Calculate photon quantities and distinguish individual photon energy from beam intensity.
Method and worked examples
The photon model treats light interactions as transfers of discrete energy and momentum. One photon has E=hf=λhc and p=cE=λh. Use the vacuum wavelength in these expressions. A photon has zero rest mass but nonzero energy and momentum; it does not require a nonzero Newtonian mass to transfer impulse. Higher frequency means greater energy per photon.
For monochromatic radiation, power P equals photon arrival rate multiplied by energy per photon. Increasing intensity at fixed frequency increases the number of photons arriving per area per time, not their individual energy. Localised detections can collectively form interference patterns. These are complementary photon and wave behaviours, so choose the model suited to the specified calculation rather than assuming one classical picture describes every observation.
Example 1
Use h=6.63×10−34Js. Find photon energy at f=5.00×1014 Hz.
E=hf=3.32×10−19J.
Example 2
A 1.00 mW beam has photon energy 4.00×10−19J. Find photon rate.