Question
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.
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 Wm–2). Take the area of the pupil to be about 0.4 cm2, and the average frequency of white light to be about 6 x 104 Hz.
Solution
Given,
Average frequency of white light, = 6 x 1014 Hz
Energy of photon, E = hv = 6.63 x 10–34 x 6 x 1014 J ≃ 4 x 10–19 J.
Photon flux corresponding to minimum intensity,
Number of photons entering the pupil per second = 2.5 x 108 x 0.4 x 10–4 s–1 = 104 s–1.
Though this number is not as large as in (a) of the previous question, but it is large enough and it is impossible to count the photons with our naked eyes.
Average frequency of white light, = 6 x 1014 Hz
Energy of photon, E = hv = 6.63 x 10–34 x 6 x 1014 J ≃ 4 x 10–19 J.
Photon flux corresponding to minimum intensity,
Number of photons entering the pupil per second = 2.5 x 108 x 0.4 x 10–4 s–1 = 104 s–1.
Though this number is not as large as in (a) of the previous question, but it is large enough and it is impossible to count the photons with our naked eyes.
