Concepts

Diffraction, Babinet's principle

Overview

When a laser beam is incident on a narrow opaque object such as a fine wire or a strand of hair, the resulting diffraction pattern is similar to the diffraction pattern of a slit of the same width as the object, in agreement with the Babinet's principle. The only difference is that the centre of the diffraction pattern of a wire looks brighter. This is because the undiffracted portion of beam adds to the intensity of the centre of the pattern.

Details

Equipment

• [1] Fine wire attached to a holder
• [1] Variable single slit
• [2] He-Ne tube laser
• [2] Optical rail
• [4] Saddle
• [2] Lab jack (if necessary)
  
• [1] Extension cord (if necessary) 

Classroom Assembly

1. Mount a laser and wire holder on opposite ends of the rail, making sure the laser is pointed away from people.
2. Plug in the laser and aim it at the wire, maximizing the intensity and spread of the diffraction pattern.
3. Use a lab jack to elevate the fringe pattern to a screen, if necessary.
4. Repeat step 1 to 3 for a single slit.
5. Turn off the lasers.

Important Notes

• Lasers can cause blindness. Use with caution.

Script

1. Turn on the two lasers.
2. Turn off the room lights to see the diffraction patterns more easily.
3. Point out the diffraction pattern is the same for a fine wire and a single slit.
4. Turn off the lasers.

Additional Resources