The photon model of light

Perhaps the strangest of all the models of light is the photon model. In Newton's 17th century "colored particle" or ray model, light consists of little particles moving very fast. Their paths form "light rays". In Huygens' model, developed at about the same time as Newton's, light was an oscillating wave — Huygen's conjectured the vibration of some material filling all space — an "ether". 

In the 1700's two models of light competed fiercely: Newton's particle model, which seemed to explain ray optics — light, shadow, the properties of lenses and mirrors, and Huygens' wave model, which was harder to apply but also worked. In the early 1800's the work of Young, Fresnel, and others made a much more convincing case for the wave model — it described complex interference phenomena in exquisite detail. And in the later 1800's Maxwell and Hertz made powerful theoretical and experimental demonstrations that what was oscillating in light were electric and magnetic fields — no vibrating matter needed. Color was determined by the frequency of the oscillating fields.

But as the 19th century progressed and more and more was learned about matter and how it interacted with light, it became increasingly clear that none of the models really could construct a reasonable description of the emission and absorption of light by matter. Two experiments were pivotal and seemed impossible to explain using the wave model: the spectrum of light emitted by hot matter (Black-body radiation), and the way ultraviolet light knocks electrons out of metals (The photoelectric effect). 

The solution was constructed by Einstein in the early years of the 20th century — in Einstein's "miracle year" of 1905 in which he solved 3 major problems and changed the way we look at much of physics. How this model was developed is a fascinating story that brings together not just both the wave and ray model of light, but our study of thermodynamics and the statistical mechanics of matter (entropy!). This is described in some detail in the follow-on article on Black-body radiation.

The result is a set of foothold principles that describe photons and their interactions very effectively. But they're hard to think about in terms of our everyday experiences! 

Karen Carleton and Joe Redish 5/2/12 and 7/10/19