36 SOCIAL SCIENTIST waves at two pointb simultaneously it might happen that at some places the crests of the first cancel the troughs of the second and we have still water. At other places the crests reinforce each other and we have large oscillations. So we find regions of large oscillations and regions of no oscillation. This phenomenon is called interference. Considering the fact that light travels in a straight line in empty space Newton supposed that light consists of particles. But light also shows interference properties. Hence physicists accepted Huyghen's postulate that light is a form of wave motion. Measurements of the interference fringes showed that the waves involved are very shorty twenty thousand of them fitting in a centimeter. In the nineteenth ceentury it was realized that not only light, but a whole range of phenomena including X-rays, thermal radiation and radio waves belonged to a wider wave phenomenon, namely, electromagnetic weaves. All these consist of oscillations of electric and magnetic fields, X-rays differing from light only in the wave length. J\'ew Century for Physics Towards the end of the last century, however, there were serious difficulties in explaining the distribution of radiation in closed cavities. To explain the way in which different kinds of radiation are distributed in a closed cavity full of thermal radiation it was necessary to study how radiation was absorbed and reflected by the walls of the container. Max Planck was able to explain the distribution by postulating that all transfers of radiation take place in small packets of energy. There cannot be fractional transfers, just as no money transaction can take place in our country of value less than a paisa. The size of each packet of energy depends on the frequency of the radiation, just as the size of the unit of money depends on the form of currency. The size of the packet is given by the simple relation E =s hY where T is the frequency and h is a constant, called Planck's constant. This postulate of the basic quantum of energy was a gift to physics made in the first year of this century. The quantum postulate that all radiation consists of packets of energy was further confirmed when Einstein used it to explain the photoelectric effect. Assuming tliat light consists of light quanta or 'photons' he explained how electrons are knocked off when light shines on metals like cesium. But this brought to the fore the old question whether light consists of corpuscles or waves. In connection with this question it must be remembered that Fresnel had shown how the straight line propagation of light can be explained in terms of waves. Also, Hamilton had shown over a hundred years ago how the motion of waves and the motion of particles are similar. Sound waves are reflected by a wall very much as a ball is rebounded. These considerations and considerations of relativity led De Broglie4 to postulate that with every moving particle is associated a wave. This remarkable assumption of matter waves was verified in 1927 when Davisson and Germer proved experimentally that a beam of electrons