The number of photons emitted per minute by a 25 W source of monochromatic light of wavelength 6000 Å is approximately: (take $h = 6.6 × 10^{-34} Js$)

$4.5 × 10^{21}$

The correct answer is Option (2) → $4.5 × 10^{21}$

Power of source: $P = 25 \, \text{W} = 25 \, \text{J/s}$

Wavelength: $\lambda = 6000 \, \text{Å} = 6000 \times 10^{-10} \, \text{m} = 6 \times 10^{-7} \, \text{m}$

Energy of one photon: $E = \frac{hc}{\lambda}$

$h = 6.63 \times 10^{-34} \, \text{Js}, \; c = 3 \times 10^8 \, \text{m/s}$

$E = \frac{6.63 \times 10^{-34} \cdot 3 \times 10^8}{6 \times 10^{-7}} \approx 3.315 \times 10^{-19} \, \text{J}$

Photons emitted per second: $N = \frac{P}{E} = \frac{25}{3.315 \times 10^{-19}} \approx 7.54 \times 10^{19}$

Photons per minute: $N_{60} = 7.54 \times 10^{19} \cdot 60 \approx 4.52 \times 10^{21}$

Answer: $\; \approx 4.5 \times 10^{21}$ photons per minute