On the torque produced by a beam of light in oblique refraction through a glass plate

Abstract

In the Presidential Address to the Physical Society in 1905* Prof. Poynting developed the idea that a beam of light must be regarded as containing a stream of momentum, and he showed that this principle may be used to solve with great ease the various cases in which a beam of light is absorbed, reflected, or refracted at a surface. This method is particularly powerful and convenient, for, once having made the assumption that light carries with it a stream of momentum, the forces can be calculated by the ordinary mechanical laws without any further appeal to the theory of wave disturbance of which we suppose light to consist. When a beam falls normally on a perfectly absorbing surface the force exerted is the momentum received per second, and it is therefore equal to the momentum in a length V of the beam, where V is the velocity of light. But the pressure exerted is equal to the energy density E of the beam—the law first deduced by Maxwell from his electromagnetic theory and now well established by the experiments of Lebedew and Nichols and Hull. It follows then that the density of the momentum in the beam must equal E/V.