Question

An image of the sun is formed by a lens of the focal length of $30cm$ 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. $2l$
• B. $4l$
• C. $I$
• D. $\frac{I}{2}$

Answer 1

The correct option is C

$I$

Step 1: Given:

The focal length of the first lens = $30cm$

The focal length of the second lens = $15cm$

Current produced = $I$

Diameter of the second lens = diameter of the first lens.

Step 2: Finding the amount of current in the case of using the second lens:

1. The phenomenon of metals releasing electrons when they are exposed to the light of the appropriate frequency is called the photoelectric effect, and the electrons emitted during the process are called photoelectrons.
2. For a light of any given frequency, the photoelectric current is directly proportional to the intensity of light
3. As the diameter of the lens remains the same, both the lens will collect the same amount of light. Therefore, the intensity of the light will remain the same.
4. As a result, the photoelectric current will also remain the same. i.e., $I$.

Hence, the answer is (C) $I$