Light is a form of electromagnetic radiation consisting of periodically varying electric and magnetic fields that travel through a vacuum at a characteristic constant speed of approximately 299,792,458 m s$^{-1}$. While the term is often used to describe only the portion of the spectrum visible to the human eye, in a broader physical sense, light encompasses the entire electromagnetic spectrum, ranging from gamma rays and x-rays at the shortest wavelengths, through ultraviolet, visible, and infrared radiation, to microwaves and radio waves at the longest wavelengths. This radiation exhibits wave-particle duality, meaning it can be described both as a wave motion characterized by frequency ($\nu$) and wavelength ($\lambda$), and as a stream of discrete packets of energy known as photons ($\gamma$). The energy of these photons is inversely proportional to the wavelength; therefore, short-wavelength radiation such as gamma rays possesses high energy, while long-wavelength radiation like radio waves possesses low energy.

The speed of light, conventionally denoted by the symbol $c$, is a fundamental physical constant representing the speed at which electromagnetic radiation propagates through a vacuum. This characteristic speed applies to the entire electromagnetic spectrum, from radio waves to gamma rays, and links the wave-like properties of radiation through the fundamental relationship between frequency and wavelength. The relation between the frequency (the number of wave crests passing a point per second) and the wavelength (the distance between successive crests) is governed by the equation

$$ c=\nu \lambda $$

which implies that frequency and wavelength are inversely proportional; therefore, radiation with a shorter wavelength, such as blue light, travels at the same speed as red light but oscillates at a higher frequency.

Historically, the fact that light has a finite speed was first demonstrated in 1675 by the Danish astronomer Ole Rømer, who observed that the intervals between eclipses of Jupiter’s moon Io varied depending on the Earth’s distance from Jupiter. While early estimates were rough, precise terrestrial measurements were later achieved in the 19th and 20th centuries by physicists such as Fizeau, Foucault, and Michelson, the latter of whom used rotating mirrors and vacuum tubes to refine the value to near its modern accepted figure.

The electromagnetic nature of light, established by James Clerk Maxwell, implies that its propagation speed is governed by the fundamental electric and magnetic properties of the medium through which it travels. The equation

$$ c= \sqrt{\frac{1}{\epsilon\mu}} $$

expresses the speed in terms of the medium’s electric permittivity $\epsilon$ and magnetic permeability $\mu$, which relate the electric displacement and magnetic induction to the electric and magnetic fields, respectively. In the vacuum of free space, these physical constants are denoted as $\epsilon_0$ (the permittivity of free space) and $\mu_0$ (the permeability of free space). When electromagnetic radiation passes through a material medium rather than a vacuum, its velocity is reduced relative to $c$ by the index of refraction $n$, which is a function of the medium’s electromagnetic properties.

In the context of Albert Einstein’s special theory of relativity, the speed of light takes on a deeper significance as a universal constant that is the same for all observers in inertial frames, regardless of their relative motion, and serves as the absolute speed limit for the propagation of energy and matter in the universe,.