COLOUR OF DISPERSOIDS 73 



reflection, diffraction, interference, etc., but to optical resonance. 

 Resonance is the production of vibrations in a body by the 

 periodic application of a stimulus which has the same period as 

 the natural period of the body. The vibrations of a tuning fork 

 may be transmitted through the air and cause to vibrate another 

 tuning fork of the same pitch. Since the resonator owes the energy 

 necessary to set it into vibration to the stimulating body it follows 

 that the stimulating body must lose energy to the resonator. 

 The particles in colloidal solution are supposed to be vibrating 

 with the same frequency as light of a certain wave-length. Con- 

 sequently, they will receive energy from the light which will tend 

 to increase their amplitude of vibration. The kinetic energy 

 of the solution will tend to increase, but any increase in kinetic 

 energy would mean increase in temperature and a slight alteration 

 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 pendulum 

 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 move- 

 ment may attain an increased velocity because of incident light. 

 Exner found that exposure to light of a suitable wave-length had 

 a positive accelerating effect. Compared with the movement as 

 a whole the alteration brought about by incident light is negligible. 



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 colour) 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, this colloidal dye absorbs red and transmits 

 blue. It reflects red. If a fine suspension is made it scatters 

 blue, i.e. appears blue when observed laterally to the plane of 

 incidence of light. By transmitted light it is red, i.e. appears 

 red when looked at against the light. 



