SAFETY NOTE: If you are hesitant to squirt around a volatile, carcinogenic liquid, a non-toxic alternative is light white paraffin oil, a squirt bottle of which is also provided.

This is a smaller, more easily portable version of demonstration L2-22, suitable for use in small classrooms. A ring of lights is repeatedly reflected by a rear mirror and a partially silvered front window, creating the illusion of lights vanishing into the distance.

Focus the TV camera on the image of its lens created by the concave mirror (or the "X" printed on a paper mounted on the lens). Move the ruler toward the mirror until it is in focus, demonstrating that the image is at that point. The pictures below show the meter stick too close to the camera, at the focus of the camera, and too close to the mirror. The background consists of a large amount of unrelated physics demonstration equipment.

The optical system in the photograph includes the following: bright point source with condenser lens, ground glass screen, wire mesh object, first iris, 8 cm focal length lens, second iris, and distant screen, as seen in the photograph at the right above.

Set the two irises to about one-third of their maximum opening to obtain a distortion free image (photograph at left below). Opening the second iris (preceding the lens) causes pincushion distortion (photograph at center below), while opening the first iris (following the lens) produces barrel distortion (photograph at right below).

The optical system in the photograph includes the following: bright point source with condenser lens, 5cm focal length plano-convex lens with flat side toward light source, copper wire screen, and 10 cm diameter 23 cm focal length convex lens. The image of the wire mesh is cast on a screen at about 10 feet from the optical cart.

To produce barrel distortion, position the wire mesh close to the first lens and position the large lens about 23 cm from the mesh to focus the image of the mesh on the screen, as shown in the photograph at the left below. The resulting barrel distortion is seen in the left center photograph.

To produce pincushion distortion, position the wire mesh about 15 cm from the first lens and position the large lens about 23 cm from the mesh to focus the image of the mesh on the screen, as shown in the right center photograph below. The resulting pincushion distortion is seen in the photograph at the right.

This demonstration also shows "curvature of field," because the focal position of the 23 cm lens is different to focus the center of the mesh than to focus the periphery.

View from within a few feet to observe chromatic aberration.

The laser beam is aligned with the photocell in the black aluminum cylinder, which is connected to a timer with large display. The camera is mounted on an optical post with the standard camera thread on one end, and positioned so that the laser beam will reach the photocell when the shutter is open. Reset the counter and cock the camera, and timer will measure the shutter opening time when the "picture is taken." Alignment is simpler if the lens is removed from single lens reflex cameras.