2.48 In astronomical observations, signals observed from the distant stars are generally weak. If the photon detector receives a total of 3.15 × 10–18 J from the radiations of 600 nm, calculate the number of photons received by the detector.

From the expression of energy of one photon (E),
E = hcλ
Where, λ = wavelength of
radiation h = Planck’s
constant c = velocity of
radiation
Substituting the values in the given expression of E

E = 6.626 x 10-34 Js3 x 108 m/s600 x 10-9 m = 3.313 × 10-19 J
Energy of one photon = 3.313 × 10-19 J
Number of photons received with 3.15 × 10-18 J energy
3.15 x 10-18 J3.313 x 10-19 J
= 9.5
≈ 10