C 190 1 



XVill. 



METHODS OF DETERMINING THE AMOUNT OF LIGHT 

 SCATTERED FROM ROUGH SURFACES. 



By W. F. BARRETT, F.R.S., 

 Pi-ofessor of Experimental Physics, Royal College of Science for Ireland. 



[Read April 20. Ordered for Publication May U. Published July 27, 1909.] 



When light falls on a rough or unpolished surface, such as a sheet of 

 ground glass, t,he reflected beam is scattered or diffused in every direction 

 owing to the irregular nature of the surface, and each point of the surface 

 thus becomes a source of light.' From whatever direction tlie surface is 

 viewed, it is thus rendered visible ; and the scattered beam is white or coloured 

 according as some wave-lengths are, or are not, absorbed. Even the whitest 

 surface absorbs some of the incident light, but the proportion of light reflected 

 from a more or less smooth surface is increased as the angle of incidence 

 becomes greater, so that with very oblique incidence a fairly smootli surface, 

 such as a sheet of white paper, can give an indistinct image of a luminous 

 body. It is obvious that the amount of light thus scattered by large surfaces, 

 such as a building or white- washed wall, is of great practical importance, more 

 especially in legal disputes where a case of " ancient lights " is concerned. 

 It was such a case upon which I was consulted, that first drew my attention 

 to the need of accurate information on the relative amounts of liglit scattered 



'■ The term ' scattered light ' is usually and properly restricted to those cases where the particles 

 of the body are small in comparison with the wave-length of light. Under such eircumstances the 

 ordinary laws of reflection are not obeyed, fur only single wavelets are formed by the scattering of 

 the incident or primary wave. Lord Eayleigh has shown that for a given ampliUide of the primary 

 wave, the amplitude of the scattered wave varies inversely as the square of the wave-length, and the 

 intensity of the scattered light varies inversely as the fourth power of the wave-length. Tliis is the 

 case when light is scattered from minute particles of smoke, or milk and water, the scattered light 

 being bluish, since the intensity varies as nbove stated. In tlie present paper I use the word 'scattered ' 

 to mean ' irregularly reflected,' the particles of the reflecting surface being comparatively coarse, and 

 thus the reflected wave is formed by a reinforcement of wavelets generated at neighbouring points of 

 the surface. In the case of opalescent surfaces, the panicles are small, and Lord Rajleigh's law 

 applies. 



