference patterns in a double-slit experiment?

Interference patterns in a double-slit experiment are caused by the superposition of waves from the two slits.

In a double-slit experiment, light is shone through two closely spaced slits, creating two separate wave sources. These waves then overlap and interfere with each other, leading to the creation of an interference pattern. This pattern is characterised by alternating bright and dark bands, known as fringes, which are observed on a screen placed behind the slits.

The bright fringes are a result of constructive interference, where the waves from the two slits arrive in phase, i.e., their peaks and troughs align. This causes their amplitudes to add together, resulting in a brighter light intensity. On the other hand, the dark fringes are due to destructive interference, where the waves arrive out of phase, i.e., the peak of one wave aligns with the trough of the other. This causes their amplitudes to cancel each other out, resulting in no light being observed.

The position of these fringes can be predicted using the principle of superposition and the wave nature of light. The path difference between the waves from the two slits determines whether they will interfere constructively or destructively. If the path difference is a whole number of wavelengths, the waves will arrive in phase and interfere constructively, creating a bright fringe. If the path difference is a half number of wavelengths, the waves will arrive out of phase and interfere destructively, creating a dark fringe.

The double-slit experiment is a fundamental demonstration of the wave-particle duality of light and other quantum particles. It shows that light and particles like electrons exhibit both wave-like and particle-like properties. When the slits are observed individually, they behave like particles, but when observed together, they interfere like waves, creating an interference pattern. This phenomenon cannot be explained by classical physics and is a cornerstone of quantum mechanics.