Question

A 100 watts bulb has an efficiency of 80%. The wavelength of the radiation emitted is 595.80 nm. Which of the following statement(s) is/are true?

• A. Number of photons emitted per second is $24\times {10}^{19}$
• B. 80 J of energy is emitted as light.
• C. If the efficiency is 50%, then the number of photons emitted is $15\times {10}^{19}$
• D. All of the above

Answer 1

The correct option is D All of the above

Efficiency of the bulb is 80%. Only 80% energy is emitted as light.

In one second, energy emitted as light
$=100\times \frac{80}{100}=80J$

Energy of one photon $=\frac{6.62\times {10}^{-34}\times 3\times {10}^8}{595.80\times {10}^{-9}}$
$=\frac{1}{3}\times {10}^{-18}$

Number of photons $=\frac{80}{\frac{1}{3}\times {10}^{-18}}=24\times {10}^{19}$

If the efficiency of the bulb = 50%
The energy of light emitted per second $=100\times \frac{50}{100}=50$ J

Number of photons $=\frac{50}{\frac{1}{3}\times {10}^{-18}}=15\times {10}^{19}$