Young's Double-Slit Experiment (AQA A Level Physics)

Double Slit Interference

• Young's double-slit experiment produces a diffraction and an interference pattern using either:

  • The interference of two coherent wave sources 

  • A single wave source passing through a double slit

• In this typical set-up for Young's double slit experiment:

  • The laser light source is placed behind the single slit

  • So the light is diffracted, producing two light sources at slits A and B

  • The light from the double slits is then diffracted, producing a diffraction pattern made up of bright and dark fringes on a screen

The typical arrangement of Young's double-slit experiment

Diffraction Pattern

• The diffraction pattern from the interference of the two sources can be seen on the screen when it is placed far away

  • Constructive interference between light rays forms bright strips, also called fringes, interference fringes or maxima, on the screen

  • Destructive interference forms dark strips, also called dark fringes or minima, on the screen

Young's double slit experiment and the resulting diffraction pattern

• Each bright fringe is identical and has the same width and intensity

• The fringes are all separated by dark narrow bands of destructive interference

The constructive and destructive interference of laser light through a double slit creates bright and dark strips called fringes on a screen placed far away

Interference Pattern

• The Young's double slit interference pattern shows the regions of constructive and destructive interference:

• Each bright fringe is a peak of equal maximum intensity

  • Each dark fringe is a a trough or minimum of zero intensity

• The maxima are formed by the constructive interference of light

• The minima are formed by the destructive interference of light

The interference pattern of Young's double-slit diffraction of light

• When two waves interfere, the resultant wave depends on the path difference between the two waves

• The wave from slit S₂ has to travel slightly further than that from S₁ to reach the same point on the screen

  • This extra distance is the path difference

The path difference between two waves is determined by the number of wavelengths that cover their difference in length

• Remember:

  • For constructive interference (or maxima), the difference in wavelengths will be an integer number of whole wavelengths

  • For destructive interference (or minima) it will be an integer number of whole wavelengths plus a half wavelength

• For the maximum in the interference pattern: 

  • There is usually more than one produced

  • n is the order of the maxima or minima; which represents the position of the maxima away from the central maximum

  • n = 0 is the central maximum

  • n = 1 represents the first maximum on either side of the central, n = 2 the next one along....

Fringe Spacing Equation

• The spacing between the bright or dark fringes in the diffraction pattern formed on the screen can be calculated using the double slit equation:

Double slit interference equation with w, s and D represented on a diagram

• In this experiment:

  • D is much bigger than any other dimension, normally several metres long

  • s is the separation between the two slits and is often the smallest dimension, normally in mm

  • w is the distance between the fringes on the screen, often in cm. This can be obtained by measuring the distance between the centre of each consecutive bright spot.

• The above equation shows that the separation between the fringes will increase if:

  • The wavelength, λ of the incident light increases

  • The distance, s between the screen and the slits increases

  • The separation, w between the slits decreases

Interference Patterns

• A source of white light diffracted through a double slit will produce the following diffraction pattern

  • It is different to that produced by a single slit or a diffraction grating

The interference pattern produced when white light is diffracted through a double slit

• Each maximum is of roughly equal width

• The central maximum is white because each wavelength interferes here constructively 

  • There are two dark narrow destructive interference fringes on either side

• All other maxima are composed of a spectrum

• Separate diffraction patterns can be observed for each wavelength of light

  • The shortest wavelength (violet / blue) would appear nearest to the central maximum because it is diffracted the least

  • The longest wavelength (red) would appear furthest from the central maximum because it is diffracted the most

• The colours look blurry and further away from the central maximum, the fringe spacing gets so small that the spectra eventually merge without any space between them

  • As the maxima move further away from the central maximum, the wavelengths of blue observed decrease and the wavelengths of red observed increase