Poisson Spot

This activity hits the spot

Introduction

Augustin Fresnel wrote the equations for light but he couldn't solve them, Poisson solved Fresnel's equations and noted that they predicted that in the center of the shadow of a ball there would be a spot of light. In this activity you will create a Poisson Spot.

Material

• a pointer laser
• a smooth ball bearing at least 3 mm (1/8 inch) in diameter
• a strong lens such as the lens from a disposable camera
• a magnet such as a refrigerator magnet
• a steel nail, 6 penny or larger
• a white screen
• a mount for the laser and lens such as a magnetic optical bench

Assembly

Tape the lens onto the front of the laser.

Mount the laser on the magnetic optical bench

Shine the laser across a dimly lit room onto a white screen. The distance from the laser to the screen should make a spot at least 4 inches in diameter.

Put the magnet onto the flat head of the nail and then balance the steel ball on the point of the nail. If your magnet is weak you'll have to use another magnetic optical bench to dangle the steel ball bearing from beneath the point of the nail.

Look at the shadow of the ball. Get close.

Notice that there is a spot of light in the center of the shadow.

What's Going On?

Light travels like a wave, and diffracts around the smooth surface of the ball. The path length from the laser to the center of the shadow is the same for all paths passing around the rim of the ball so the light interferes constructively creating a spot of light amid shadow.

Going Further

Examine the shadows of other objects.

Look at the shadow of the hole in the center of a washer.

Notice the rings of darkness inside the shadow of the hole.