Question

The number of photons emitted per second by a 62 W source of monochromatic light of wavelength 4800 $A^0$ is :

• A. $1.5\times {10}^{19}$
• B. $1.5\times {10}^{20}$
• C. $2.5\times {10}^{20}$
• D. $4\times {10}^{20}$

Answer 1

The correct option is B $1.5\times {10}^{20}$

Power of the source $=$ 62 watt
Energy emitted in a second $=$ 62 joules
So, number of photons emitted in a second $=\frac{62\lambda }{hC}$

$=\frac{62\times 4800\times {10}^{-10}}{6.626\times {10}^{-34}\times 3\times {10}^8}$ photons / sec
$=1.4962\times {10}^{20}$ photons / sec
$\approx 1.5\times {10}^{20}$ photons /sec

So, the answer is option (B).