Energy, Linear Momentum, and Angular Momentum of Light: What Do We Measure?

Olivier Emile, Janine Emile

The most commonly observed quantity related to light is its power or, equivalently, its energy. It can be either measured with a bolometer, a photodiode, or estimated with the naked eye. Alternatively, people can measure the light impulse or linear momentum. However, linear momentum is characterized by its transfer to matter, and its precise value is, most of the time, of little use. Energy and linear momentum are linked and can be deduced from each other, from a theoretical point of view. Because the linear momentum measurement is more difficult, energy is the most often measured quantity. In every physical process, angular momentum, like energy and linear momentum, is conserved. However, it is independent and cannot be deduced from the energy or the linear momentum. It can only be estimated via its transfer to matter using a torque observation. Nevertheless, experimentally, the torque is found to be proportional to the optical power. This leads to a need for a quantum interpretation of the optical field in terms of photons. Clear experimental evidences and consequences are presented here and debated.

DOI