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Question
An optical device has been given to a student and he determines its focal length by focusing the image of the sun on a screen placed 24 cm from the device on the same side as the sun. Select the correct statement about the device.
(A) Convex mirror of focal length 12 cm
(B) Convex lens of focal length 24 cm
(C) Concave mirror of focal length 24 cm
(D) Convex lens of focal length 12 cm
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Solution
The optical device is a concave mirror because when light rays from a distant object (here it is Sun) fall on this mirror, its image is formed at the focus of the mirror on the same side as the object. Thus in the given case, distance between the mirror and the screen is the focal length of the concave mirror. Hence, the focal length of the given device (concave mirror) is 24 cm.
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