Young’s Double-Slit Experiment (DP IB Physics: SL): Revision Note

Young's Double Slit Experiment

• 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

• Lasers are the most common sources used in Young's double slit experiment because the waves must be:

  • Coherent (have a constant phase difference and frequency)

  • Monochromatic (have the same wavelength)

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

  • The light source is placed behind the single slit

  • The light is then diffracted to produce two sources in the double slit at 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

• In the ideal two-source model, each bright fringe has the same width and intensity

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:

  • In the ideal two-source model, each bright fringe is a peak of equal maximum intensity

  • Each dark fringe is 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 the conditions for interference as explained in the previous revision note on double source interference

  • For constructive interference (or maxima):

path difference = $n\lambda$

• For destructive interference (or minima):

path difference = $\left(n+\frac{1}{2}\right)\lambda$

• For the maxima 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....

Double Slit 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:

$s=\frac{\lambda D}{d}$

• Where:

  • s = separation between successive fringes on the screen (m)

  • λ = wavelength of the waves incident on the slits (m)

  • D = distance between the screen and the slits (m)

  • d = separation between the slits (m)

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

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

  • The wavelength of the incident light increases

  • The distance between the screen and the slits increases

  • The separation between the slits decreases