An image of the sun is formed by a lens, of the focal length of 30 cm, on the metal surface of a photoelectric 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 15 cm. The photoelectric current in this case is

Options

A. `4I`
B. `2I`
C. `I/2`
D. `I`

Answer

A

Explanation

Photoelectric current `prop (1)/(d^(2))`
In this case `d=f`. The parallel beam of sunlight is brought to the focus .
Thus `I prop (1)/(f^(2)) :.(I_(2))/(I_(1))=(f_(1)^(2))/(f_(2)^(2))=(30xx30)/(15xx15)=4`
`:.I_(2)=4I_(1)` or `4I`