Question

Assuming that a $25$ watt bulb emits monochromatic yellow light of wavelength $0.57\, \mu$. The rate of emission of quanta per second will be:

• A. $5.89 \times 10^{15} s ^{-1}$
• B. $7.28 \times 10^{17} s ^{-1}$
• C. $5 \times 10^{10} s ^{-1}$
• D. $7.18 \times 10^{19} s ^{-1}$

Answer 1

Answer: (D)

Let $n$ quanta are evolved per second

$n\left[\frac{h c}{\lambda}\right]=25 \,J / s$

$n \frac{6.626 \times 10^{-34} \times 3 \times 10^{8}}{0.57 \times 10^{-6}}=25$

$n=7.18 \times 10^{19} s^{-1}$