Question

A 100 W incandescent lightbulb emits about 5 W of visible light. (The other 95 W are emitted as infrared radiation or lost as heat to the surroundings.) The average wavelength of the visible light is about 600 nm, so make the simplifying assumption that all the light has this wavelength. How many visible-light photons does the bulb emit per second?

Short answer

No. of visible-light photons emitted per second is$1.51\times {10}^{19}$

Step-by-step solution

Step 1. Given information is :A 100 W incandescent lightbulb is emitting about 5 W of visible light. Average wavelength of the visible light is about 600 nm.

We need to find out the no. of visible-light photons emitted per second.

Step 2. Using the expression for rate of emission of photon

$$\begin{gathered} R=\frac{P}{hf} \\ f=\frac{c}{\lambda } \\ R=\frac{P\lambda }{hc} \\ R=\frac{\left(5W\right)\left(600\times {10}^{-9}m\right)}{\left(6.63\times {10}^{-34}\right)\left(3\times {10}^{8}m/s\right)} \\ R=1.51\times {10}^{19}photons/s \end{gathered}$$