PURPOSE: To encourage thought regarding how an image is formed.

DESCRIPTION: An arrow/circle cross object is imaged on a screen by a convex lens, as seen in the center photograph above. The experimental setup is shown at the left, with the object at the left, the lens in the center, and the image screen at the right in the photograph. A paper baffle is then stuck onto the lens, as seen in the photograph at the right, blocking half of the light passing through the lens. What happens to the image? Will it remain unchanged; will it become brighter; will it become fainter; will the left side be gone, or will the right side be gone? Shown below are the image without the baffle on the focusing lens (left) and with the baffle on the focusing lens (right). The image with the baffle in place is clearly fainter than the original image, as can be seen.

SUGGESTIONS: Let the class vote before you insert the baffle.

See Question of the Week #111 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown)

EQUIPMENT: Light source with small arrow/circle object baffle, 15cm focal length convex lens, and screen on optical rail, as photographed.

SETUP TIME: 5 minutes.