Light as Photons

 

In 1905, Einstein, building on the pioneering work of Max Planck's proposed that light could indeed exist as discrete bundles of energy, which we now call photons. 

According to Einstein, the energy E associated with a single photon is

E = hf ............ 1

In 1917, Einstein went further and assigned to each photon a linear momentum of magnitude

 p = p/λ .......... 2

In these equations f and λ are, respectively, the frequency and the wavelength of the incident light, and h is a constant whose value is, to four significant figures,

h = 6.626 X 10³⁴ J-s.

The Planck constant, as this quantity is now called, proves to be the fundamental constant of quantum physics. A glance at Equations 1 and 2 suggests the subtlety of the nature of light. On the left sides of these equations we have energy and momentum, properties that we associate with particles. On the right sides we have frequency and wavelength, properties that we associate with waves. Both “particle” and “wave” are concepts that the human mind has developed over many millennia from familiarity with objects such as tossed pebbles on the one hand and water waves on the other. There is no way to extend such classical concepts unchanged to the quantum world. Do not try! In the quantum realm, physicists have learned to avoid all such concrete mental pictures unless the nature of an experiment makes it helpful to do so.