Young's Double Slit — Definition

Definition

Imagine you have a single source of light, like a tiny bulb. If you place a screen with two very narrow, closely spaced slits in front of this bulb, the light passing through these two slits will behave as if there are now two separate, identical light sources.

These two 'new' sources are special; they are called 'coherent sources' because they emit light waves that have a constant phase difference between them and the same frequency. When the light waves from these two coherent slits travel and eventually meet on another screen placed further away, something fascinating happens: they don't just add up their brightness uniformly.

Instead, they create a pattern of alternating bright and dark bands, known as 'interference fringes'.

The bright bands, or 'maxima', occur where the crests of waves from one slit meet the crests of waves from the other slit (or trough meets trough). When this happens, the waves reinforce each other, leading to a much brighter spot.

This is called 'constructive interference'. The dark bands, or 'minima', occur where the crest of a wave from one slit meets the trough of a wave from the other slit. In this case, the waves cancel each other out, resulting in a dark spot.

This is called 'destructive interference'.

Young's Double Slit Experiment (YDSE) is precisely this setup. It's a classic experiment in physics that beautifully demonstrates that light behaves like a wave. The pattern of bright and dark fringes on the screen is direct evidence of light waves interfering with each other, much like ripples on water.

The spacing of these fringes, called 'fringe width', depends on several factors: the wavelength of the light used, the distance between the two slits, and the distance from the slits to the screen. By observing and measuring these fringes, we can even determine the wavelength of light, which was a remarkable achievement in Young's time.

It's a cornerstone experiment that solidified our understanding of light's wave nature.