Nature of light

For a long time, light was understood simply as 'that form of radiant energy which makes objects visible by stimulating the retina of the eye' -- a definition based purely on its EFFECT, not its actual nature. Physics eventually pinned down what light truly is through two complementary lines of evidence that together define the modern picture.

At the beginning of the 20th century it was established that light is an electromagnetic (EM) wave, propagating even through a vacuum (unlike sound, which needs a medium) at the universal constant speed $c = 299792458\ \text{m s}^{-1}$. Einstein's special theory of relativity shows this is the maximum possible speed for anything in the universe; for ordinary calculations it is used as $c \approx 3\times10^{8}\ \text{m/s}$. Later, quantum theory showed that light ALSO has a particle character: it consists of discrete energy-carrying particles called photons, a fact confirmed experimentally using countable numbers of photons. So the modern, dual understanding of light is that it consists of energy-carrying photons that are simultaneously guided by the rules of electromagnetic wave propagation.

Inside any material medium, the speed of an EM wave is set by that medium's permittivity $\varepsilon$ and permeability $\mu$ (its electric and magnetic properties), and every material medium slows light down relative to vacuum. This slowing is captured by the ABSOLUTE REFRACTIVE INDEX of the medium, $n = \dfrac{c}{v}$ -- the ratio of light's speed in vacuum to its speed in that medium -- a dimensionless property of the medium alone. …