Maxwell. 305 



The question was examined by Maxwell* from the point 

 of view of the electromagnetic theory of light ; which readily 

 furnishes reasons for the existence of light-pressure. For 

 suppose that light falls on a metallic reflecting surface at 

 perpendicular incidence. The light may be regarded as con- 

 stituted of a rapidly-alternating magnetic field ; and this must 

 induce electric currents in the surface layers of the metal. But 

 a metal carrying currents in a magnetic field is acted on by a 

 ponderomotive force, which is at right angles to both the 

 magnetic force and the direction of the current, and is there- 

 fore, in the present case, normal to the reflecting surface : 

 this ponderomotive force is the light-pressure. Thus, according 

 to Maxwell's theory, light-pressure is only an extended case of 

 effects which may readily be produced in the laboratory. 



The magnitude of the light-pressure was deduced by 

 Maxwell from his theory of stresses in the medium. We have 

 seen that the stress across a plane whose unit-normal is N is 

 represented by the vector 



(D . N) . E - J (D . E) . N + (B . N) . H - ~ (B . H) . N. 



47T O7T 



Now, suppose that a plane wave is incident perpendicularly on 

 a perfectly reflecting metallic sheet: this sheet must support 

 the mechanical stress which exists at its boundary in the 

 aether. Owing to the presence of the reflected wave, D is zero 

 at the surface ; and B is perpendicular to N, so (B . N) vanishes. 

 Thus the stress is a pressure of magnitude (l/8?r) (B . H) 

 normal to the surface : that is, the light-pressure is equal to 

 the density of the aethereal energy in the region immediately 

 outside the metal. This was Maxwell's result. 



This conclusion has been reached on the assumption that 

 the light is incident normally to the reflecting surface. If, on 

 the other hand, the surface is placed in an enclosure completely 

 surrounded by a radiating shell, so that radiation falls on it 

 from all directions, it may be shown that the light-pressure is 

 measured by one-third of the density of aethereal energy. 



* Maxwell's Treatise on Electricity and Magnetism, 792. 

 X 



