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Question:

A student used a device (X) to obtain/focus the image of a well illuminated distant building on a screen (S) as shown in the diagram. Select the correct statement about the device (X).

This device is a convex lens of focal length 4 cm.

This device is a convex mirror of focal length 8 cm.

This device is a concave lens of focal length 8 cm.

This device is a convex lens of focal length 8 cm.

Solution:

The convex lens converges all the rays coming from the distant object at a point on the screen placed at its focus behind the lens.Thus according to the figure, the device X must be a convex lens of focal length 8 cm.