A 100W sodium lam

(a) Energy of photon E = hv = hc/λ

Where h = plank’s constant = 6.63 × 10^-34 J. sec

ν = frequency of light

c = speed of light = 3 × 10⁸ m/s

λ = wavelength of light

E = 6.63 × 10^-34 × 3 x10⁸ / 589 × 10^-9

= 3.38 × 10^-19 J

Energy of a photon is equal to 3.38 × 10^-19 J.

(b) Rate of photon falling on the sphere = power of lamp/energy per photon

∴ Rate of photon = 100 / 3.38 × 10^-19J = 3.0 × 10²⁰ photons/sec.

Rate at which photons are falling on the sphere is equal to 3.0 × 10²⁰ photons/sec.