Describe in a couple sentences the evidence showing that light is an electromagnetic wave. (Address both the "electromagnetic" and the "wave" nature of light).
 
 

My Solution

"Light is an electromagnetic wave" means that what we perceive as light (the signal received by our retinae) is the effect of oscillating electric (E) and magnetic (B) fields that propagate as a wave from a light source through space to our eyes.

We know that the signal (and the energy) contained in light propagates like a wave because we can demonstrate typical wavelike phenomena: Diffraction and interference. In particular, interference of two light beams reflected from both sides of a thin film or passing through two narrow slits creates the typical patterns of dark and bright fringes which reveal the importance of phase differences for different paths. The exact dependence on the color (wavelength) of the light, on the spacing of the sources, and on the path length differences can only be explained by assuming that light is a wave with wavelengths of the order of 400 nm — 700 nm. Additional evidence for the wave nature of light comes from the applicability of Huygen’s principle (light can be "bend" around sharp edges because every point along the wave front acts as a new source of circular waves) and from the correlation between the signal velocity in a medium (c/n) and the angle of refraction as given by Snell’s law. [Note that "polarization" and "energy transport" are not, by themselves, valid proofs of the wave nature of light — these phenomena would also occur if light were transmitted by rods flying through space].

The strongest argument for the electromagnetic nature of light is that it can be described with great detail and high accuracy by Maxwell’s equations for electric and magnetic fields. These equations (which had been derived and confirmed independently by observations of charges, magnets and electric circuits) imply the existence of electromagnetic waves that travel with a specific speed in vacuum: (1/e_om_o)^1/2 = 2.9979…^.10⁸ m/s. Existence of these waves and their creation through electric currents and moving charges has been established over a wide range of radio frequencies. The empirical value of the speed of light is in perfect agreement with the prediction, which supports the view that light is just another (shorter-wavelength) manifestation of the same phenomenon. In addition, one can demonstrate that light is indeed generated by moving charges (for example, state transitions of electrons inside atoms, vibrations of hot molecules and solids — in light bulbs and the sun -, and directly in the case of synchrotron light which is produced by a circulating beam of electrons inside an accelerator). Secondly, it acts on charges (examples are the photoelectric effect - direct emission of electrons from Alkali metals -, solar panels, photosynthesis and the stream of charged particles flowing away from the sun due to the "light pressure" exerted on them). Finally, its propagation is influenced by electromagnetic properties of materials (conductive materials reflect light, and the phase velocity is reduced in materials with high dielectric constant). [Note however that light is not itself "charged" — since it is made up from electric and magnetic fields which act on charges, but do not carry charge themselves].

One example from each of the two above paragraphs above would count as a correct answer for this Quiz.