Concepts

Diffraction pattern off of small spheres

Overview

When a laser beam is incident on many small objects such as lycopodium powder, the resulting diffraction pattern is similar to the diffraction pattern of a small circular aperture (Airy's disk pattern). The difference is that in a lycopodium powder diffraction, light gets diffracted from the edges of the tiny powder particle, creating irregular speckled pattern. When a focusing lens is placed in front of the laser, the laser beam is incident on fewer particles. This results in the appearance of bigger irregular spots. The laser can be focused on the surface of a scotch tape with somewhat similar results.

Details

Equipment

• [1] Slide with lycopodium powder
• [1] Slide with scotch tape
• [1] He-Ne tube laser
• [1] Lens (~3 cm focal length)
  
• [1] Slide mount
• [3] Saddle
• [1] Optical rail
• [1] Blank piece of white paper
• [1] Roll of tape
  
• [2] Lab jack (if necessary)
• [1] Extension cord (if necessary)

Classroom Assembly

1. Mount the laser and and the slide containing lycopodium powder on opposite ends of the rail, making sure the laser is pointed away from people.
2. Without using the focusing lens, plug in the laser and aim it at the sample.
3. Use the lab jacks to elevate the fringe pattern to a screen, if necessary.
4. Tape the piece of paper to the wall as a viewing screen for the diffraction pattern.
   
5. 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 Airy's disk shape of lycopodium powder diffraction along with irregular speckled pattern.
4. Add the focusing lens and adjust its position to focus the laser beam on the sample. You should be able to get diffraction off a single particle.
   
5. Point out the difference in the diffraction pattern.
6. Repeat the experiment using a slide with scotch tape attached to it.
7. Turn off the laser.

Additional Resources