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Question: Answered & Verified by Expert
A 60 W bulb is placed at a distance of 4 m from you. The bulb is emitting light of wavelength 600 nm uniformly in all directions. In 0.1 s, how many photons will enter your eye, if the pupil of the eye has a diameter of 2 mm? [Take hc=1240 eV nm]
PhysicsWave OpticsNEET
Options:
  • A 2.84 x 1012
  • B 2.84 x 1011
  • C 9.37 x 1011
  • D 6.84 x 1011
Solution:
2568 Upvotes Verified Answer
The correct answer is: 2.84 x 1011
The intensity of light at the location of your eye is

           I = P 4 π r 2 = 6 0 4 π × 4 2 W m - 2

The energy entering into your eye per second is

           P 1 = I × π d 2 4

where d is the diameter of pupil

           P 1 = 6 0 4 π × 4 2 × π × 2 × 1 0 - 3 2 4

              = 9.375 × 1 0 - 7 Js - 1

Let n be the number of photons entering into the eye per second, then

                           P 1 = n × hc λ

           9.375 × 1 0 - 7 = n × 1 2 4 0 × 1.6 × 1 0 - 1 9 6 0 0

                              n = 2.84 × 1 0 1 2 photon s - 1

So, the number of photons entering the eye in 0.1 s = 0.1 n = 2.84 x 1011

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