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Laser Emitting Photons Problem

Related Topics

Wize University Chemistry Textbook > Early Atomic Theory to Quantum Theory

Electromagnetic Radiation

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A laser emits a light beam with a power of 5 mW. The wavelength of the light is 374 nm. How many photons per second does this laser emit? (3mW=3mJ/s) Hint: Calculate the energy of a single photon


Power=5mW (light beam) based on conversion provided I know this is equal to 5mJ/s
wavelength=374nm -> convert to m: 1nm=1x10-9m, so 374x10-9m
Photons/second emitted?

Since we know the wavelength of the light, which equations come in handy when dealing with light?


E=hv and c=lamda(v)

We need to figure out the energy of a single photon, then since we know that in total the light beam emits 5mJ/s we can figure out how many photons of light corresponds to 5mJ of energy each second.

Find the frequency of the photon:


Solve for the energy of each photon:


5 mW/s means 0.005 W/s, or 5mJ/s of energy. How many joules of energy is this?


Now since we know that the energy of a single photon is 5.32x10-9J and we know the total light beam has 5x10-3J/s we can solve for how many photons of light there are each second:


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