DP IB Physics: HL

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First teaching 2014

Last exams 2024

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9.3.1 Young’s Double-Slit Experiment

Test Yourself

Young’s Double-Slit Experiment

  • Young’s double-slit experiment demonstrates how light waves can produce an interference pattern
  • The setup of the experiment is shown below:

Double slit experiment diagram, downloadable AS & A Level Physics revision notes

Young’s double-slit experiment arrangement

  • When a monochromatic light source is placed behind a single slit, the light is diffracted producing two light sources at the double slits A and B
  • Since both light sources originate from the same primary source, they are coherent and will therefore create an observable interference pattern
    • Both diffracted light from the double slits create an interference pattern made up of bright and dark fringes
  • The distance between the fringes can be calculated using the double-slit equation:

9-3-1-fringe-spacing

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

Investigating Young’s Double-Slits Experimentally

The overall aim of this experiment is to investigate the relationship between the distance between the slits and the screen, D, and the fringe width, s

  • Independent variable = Fringe width, s
  • Dependent variable = Distance between the slits and the screen, D
  • Control variables
    • Laser wavelength, λ
    • Slit separation, d

Method

9-3-1-youngs-double-slit-apparatus9-3-1-youngs-double-slit-apparatus

The setup of apparatus required to measure the fringe width s for different values of D

  1. Set up the apparatus by fixing the laser and the slits to a retort stand and place the screen so that D is 0.5 m, measured using the metre ruler
  2. Darken the room and turn on the laser
  3. Measure from the central fringe across many fringes using the vernier callipers (or ideally, a travelling microscope) and divide by the number of fringe widths to find the fringe width, s
  4. Increase the distance D by 0.1 m and repeat the procedure, increasing it by 0.1 m each time up to around 1.5 m
  5. Repeat the experiment twice more and calculate and record the mean fringe width, s, for each distance D
  • An example table might look like this:

9-3-1-youngs-double-slit-table

Analysing the Results

  • The fringe spacing equation is given by:

s equals fraction numerator lambda D over denominator d end fraction

  • Where:
    • s = the distance between each fringe (m)
    • λ = the wavelength of the laser light (m)
    • D = the distance between the slit and the screen (m)
    • d = the slit separation (m)

  • Comparing this to the equation of a straight line: y = mx
    • y = s (m)
    • x = D (m)
    • Gradient = lambda over d (unitless)

  • Plot a graph of s against D and draw a line of best fit
  • The wavelength of the laser light is equal to the gradient multiplied by the slit separation, as shown by the graph:

9-3-1-youngs-double-slit-graph

Evaluating the Experiment

Systematic errors:

  • Ensure the use of the set square to avoid parallax error in the measurement of the fringe width
  • The distance between fringes is very small due to the short wavelength of visible light
    • A monochromatic light source must be used so that the fringes easier to observe

Random errors:

  • The fringe spacing can be subjective depending on its intensity on the screen, therefore, take multiple measurements of s (between 3-8) and find the average
  • Use a Vernier scale to record distances s to reduce percentage uncertainty
    • Use a travelling microscope, if available, for the greatest accuracy
  • Reduce the uncertainty in s by measuring across all visible fringes and dividing by the number of fringes
  • Conduct the experiment in a darkened room, so the fringes are clear

Safety Considerations

  • Lasers should be Class 2 and have a maximum output of no more than 1 mW
  • Do not allow laser beams to shine into anyone’s eyes
  • Remove reflective surfaces from the room to ensure no laser light is reflected into anyone’s eyes

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Lindsay Gilmour

Author: Lindsay Gilmour

Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.