Question

A 100 W point source emits monochromatic light of wavelength $6000\stackrel{o}{A}$. Calculate the total number of photons emitted by the source per second.

• A. $5\times {10}^{20}$
• B. $8\times {10}^{20}$
• C. $6\times {10}^{20}$
• D. $3\times {10}^{20}$

Answer 1

The correct option is D $3\times {10}^{20}$

The energy of the light $E=\frac{hc}{\lambda }=\frac{(6.62\times {10}^{-34})(3\times {10}^8)}{6000\times {10}^{-10}}=3.31\times {10}^{-19}J$

We know that power is the energy per second. Thus, the energy per second is given $100J$

Thus, number of photons per second $=\frac{100}{3.31\times {10}^{-19}}=3.02\times {10}^{20}\sim 3\times {10}^{20}$