PURPOSE: Make a faint object more visible.

DESCRIPTION: This demonstration uses the technique of dark field imaging to see a fingerprint on a microscope slide.

The laser beam is defocused using an 8 mm convex lens so that a uniform spot falls on a screen (at the right in the picture, not shown). A fine wire mesh placed to the left of the 12.7 cm focal length projection lens is focused on the distant screen. The laser beam is focused by the projection lens at a point where a vertical razor edge is positioned on a horizontal cross carriage. The mesh is removed and replaced by a microscope slide with a fingerprint on it; to make the fingerprint touch your finger into vaseline, then wipe most of it off so that you get a nice clear fingerprint on the slide. Adjust the razor edge so that the direct light from the laser is cut off, leaving only the light scattered by the fingerprint, which forms an image of the fingerprint on the screen.

This can be seen nicely using a ground glass screen and TV camera.



EQUIPMENT: Laser, 8mm focal length magnetic convex lens, slide holder with glass slide with vaseline fingerprint, 12.7 cm focal length projection lens, razor edge on adjustable cross-carriage, and screen.

SETUP TIME: 20 min.

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