Question

A 100-watt lightbulb radiates energy at a rate of 100 J/s. (The watt, a unit of power, or energy over time, is defined as 1 J/s.) If all of the light emitted has a wavelength of 550nm, how many photons are emitted per second? (Assume three significant figures in this calculation.)

Answer 1

Answer:

$N=2.18\cdot 10^{20}photons/s$

Explanation:

The energy of a photon is: $E=h\nu=h\frac{c}{\lambda}$ when h is the Planck constant.

$h=6.63\cdot 10^{-34}Js$    

$c=3.8\cdot 10^{8}m/s$

Then, we calculate the energy of a photon with a wavelength equal to 550 nm.

$E= 4.58\cdot 10^{-19}J$

To find the number of photons we can use this equation:

$N=\frac{P}{E}=\frac{100}{4.58\cdot 10^{-19}}=2.18\cdot 10^{20} photons/s$

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