6D20.50 Crossed Gratings and Laser


Fraunhofer diffraction, Ronchi ruling


When a laser beam is incident on a Ronchi ruling, a single diffraction pattern is observed. As the ruling is rotated, the diffraction pattern rotates with the ruling. If the second Ronchi ruling, identical to the first ruling, is inserted and aligned such that it is parallel with the first ruling, the diffraction pattern does not change. However, as the one of the rulings is rotated so that the angle between rulings goes from 0 to 90°, a 2D-array of spots forms.



  • [2] Slide with Ronchi ruling attached
  • [1] He-Ne tube laser
  • [1] Fixed slide mount
  • [1] Rotating slide mount with Ronchi ruling attached
  • [3] Saddle
  • [1] Optical rail
  • [1] Lab jack (if necessary)
  • [1] Extension cord (if necessary) 

Classroom Assembly

  1. Mount the laser and and the slide containing Ronchi ruling on opposite ends of the rail, making sure the laser is pointed away from people. Use the rotating slide mount to hold the ruling.
  2. Plug in the laser and aim it at the ruling.
  3. Use the lab jack to elevate the pattern to a screen, if necessary.
  4. Turn off the laser.

Important Notes

  • Lasers can cause blindness. Use with caution.


  1. Turn on the laser.
  2. Turn off the room lights to see the diffraction pattern more easily.
  3. Point out the single diffraction pattern from one Ronchi ruling.
  4. Rotate the ruling. Notice the diffraction pattern rotates as the ruling is rotated.
  5. Return the ruling to its original position.
  6. Mount the second Ronchi ruling on the optical rail and align it so that it is parallel with the first ruling.
  7. Notice the diffraction pattern does not change.
  8. Rotate the first ruling. Point out how diffraction pattern changes as the ruling is rotated.
  9. Turn off the laser.


Additional Resources


  • PIRA 6D20.50


  • Don't attempt this at home!

Last revised

  • 2017


Related AV

Related demos

  • Lycopodium Powder Diffraction


If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.