Question

An image of sun is formed by a lens of focal length 30 cm on the metal surface of a photo-electric cell and a photoelectric current (I) is produced. The lens forming the image is then replaced by another of the same diameter but of focal length of 15 cm. The photoelectric current in this case is……….. 

(A) (1/2) 

(B) 1 

(C) 2I 

(D) 4I

Answer 1

The correct option is (B) 1.

Explanation:

Given: diameter on lens remains same.

Hence equal diameters will collect same amount of light 

∴ intensity of light will remains the same 

∴ photoelectric current will also remain the same.