First slide

Quantum Theory of Light

Question

If 5% of the energy supplied to a bulb is radiated as visible light, how many quanta are emitted per sec by a 100 watt lamp ? Assume wavelength of visible light as 5.6 x10 ^-5cm

Moderate

A

1.4 x 10¹⁹
B

2.0 x 10^-4
C

1.4 x 10^-19
D

2.0x10₄

Solution

Energy radiated as visible light
$= \frac{5}{100} \times 100 = 5 J / s$
Let n be the number of photons emitted per second. Then
$nhv = n \frac{hc}{λ} = 5 n = \frac{5 λ}{hc} = \frac{5 \times 5 \cdot 6 \times 10^{- 7}}{(6 \cdot 2 \times 10^{- 34}) (3 \times 10^{8})} = 1 \cdot 4 \times 10^{19}$

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