Question

${10}^{20}$ photons of wavelength $660nm$ are emitted per second from a lamp. What is the wattage of the lamp if Planck's constant is equal to$6.6\times {10}^{-34}$ Js?

• A. 30 W
• B. 60 W
• C. 100 W
• D. 500 W

Answer 1

The correct option is A 30 W

Energy $E$ of photons of frequency $\nu$ is given by $E=h\nu$. If the lamp emits $n$ photons per second, then the power of the lamp is $P=nh\nu$.
Now, $\nu =\frac{c}{\lambda }$
Therefore, $P=\frac{nhc}{\lambda }$
$=\frac{{10}^{20}\times 6.6\times {10}^{-34}\times 3\times {10}^8}{660\times {10}^{-9}}=30W$
Hence, the correct choice is (a).