physics

• Path difference and coherence

  Coherent waves - wave sources of the same type and a single frequency

  • Ensures they maintain a constant phase relationship

  Path difference - the difference in length of the paths travelled by different waves
• Core practical (2) - investigating interference effects

  • When monochromatic light is passed through a diffraction grating with hundreds of slits per millimetre, the interference pattern is sharp
  • More accurate measurements
  • Resolution of measuring equipment:
  • Metre ruler = ±1 mm
  • Vernier Callipers = ±0.01 mm

  
• Young’s double slit experiment

  Aim  

  The overall aim of this experiment is to investigate the relationship between the slit-screen distance, D, and the fringe width, w

  Test variables

  • Independent variable = Distance between the slits and the screen, D
  • Dependent variable = Fringe width, w
  • Control variables = Wavelength of laser light, λ; slit separation, s 
• Method 

  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 and divide by the number of fringe widths to find the fringe width, w
  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 w for each distance D

  
• Analysing results

  • Fringe spacing equation
  • w = wavelength x D / s
  • Comparison to y=mx+c
  • y=w
  • x=D
  • Gradient = wavelength / s
• Interference by diffraction grating

  Aim

  To calculator the wavelength of the laser light using a diffraction grating

  Test variables

  • Independent variable = Distance between maxima, h
  • Dependent variable = The angle between the normal and each order, θn 
  • Control variables = distance between slits and screen, D; laser wavelength, ; slit separation, d
• 1. Place the laser on a retort stand and the diffraction grating in front of it
  2. Use a set square to ensure the beam passes through the grating at normal incidence and meets the screen perpendicularly
  3. Set the distance D between the grating and the screen to be 1.0 m using a metre ruler
  4. Darken the room and turn on the laser
  5. Identify the zero-order maximum
  6. Measure the distance h to the nearest two first-order maxima using a vernier calliper
  7. Calculate the mean of these two values
  8. Measure distance h for increasing orders
  9. Repeat with diffraction grating with different number of slits per mm

  
• Analysing results

  • Diffraction grating equation
  • n x wavelength = dsin(x)
  • Angle calculated with trigonometry
  • tan(x)= h/D ->  x = tan-1(h/D)