A parallel beam of light of wavelength 900 nm and intensity 100 Wm - 2 is incident on a surface perpendicular to the beam. The number of photons crossing 1 cm 2 area perpendicular to the beam in one second is

Step-by-step solution

Wavelength of incident beam λ = 900 × 10 - 9   m Energy of a single photon E = h c λ . Intensity is defined as power per unit area. Therefore, Intensity of incident beam I = E t A If the number of photons crossing the given area per second is n , then n = E net   E single   pholon   = I A λ h c ⇒ n = 100 1 × 10 - 4 900 × 10 - 9 6 . 62 × 10 - 34 × 3 × 10 8 = 4 . 5 × 10 16 .