8 4 D O C . 7 7 E S S E N C E O F Q U A N T U M M E C H A N I C S Mr. de Broglie.— This makes it possible to give the trajectory the photons fol- low and to determine precisely their direction thus, one can consider the photon as a material point that has a position and a velocity. Mr. Kramers.— I don’t quite see, for my part, any advantage there might be for the description of the experiments in making a picture where the photons follow well-defined trajectories. Mr. Einstein.— During reflection on a mirror, Mr. L. de Broglie admits that the photons move parallel to the mirror, with a velocity c sin0 but what happens if the incidence is perpendicular? Do the photons then have zero velocity, as expressed in the formula (0 = 0)? Mr. Piccard.— Yes. In the case of the reflection, one must admit that the com- ponent of the velocity of the photons parallel to the mirror is invariable. In the re- gion of interference, the component perpendicular to the mirror disappears. The more the incidence increases, the more the photons slow down. So one gets com- pletely immobile photons in the limiting case of the perpendicular incidence. Mr. Langevin.— And so, therefore, in the region of interference, the photons no longer have the velocity of light hence, don’t they always, have the velocity c? Mr. de Broglie.— No, in my theory the velocity of the photons is only equal to c outside the whole range of interference, since the luminous radiation propagates only in empty space. As soon as there are phenomena of interference, the velocity of the photons becomes lower than c. [p. 267]