Light is the visible portion of the electromagnetic spectrum - the narrow band of radiated energy to which the human eye is sensitive and which forms the fundamental raw material of all photography. It occupies a specific range of wavelengths within the broader electromagnetic spectrum, spanning approximately 400 to 720 nanometres - equivalently expressed as 4000 to 7000 Ångströms - from the shortest visible wavelengths at the violet end of the spectrum through to the longest visible wavelengths at the deep red end. Radiation of wavelengths shorter than approximately 400nm falls into the ultraviolet region, invisible to the human eye but detectable by many photographic emulsions and digital sensors, while radiation of wavelengths longer than approximately 720nm falls into the infrared region, also invisible to the naked eye but similarly recordable by certain photographic and digital imaging systems.
Within the visible range, different wavelengths of light are perceived by the human eye as different colours. The shortest visible wavelengths around 400 to 450nm are perceived as violet and deep blue, progressing through blue, cyan, and green in the middle of the visible range at around 500 to 570nm, through yellow and orange, and reaching red at the longest visible wavelengths around 620 to 720nm. White light, such as that produced by the sun at midday, is a mixture of all visible wavelengths in approximately equal proportions, while coloured light results from the dominance of a particular wavelength range within the mixture.
Light exhibits a dual nature that has been fundamental to the development of both optical science and quantum physics. It behaves simultaneously as a wave - exhibiting the characteristic wave phenomena of interference, diffraction, and polarisation that underpin many optical devices and photographic techniques - and as a stream of discrete energy packets known as photons, each carrying a quantum of energy inversely proportional to its wavelength. It is the particle nature of light that is directly relevant to the photochemical processes of photography, as the energy carried by individual photons is what initiates the chemical reactions within a photographic emulsion or generates the electrical signals within a digital image sensor that ultimately form a photographic image.
The speed of light in a vacuum is approximately 299,792 kilometres per second - one of the fundamental constants of the physical universe - and it is the reduction of this speed as light passes through optical media such as glass that causes refraction, the bending of light rays at air to glass interfaces that is the foundational principle of all photographic lenses and optical systems. The degree to which a given optical material slows and refracts light is described by its refractive index, and the careful selection and combination of optical glasses with different refractive indices is central to the design of corrected photographic lenses.
In practical photographic terms, understanding and controlling light - its direction, quality, intensity, colour temperature, and spectral composition - is the most fundamental skill in photography. The way light falls on a subject determines its apparent form, texture, colour, and mood in the final image, and the photographer's ability to observe, interpret, and manipulate light is what distinguishes a technically competent photograph from a truly compelling one.