Question

An image of the sun is formed by a lens of focal length $30cm$ 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 $15cm$. The photoelectric current in this case is:

• A. $\frac{I}{2}$
• B. $I$
• C. $2I$
• D. $4I$

Answer 1

The correct option is B $I$

Since image is formed at focus in case of incident light are parallel rays as we can assume in case of sum. So, the distance between lens and metal is equal to focal length i.e. 30 cm. So, total intensity of the sum rays on the lens is reaching at the metal surface, at focus. Now when focal length is reduced to 15cm, first rays will comerge at 15 cm them again diverge so, that the intensity available at the lens is equal to the intensity of the light at 30 cm. So, there is no change in intensity, so, there will not be any change in current due to photoelectrons.