Question

A bulb lamp emits light of mean wavelength $$4500 \mathring{A}$$. The lamp is rated at $$150 W$$ and $$8\%$$ of the energy appears as emitted light. How many photons are emitted by the lamp per second?

Answer 1

The energy of photon $$= \dfrac{hc}{\lambda}$$
$$= \dfrac{(6.63 \times 10^{-34})(3 \times 10^8)}{4500 \times 10^{-10}}$$
According to the given problem, power of the lamp is $$150 W$$.
As $$8\%$$ of the light energy is utilized for emission of light, hence the utilized energy for emission per second is given by
Useful power $$= 150 \times \dfrac{8}{100} = 12 W$$
$$= 12 Js^{-1}$$
Therefore, number of photons emitted per second by the lamp
$$\dfrac{useful \, power}{energy \, of \, photon} = \dfrac{12 \times 4500 \times 10^{-10}}{(6.63 \times 10^{-34})(3 \times 10^8)}$$
$$= 27.17 \times 10^{18}$$