Light as a Wave

Modern physics identifies light specifically as a TRANSVERSE electromagnetic wave: it consists of oscillating electric and magnetic fields that are mutually perpendicular to each other, and both perpendicular to the wave's direction of propagation. Because these oscillating fields do not require any material medium to sustain them, electromagnetic waves -- unlike sound or water waves -- can travel through a perfect vacuum, exactly as they do across the emptiness of space from the Sun and stars to reach us.

The speed of light in any material medium, v, is governed by the medium's REFRACTIVE INDEX, n, itself determined by the medium's electric permittivity and magnetic permeability. Refractive index is formally defined as the ratio of light's speed in vacuum, c, to its speed in that medium: $n = c/v$. Vacuum, by this definition, has $n = 1$ exactly, and air is close enough to vacuum in its optical properties that its refractive index is very well approximated as 1 too.

Electromagnetic waves span an enormous range of wavelengths -- from far smaller than a femtometre ($10^{-15}$ m, such as gamma rays) to far larger than a kilometre (radio waves) -- with the different named bands (gamma/X-rays, ultraviolet, visible, infrared, microwave, radio) simply corresponding to different, increasing wavelength ranges along this one spectrum. The narrow band of VISIBLE light that the human eye can detect spans roughly 400-700 nanometres, with violet at the short-wavelength end and red at the long-wavelength end; different wavelengths within this range are perceived by the eye as different colours, and a mixture of all of them together (in the right proportions) is perceived as white light. …