The number of photons per second on an average emitted by the source of monochromatic light of wavelength 600 nm, when it delivers the power of 3.3 x 10⁻³ watt will be : (h = 6.6 x 10⁻³⁴ Js)

The problem provides:

Power of the light source: P = 3.3 × 10^-3 W

Wavelength of the monochromatic light: λ = 600 nm = 600 × 10^-9 m

Planck’s constant: h = 6.6 × 10^-34 J·s

Speed of light: c ≈ 3 × 10⁸ m/s

First, find the energy of a single photon using the relation

Insert the numerical values:

Simplify the powers of ten:

Since 600 = 6 × 10², the denominator becomes 6 × 10² × 10^-9 = 6 × 10^-7. Thus

Evaluating the numeric factor 19.8 ÷ 6 ≈ 3.3 and combining the powers of ten gives

Now compute the number of photons emitted per second, N, by dividing the total power by the energy per photon:

Substituting the known values:

The factor 3.3 cancels, leaving only the powers of ten:

Therefore, the light source emits approximately 10¹⁶ photons each second.

The correct answer is 10¹⁶ photons per second.