6C10.XX Fraunhofer to Fresnel Diffraction

Concepts

Interference, diffraction

Overview

A laser beam is shone onto a variable single slit. The width of the slit determines the type of diffraction produces on the screen. When the slit is very narrow, the Fraunhofer diffraction appears on the screen. When the slit width is increased, the Fresnel diffraction appears.

Fresnel diffraction

Fraunhofer diffraction

Details

Equipment

  • [1] Variable single slit
  • [1] Laser with beam expander
  • [1] Optical rail
  • [1] Slide mount
  • [1] Saddle
  • [2] Lab jack
  • [1] Extension cord (if necessary) 

Classroom Assembly

  1. Place the laser on top of the lab jacks, making sure the laser is pointed away from people.
  2. Place the single slit on the optical rail.
  3. Plug in the laser and aim it at the slit.
  4. Adjust the width of the slit in two dimensions such that the Fraunhofer diffraction pattern is visible.
  5. Use the lab jacks to elevate the fringe pattern to a screen, if necessary.
  6. Turn off the laser.

Important Notes

  • Lasers can cause blindness. Use with caution.

Script

  1. Turn on the laser.
  2. Turn off the room light to see the diffraction pattern more easily.
  3. Point out the Fraunhofer diffraction pattern.
  4. Increase the slit width by turning the two knobs of the variable slit.
  5. Point out the Fresnel diffraction pattern.
  6. Turn off the laser.
 

Additional Resources

References

  • PIRA 6C10.15

Disclaimer

  • Don't attempt this at home!

Last revised

  • 2017

Technicals

Related AV

Related demos

  • Variable Single Slit

 

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.