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

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:

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

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:

Analysing the Results

• The fringe spacing equation is given by:

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