Unit: Optics
Year: 1993 Question#: 90
Question: When light rays pass through the film in a movie projector, an image of the film is produced on a screen. In order to produce the image on the screen, what type of lens does the projector use and how far from the lens must the film be placed?
(1 ) converging lens, at a distance greater than the focal length
(2 ) converging lens, at a distance less than the focal length
(3 ) diverging lens, at a distance greater than the focal length
(4 ) converging lens, at a distance less than the focal length
What is this question really asking?
Explanation by: Danica A
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Answer 1
converging lens, at a distance greater than the focal length.
CORRECT
A converging lens with an object at a greater distance than the focal length will form a real image on the opposite side of the lens (see ray trace below). The image must be real and it must be on the opposite side of the lens for the image to be visible to us. Therefore this is the correct answer.
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Answer 2
converging lens, at a distance less than the focal length
INCORRECT
A converging lens with an image at a distance less than the focal length forms a virtual image on the same side of the lens (see diagram below). A movie image needs to be real and on the opposite side of the lens.
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diverging lens, at a distance greater than the focal length
INCORRECT
A diverging lens will always form a virtual image on the same side of the lens (see below). This cannot be correct because the object's image must be real and on the opposite side of the lens for the movie viewers to see the image.
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diverging lens, at a distance less than the focal length
INCORRECT
A diverging lens will always form a virtual image on the same side of the lens (see below). This cannot be correct because the object's image must be real and on the opposite side of the lens for the movie viewers to see the image.
return to top What's this question really asking?
What type of lens and object distance will produce a real image on the opposite side of the lens? return to top
