Preparation and setup

• A partially darkened laboratory is required ensuring lasers are used safely.

• Set up the apparatus by fixing the laser and the slits to a retort stand and place the screen so that the distance from the double slit to the screen is 0.5 m (D), measured using the metre ruler

• Darken the room and turn on the laser

Conducting the experiment

• Carefully adjust the position of the laser until the light spreads evenly over the two slits. An interference pattern should be visible on the screen.

• Measure from the central fringe across many fringes using the vernier callipers and divide by the number of fringe widths to find the fringe width, w

Repeat the experiment?

• 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

• Repeat the experiment twice more and calculate and record the mean fringe width w for each distance D

Calculations and analysis

• The fringe width (or fringe spacing), w, can be measured by measuring across a large number of visible fringes. (Take care when counting – counting from the first bright fringe to the tenth bright fringe would represent nine fringe widths!).

• Use the metre ruler to measure D.

• A measurement of the slit separation, s, is required. The value could be measured with vernier callipers or travelling microscope. If a travelling microscope is used it must only be used to measure slit separation and not the fringe width. Alternatively the manufacturer may quote the value on the slide.

• Use the equation λ= w s / D

• Alternatively, the value of D could be changed from approximately 0.5 m to 1.5 m and the fringe width, w, measured for each value of D.

• A graph of w on the y-axis against D should be a straight line through the origin, with gradient = λ /s.

Method - Diffraction grating

Preparation and setup

• Swap out the single slit with the plane transmission diffraction grating.

Conducting the experiment

• Carefully adjust the position of the diffraction grating so that the diffraction grating is perpendicular to the beam of light from the laser. (A large set square might be useful).

• The diffraction pattern should be visible on the screen. The number of orders shown will depend on the line spacing of the diffraction grating.

Repeat the experiment?

• Use a set square to ensure the beam passes through the grating at normal incidence and meets the screen perpendicularly

• Set the distance D between the grating and the screen to be 1.0 m using a metre ruler

• Darken the room and turn on the laser

• Identify the zero-order maximum (the central beam)

• Measure the distance h to the two nearest first-order maxima (i.e. n = 1, n = 2) using a vernier calliper

• Calculate the mean of these two values

• Measure distance h for increasing orders

• Repeat with a diffraction grating that has a different number of slits per mm

Calculations and Analysis

• The angles Ɵ1 and Ɵ2 can be determined by measuring the distances h1, h2 and D. (This gives the tangent of the angles, and hence the angles can be calculated).

• The formula nλ = d sinƟ can be used to determine the wavelength of the laser light.
  n is the order of the diffraction pattern
  d is the grating spacing = 1/number of lines per metre
  λ is the wavelength of light

• The values of Ɵ for each order, both above and below the zero order, should be measured. A mean value for λ can be calculated from the data.