Question

A thin lens of focal length f and its aperture has a diameter d. It forms an image of intensity I. Now the central part of the aperture upto diameter (d/2) is blocked by an opaque paper. The focal length and image intensity would change to

• A. f/2, l/2
• B. f, l/4
• C. 3f/4, l/2
• D. f, 3l/4

Answer 1

The correct option is D f, 3l/4

Covering a part of the surface will not change the focal length of the lens, as the focal length depends only on the refractive index of the lens and the radius of curvature of the lens.
For the intensity, the amount of light blocked when half of the lens will be covered by an opaque cloth, = ${\left(\frac{1}{2}\right)}^2$
Thus, the intensity of light gets reduced to $i-\frac{i}{4}=\frac{3i}{4}$