WAVE NATURE OF ELECTRON 245 electricity corpuscle owes its appearance to Sir J. J. Thomson and you will all be familiar with H. A. Lorentz, s use of it in his theory of electrons Some thirty years ago, physics was hence divided into two: firstly the physics of matter based on the concept of corpuscles and atoms which were supposed to obey Newtons classical laws of mechanics, and secondly radia- tion physics based on the concept of wave propagation in a hypothetical continuous medium, i.e. the light ether or electromagnetic ether. But these two physics could not remain alien one to the other; they had to be fused together by devising a theory to explain the energy exchanges between mat ter and radiation-and that is where the difficulties arose. While seeking to link these two physics together, imprecise and even inadmissible conclu- sions were in fact arrived at in respect of the energy equilibrium between matter and radiation in a thermally insulated medium: matter, it came to be said, must yield all its energy to the radiation and so tend of its own accord to absolute zero temperature! This absurd conclusion had at all costs to be avoided. By an intuition of his genius Planck realized the way of avoiding it: instead of assuming, in common with the classical wave theory, that light source emits its radiation continuously, it had to be assumed on the contrary that it emits equal and finite quantities, quanta. The energy of each quantum has, moreover, a value proportional to the frequency u of the ra- diation. It is equal to h, h being a universal constant since referred to as Plancks constant The success of Planck's ideas entailed serious consequences. If light is emit- ted as quanta, ought it not, once emitted, to have a granular structure? The existence of radiation quanta thus implies the corpuscular concept of light On the other hand, as shown by Jeans and H. Poincare, it is demonstrable that if the motion of the material particles in light sources obeyed the laws of classical mechanics it would be impossible to derive the exact law of black body radiation, Plancks law. It must therefore be assumed that traditiona dynamics, even as modified by Einstein s theory of relativity, is incapable of accounting for motion on a very small scale The existence of a granular structure of light and of other radiations was confirmed by the discovery of the photoelectric effect. If a beam of light or of X-rays falls on a piece of matter, the latter will emit rapidly moving elec trons. The kinetic energy of these electrons increases linearly with the fre- quency of the incident radiation and is independent of its intensity. This phenomenon can be explained simply by assuming that the radiation is com- posed of quanta lo capable of yielding all their energy to an electron of theWAVE NATURE OF ELECTRON 245 electricity corpuscle owes its appearance to Sir J. J. Thomson and you will all be familiar with H. A. Lorentz’s use of it in his theory of electrons. Some thirty years ago, physics was hence divided into two: firstly the physics of matter based on the concept of corpuscles and atoms which were supposed to obey Newton’s classical laws of mechanics, and secondly radia￾tion physics based on the concept of wave propagation in a hypothetical continuous medium, i.e. the light ether or electromagnetic ether. But these two physics could not remain alien one to the other; they had to be fused together by devising a theory to explain the energy exchanges between mat￾ter and radiation - and that is where the difficulties arose. While seeking to link these two physics together, imprecise and even inadmissible conclu￾sions were in fact arrived at in respect of the energy equilibrium between matter and radiation in a thermally insulated medium: matter, it came to be said, must yield all its energy to the radiation and so tend of its own accord to absolute zero temperature! This absurd conclusion had at all costs to be avoided. By an intuition of his genius Planck realized the way of avoiding it: instead of assuming, in common with the classical wave theory, that a light source emits its radiation continuously, it had to be assumed on the contrary that it emits equal and finite quantities, quanta. The energy of each quantum has, moreover, a value proportional to the frequency v of the ra￾diation. It is equal to hv, h being a universal constant since referred to as Planck’s constant. The success of Planck’s ideas entailed serious consequences. If light is emit￾ted as quanta, ought it not, once emitted, to have a granular structure? The existence of radiation quanta thus implies the corpuscular concept of light. On the other hand, as shown by Jeans and H. Poincaré, it is demonstrable that if the motion of the material particles in light sources obeyed the laws of classical mechanics it would be impossible to derive the exact law of black￾body radiation, Planck’s law. It must therefore be assumed that traditional dynamics, even as modified by Einstein’s theory of relativity, is incapable of accounting for motion on a very small scale. The existence of a granular structure of light and of other radiations was confirmed by the discovery of the photoelectric effect. If a beam of light or of X-rays falls on a piece of matter, the latter will emit rapidly moving elec￾trons. The kinetic energy of these electrons increases linearly with the fre￾quency of the incident radiation and is independent of its intensity. This phenomenon can be explained simply by assuming that the radiation is com￾posed of quanta hv capable of yielding all their energy to an electron of the