First slide

Quantum Theory of Light

Question

10²⁰ photons of wavelength 660 nm are emitted per second from a lamp. What is the wattage of the lamp ? Planck's constant = 6.6 x10^-34 J-s

Easy

A

15 W
B

30 W
C

45 W
D

60 W

Solution

The power of the lamp is given by P = n h v(n = number of photons emitted by lamp)

$= \frac{nhc}{λ} = \frac{10^{20} \times (6 \cdot 6 \times 10^{- 34}) \times (3 \times 10^{8})}{660 \times 10^{- 9}} = 30 W$

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