16, Radiation Pressure, 
The fallacy in Larmor’s proof. 
By Exper Barter, B.A., I.C.S8. 
ore energy. It is shewn that in this case,—‘< th ZY 
transmitted per unit time is increased by reflection, and this 
increase per unit time can arise o m work done the 
energy density in the medium in front of the reflector. 
Lord Rayleigh however has shown (Phil. Mag. X, 1905. 
es i ay exert positive, zero or 
negative pressure, according to the relation between the pres- 
sure and density of the gas. The case where no pressure is 
exerted is that in which the wave is propagated without 
change of form. This is exactly the case considered by Sir 
Joseph Larmor. 
It seems necessary therefore to enquire into the validity 
of the assumption that the work done by the advancing re- 
flector is done against a pressure due to the radiation, i.e. is 
a measure of the average pressure exerted by the radiation on 
a stationary reflector. 
The condition that the waves shall be completely reflected 
is that the medium behind the advancing reflector shall al- 
ways be undisturbed. The reflector considered in Sir Joseph 
Larmor’s treatment does not move the medium as a whole 
with it, but moves through the medium, leaving that behind 
it, unaffected by the strain in front of it. If it moved through 
an undisturbed medium, it would exert no force on the 
medium. If the medium in front is strained it must still be 
passed through to the back of the reflector at such a rate that 
it emerges unstrained, ie. at the same rate as if the whole 
medium wasundisturbed. In advancing thus the reflector does 
