Discussion
A 150 watt bulb emits monochromatic light of wavelength 662 nm. How many number of photons emitted per second by the bulb?
(Planck's constant, h = \(6.62 \times 10^{-34}\) J s, c = \(3 \times 10^{8}\) m \(s^{-1}\))
Answer: D
The bulb emits \(5 \times 10^{20}\) photons per second.
Energy of one photon:
\[ \begin{aligned} E &= \frac{hc}{\lambda} = \frac{6.62 \times 10^{-34} \times 3 \times 10^{8}}{662 \times 10^{-9}} \\ &= 3 \times 10^{-19}\ \text{J} \\ n &= \frac{150}{3 \times 10^{-19}} = 5 \times 10^{20}\ \text{s}^{-1} \end{aligned} \]
The numbers are chosen so that 6.62 and 662 cancel neatly. The other options come from slips in the photon energy.
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