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Solution


Q.

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.
1 Convex mirror of focal length 12 cm  
2 Convex lens of focal length 24 cm  
3 Concave mirror of focal length 24 cm  
4 Convex lens of focal length 12 cm  

Ans.

(3)

The optical device is a concave mirror because when light rays from a distant object (like Sun) fall on this mirror, its image is formed at the focus of the mirror on same side as the object. Thus, the distance between the mirror and the screen is the focal length of the concave mirror. Hence, focal length of given device is 24 cm.