# Principle of Superposition

When two waves meet at a point then the resultant displacement at that point is a vector sum of the displacements.

### Young’s Double Slit Experiment

The principle of superposition is best represented using Young’s double slit experiment. This is when light is shined through a double slit creating light and dark fringes on a screen. This provides evidence that light travels in waves.

If a monochromatic lamp is used instead of a laser, a source slit can be used, which turns an incoherent wave train into a coherent source (lasers are already coherent). This causes the light to be diffracted so the same wave trains reach the two slits.

### Coherence

Two sources are considered coherent when they are in phase or have 1 wavelength phase difference. They must also have the same frequency and wavelength.

Constructive interference occurs at the light fringes. This is when two waves arrive in phase. This means they have a whole number of wavelength path difference.

Destructive interference occurs at the dark fringes, when two waves arrive in anti-phase or a half cycle phase difference therefore the path difference equals a whole number of wavelength + ½ a wavelength.

### Calculations

Young’s double slit experiment also allows the wavelength of light to be calculated using the following equation: