Determine the Wavelength of Light from a Laser or other Light Source using a Diffraction Grating

Equipment  

  • Diffraction grating 
  • Laser 
  • Screen 
  • Ruler

Method

1. Shine the laser through the diffraction grating onto the screen. 

2. Measure the distance between the central fringe and the one beside it (1st order - see below). 

3. Measure the distance between the grating and the screen.

diagram                diagram

Calculations

  • The formula associated with diffraction gratings is: d sinθ λ = n Where d is the distance between the slits, θ is the angle to the normal made by the maximum, n is the order and λ is the wavelength. 
  • To find tan θ divide the distance between the central fringe and the one beside it by the distance between the grating and the screen (tanθ=opp/adj) then use inverse tan ( θ) tan−1 to find θ. 
  • To find d, read the information on the packaging of the grating - it will say how many lines per mm. Note that if it has 350 lines/mm that is 350,000 lines/m and so 1/350,000 is the slit spacing. 
  • We measured the distance to the first order hence n = 1. 
  • Substitute all these values into dsinθλ = (n is not included as n = 1) to find the wavelength of the laser.

Notes

  • Also calculate the wavelength using 2nd and 3rd order measurements and find the average of these values for the mean wavelength. 
  • Vary different properties such as the number of lines in the diffraction grating and the wavelength of the light to see how they affect θ.

Safety

  • When using lasers, ensure that you never look directly at the beam as doing so cause permanent eye damage 
  • Always display a laser in use sign so that everyone is aware that a laser is being used. 
  • Never shine it off shiny surfaces since it may reflect into someone’s eyes.