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Solution


Q.

A teacher sets up the stand carrying a convex lens of focal length 15 cm at 42.7 cm mark on the optical bench. He asks four students A, B, C and D to suggest the position of screen on the optical bench so that a distinct image of a distant tree is obtained immediately on it. The positions suggested by the students were as:

(A) 12.7 cm
(B) 29.7 cm
(C) 57.7 cm
(D) 72.7 cm

The correct position of the screen was suggested by:
1 A  
2 B  
3 C  
4 D  

Ans.

(3)

Focal length of convex lens

f=15cm

It is placed at 42.7 cm mark on optical bench.

When we want the distant objects to be focused, we get image at focus of convex lens.

Because rays from distant object are parallel rays and they get converged at focus.

∴ Position of screen =  42.7 + 15 = 57.7   cm 42.7+15=57.7cm