Physics Demo Number: 023

Approximate Run Time: 15 min

Poisson’s Spot (Bright spot in center of shadow)

Demo Description

One uses the 5 mW He-Ne laser with a beam expander on it to illuminate a small ball bearing (or yellow spherical pin head) held on top of a small steel post by a magnetic field.

When one arranges the beam and ball appropriately:

[laser post in hole10 cm from one end of support, sphere post in hole 19 cm from other end] ,

as in photos l and 2, one should arrive at a shadow of the sphere,where one can see a bright spot at the center of the shadow, for the shadow on a surface about 3 or 4 m from the sphere.

Utilization of the gooseneck video camera to view and project the shadow onto the lecture hall interface, will allow the whole class to see the bright spot.

Scientific Principles

Show that the shadow of a circular obstacle contains a bright spot at its center.

Equipment

• 5mw HeNe laser

• Gooseneck video camera

• Bearing ( may be replaced by yellow headed quilting pin)

• Aluminum-and-steel support post

• Ceramic magnet

Equipment Location

• Kit 023 on [C-3-4]

• Black gooseneck video camera is on steel cart behind desk.

• Laser on [C-2-4]

1

2

3

Instructions

The accompanying pictures show use of apparatus to create a Poisson spot in the center of the shadow cast on a piece of cardboard by a small ball bearing interrupting the expanded beam of the big 5mw He Ne laser.

Photo 4 shows the screen of the TV monitor seen in Photo 3.

The monitor is displaying the picture from the camera.

The black gooseneck video camera can capture the shadow and spot to the video projector in the lecture hall.

This equipment and set up require a lot of pre-class tryouts and practice runs.

4

Writeup created by David A. Burba

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