Quantum Physics Tutorial 7 - Radiation Pressure


Radiation Force

Radiation Pressure


We know that pressure is defined as force per unit area:


It may be a surprise, but there is a pressure from the photons arriving from the Sun.   This is called radiation pressure.  Like any other pressure, its units are Pa.


It has a value of about 10 10-6 Pa, which is not exactly going to crush you.  Atmospheric pressure, for comparison, is about 1 105 N m-2.


Question 1

How much less is the atmospheric pressure than the radiation pressure?



All electromagnetic radiation results in radiation pressure.



Radiation Force

When radiation strikes a surface, momentum from the particles is transferred to the surface.  The change in momentum leads to a force.


You might (correctly) think that photons have no mass (hence zero momentum), but relativity allows them to have momentum by this general equation:


E2 = m2c4 + p2c2


This is consistent with mass and energy being equivalent.


When mass is zero, we get the following result:


E = pc


And by rearranging:



We also know that force is defined as rate of change of momentum (Newton II):


Dp = FDt


We can combine the two equations above by writing:


And then we write an expression for F:


We can see that there is an energy time component.  This is power, so we can write:



Question 2

A light source has a power of 6.0 W.  What is the force from its photons?




Radiation Pressure

We know that:


So we can divide the equation above by the area:


Power divided by area gives intensity, so we can write:


where  I is the intensity (W m-2).


Question 3

The intensity of light striking the Earth's surface at a certain spot is 550 W m-2.  What is the radiation pressure?



The wavelength does not matter, because the lower the photon energy, the more photons are needed for a given power.


Note that both pressure and momentum have the same code, p.

Make sure you know which context you are using p in.


Radiation pressure can be used for optical refrigeration which the vibration of atoms can be reduced as they interact with incoming photons from a laser.