OPTICAL RESONANCE 70 



in frequency. This opens up the possibility of considerable 

 energy changes in comparatively short times. 



What effect will be produced when the rates of vibration are nearly but 

 not quite the same ? If two pendulum-controlled clocks which are keeping 

 nearly the same time when on separate stands are placed on the same stand 

 they will keep time exactly. Both pendulums transmit vibrations to the 

 stand, and so to one another. The faster ])endulum exerts a periodic force 

 on the slower pendulum and is itself slowed by the loss of energy. In the 

 same way the slower pendulum tends to cause forced vibrations in the stand 

 and so influence the faster pendulum. Finally the two pendulums (and stand) 

 vibrate at periods exactly the same. Is it possible that light may cause 

 forced vibrations of colloidal particles ? 



Certain investigators have claimed that the Brownian movement may 

 attain an increased velocity because of incident light. Exner found that 

 exposure to light of a suitable wave-length had a slight but a positive 

 accelerating effect. 



One effect of optical resonance is the production of surface 

 colours. When light of a certain wave-length is strongly absorbed 

 by particles, they may also reflect that light " selectively." For 

 instance, magenta crystals (aniline dye) transmit red but reflect 

 green. If the particle is made small enough it will scatter the 

 light that it previously transmitted, and will transmit, of course, 

 the light that is not scattered. This is readily carried out with 

 indigo. In mass, i.e. when the particles are large, this colloidal 

 dye appears red when observed laterally to the plane of incidence 

 of light. By transmitted light it is blue, i.e. appears blue when 

 looked at against the light. If a fine suspension is prepared it 

 reflects blue and transmits red, 



{b) Faraday-Tyndall Phenomenon. An examination of the 

 optical properties of these various disperse systems makes it clear 

 that there is a regular gradation in the size of the particles dispersed, 

 which passes from the easily visible suspension to the invisible 

 solute. If the size of a particle is decreased below 200^^, it caimot 

 be seen even by the most powerful microscope made, or that could 

 ever be made. The particle is ultra-microscopic because its 

 diameter is less than half the wave-length of light. If mono- 

 chromatic light with a very short wave-length, say 2,000 A.u., were 

 to be used with a suitable microscope, particles of lOO^UjU. and 

 greater could be photographed. Such a microscope with quartz 

 lenses has been constructed by Barnard. 



The tiny particles may also be made apparent in much the same 

 way as the innumerable specks of dust floating in the air become 

 sparkling motes dancing in a ray of sunlight which has penetrated 

 into a partially darkened room. When a strong beam of light is 

 sent through a rectangular cell containing pure water, the beam 

 may be rendered visible before and after its passage through the 



