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Solution


Q.

A student determines the focal length of a device ‘X’ by focusing the image of a distant object on a screen placed 20 cm from the device on the same side as the object.
The device ‘X’ is:
1 Concave lens of focal length 10 cm  
2 Convex lens of focal length 20 cm  
3 Concave mirror of focal length 10 cm  
4 Concave mirror of focal length 20 cm  

Ans.

(4)

In case of the concave mirror, the image will be formed on the same side of the screen at its focus point when object is distant.