First slide

Electromagnetic radiation - Dual nature of radiation and matter

Question

If 5% of the energy supplied to a bulb is irradiated as visible light, how many quanta are emitted per second by a 100 W lamp? Assume wavelength of visible light as 5.6
x 10^-5 cm.

Easy

A

$1.4 \times 10^{19}$
B

$3 \times 10^{3}$
C

$1.4 \times 10^{- 19}$
D

$3 \times 10^{4}$

Solution

Energy radiated as visible light ,is $\frac{5}{100} \times 100 = 5 {Js}^{- 1}$
Let n be the number of photons emitted per second. Then, $\frac{n h c}{λ} = 5$
$∴ n = \frac{5 λ}{h c} = \frac{5 \times 5 .6 \times 10^{- 7}}{(6 .62 \times 10^{- 34}) (3 \times 10^{8})} = 1 .4 \times 10^{19}$

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