Foundations and theories of light

Electromagnetic spectrum and visible light range

Visible light is electromagnetic radiation that the human eye can perceive. It occupies the visible spectrum, typically defined as wavelengths from about 400–700 nm (corresponding to frequencies roughly 750–420 THz). Visible light lies between infrared (longer wavelengths, lower frequencies) and ultraviolet (shorter wavelengths, higher frequencies), and is part of the broader category of optical radiation. More generally, the electromagnetic spectrum is classified by wavelength into radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The behavior of electromagnetic radiation depends on its wavelength and the energy carried per quantum (photon). In the visible range, photon energies are sufficient to trigger molecular changes in the eye (retinal), producing vision. Below the visible range, infrared photons are generally too low in energy to cause the needed molecular change, while above it, ultraviolet is largely blocked by the cornea and lens and can damage retinal cells; some animals can detect infrared or ultraviolet via different biological mechanisms. Light’s key properties include intensity, direction of propagation, wavelength/frequency spectrum, and polarization. In vacuum, all electromagnetic radiation travels at the speed of light, exactly 299,792,458 m/s. Although everyday light interactions can often be explained with geometrical optics, electromagnetic radiation also exhibits wave–particle duality: it can be described using wave behavior (e.g., interference) and photon behavior (e.g., detection with specialized equipment).

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Speed of light in vacuum

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