PURPOSE: To determine the location of a real image in a complex lens-mirror system.

DESCRIPTION: A lens has a focal length twice that of a convex mirror. The lens is positioned at a distance of twice the focal length of the mirror (the focal length of the lens) in front of the mirror. An object is placed a distance of twice the focal length of the lens in front of the lens. The image position, at the same point as the object position, can be determined by calculation and verified by this demonstration.

SUGGESTIONS: This was a question on an examination for engineering physics, after which the students viewed the image using this setup to see that the system actually works!

REFERENCES: (PIRA unknown.) See Demonstration Reference File for further information, including the exam question and the complete solution.

EQUIPMENT: 115VAC 25 watt clear "C" filament bulb on offset holder, light baffle, 60cm focal length convex lens, 30cm focal length convex mirror, and small cardboard screen, on two meter optical rail.

SETUP TIME: 5 min.

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