[ 169 j 



XVI. Note on the Tangential Stress due to L'ujht incident 

 obliquely on an Absorbing Surface. Btj J. H. Poynting, 

 Sc.jD.j F.Jri.S., Mason Professor of Physics, Birmmgham*. 



npHE existence of pressure on a surface due to the incidence 

 I of a normal beam of light, first deduced as a consequence 

 of the electromagnetic theory by Maxwell, has been fully 

 confirmed by the experiments of Lebedew, and quite inde- 

 pendently by the exact work of Nichols and Hull. These 

 experiments show that the pressure exists and that it is equal 

 to the energy per c.c. or to the energy-density in the incident 

 beam. 



In so far as it produces this pressure we may regard the 

 beam as a stream of momentum, the direction of the momentum 

 being along the line of propagation, and the amount of 

 momentum passing per second through unit area cross-section 

 of the beam being equal to the density of the energy in it. 

 Let E denote this energy-density. If the beam is inclined 

 at to the normal to a surface on which it falls, the mo- 

 mentum stream on to unit area of the surface is E cos per 

 second, and this is the force which the beam will exert in its 

 own direction. If the beam is entirely absorbed, the result is a 

 pressure E cos 2 6 along the normal and a tangential stress in 

 the plane of incidence E sin cos 6 — \ E sin 20. If fju 

 of the incident beam is reflected, the normal pressure is 



(1+ft) E cos 2 0, and the tangential stress is -~~^ E sin 20 f. 



When there is absorption the tangential stress has a maximum 

 value at 15° if fi is constant. When there is no absorption 

 the tangential stress disappears. 



The tangential stress is much more easily detected than the 

 normal pressure. For the action of the gas surrounding the 

 surface is normal to it and is with difficulty disentangled 

 from the normal light-pressure. But the gas action is at 

 right angles to the tangential stress, and it is merely necessary 

 to arrange a surface free to move in its own plane to eliminate 

 the action of the normal forces and to reveal the tangential 

 stress. 



With the assistance of my colleague Dr. Guy Barlow, to 

 whom I am much indebted for help in the work, I have made 

 the following experiment to show the existence of the stress. 



* Read at Section A, British Association, Cambridge, August li)04. 

 Communicated b\ the Author. 



t These expressions are given in "Radiation in the Solar System," 

 Phil. Trans. A 202. p. 539. 



