The minimum intensity of light to be detected by human eye is 10 to the power-10 W/m . The number of photons of wavelength 5.6 × 10 to the power -7 m entering the eye, with pupil area 10 to the power -6 m , per second for vision will be nearly

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Q. The minimum intensity of light to be detected by human eye is

10^{- 10} W / m^{2}

. The number of photons of wavelength

5.6 \times 10^{- 7} m

entering the eye, with pupil area

10^{- 6} m^{2}

, per second for vision will be nearly

Estimating the following two numbers should be interesting. The first number will tell you why radio engineers do not need to worry much about photons! The second number tells you why our eye can never ‘count photons’, even in barely detectable light.

(a) The number of photons emitted per second by a Medium wave transmitter of 10 kW power, emitting radiowaves of wavelength 500 m.

(b) The number of photons entering the pupil of our eye per second corresponding to the minimum intensity of white light that we humans can perceive (∼10⁻¹⁰ W m⁻²). Take the area of the pupil to be about 0.4 cm², and the average frequency of white light to be about 6 × 10¹⁴ Hz.