Question

A convex lens of focal length 40 cm forms an image of an extended source of light on a photo-electric cell. A current \( I \) is produced. The lens is replaced by another convex lens having the same diameter but focal length 20 cm. The photoelectric current now is:

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

Answer 1

The Correct Option is C

The intensity of light incident on the photoelectric cell is proportional to the lens’s focal length and area. Since the lens diameter is unchanged, the area remains the same. Reducing the focal length from 40 cm to 20 cm increases the intensity of light four times (since light concentration doubles due to focal length halving). Therefore, the current I will double:
So, New current = \(2I\)