Watts per photon

Fun conversion of the day: photon count to watts. This is high school physics, but it’s a good reminder of scale.

Let’s take a decent flux, say 50 million green (520 nm) photons per second. The energy (units = J) is equal to the frequency (units = 1/s) times Planck’s constant (units = J*s). The wattage is the energy per unit time (watts = J/s).

Wattage of flux = 50e6 green photons per second * (c / 520e-9) * 6.63e-34
(c = speed of light = 3e8 m/s)

= 1.9e-11 watts

So it’s clear why thorough lightproofing is necessary. There are hundreds of watts of overhead lighting. That’s 13 orders of magnitude more light than you’re trying to measure. And if you’re working with dim samples, the fluxes can be orders of magnitude less than assumed in this calculation.

(Hat tip to CW.)