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 \times 10^{ - 5} $ cm)

• A. $ 1.4 \times 10^{ 19 } $
• B. $ 3 \times 10^3$
• C. $ 1.4 \times 10^{ - 19 } $
• D. $ 3 \times 10^4$

Answer 1

Answer: (A)

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 = E = 5
$ \therefore n = \frac{ 5 \lambda }{ hc } = \frac{ 5 \times 5.6 \times 10^{ - 7 }}{ ( 6.62 \times 10^{ - 34 } ) \, ( 3 \times 10^8 ) } $
= $ 1.4 \times 10^{ 19 } $