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

Answer

A

Explanation

Photoelectric current `prop (1)/(d^2)`
In this case d =f . The paralled beam of sunlight is brought to the focus
Thus, `I prop (1)/(f^2)`
`therefore (I_2)/(I_1) = (f_1^2)/(f_2^2) = (30 xx 30)/(15 xx 15) =4 " " I_(2) =4I_(1) or 4 I`