Question

A 40 watt bulb converts 6% of its power to red light (wavelength $6500\dot{A}$). The number of red lights photons emitted by the bulb per second is

• A. $100$
• B. $4\times {10}^{18}$
• C. $8\times {10}^{18}$
• D. $13\times {10}^{18}$

Answer 1

Answer: (C)

$8\times {10}^{18}$

In 1 second the bulb would consume 40J of energy, out of which only 6% is radiated as red light
In other words only 2.4J of energy is red light.

Each photon of red light has (E) = $\frac{hc}{\lambda }$ ; where h is plancks constant, and c is the speed of light.

Number of photons is equal to the total energy divided by the energy of each photon = $\frac{2.4\lambda }{hc}$

= $\frac{2.4\ast 6500\ast {10}^{-10}}{6.64\ast {10}^{-34}\ast 3\ast {10}^8}$ = $8\ast {10}^{18}$

So, the answer is option (C).