Navigate

A lens having focal length f and aperture of diameter d forms an image of intensity I

A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperature of diameter d/2 in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively:

Options

(a) f and I/4
(b) 3f/4 and I/2
(c) f and 3I/4
(d) f/2 and I/2

Correct Answer:

f and 3I/4

Explanation:

By covering aperture, focal length does not change. But intensity is reduced by 1/4 times, as aperture diameter d/2 is covered.
I’ = I – I / 4 = 3I / 4
New focal length = f and intensity = 3I / 4

AIPMT2010
admin:

Related Questions

  1. The voltage of clouds is 4×10⁶ volt with respect to ground. In a lightening
  2. A sphere of 4cm radius is suspended with in a hollow sphere of 6cm radius. If the inner
  3. In the Bohr’s model of hydrogen atom, the ratio of the kinetic energy to the total
  4. The depth at which the effective value of acceleration due to gravity is g/4
  5. If voltage across a bulb rated 220 volt-100 Watt drops by 2.5% of its rated value