A nanometer is an extremely small unit of length in the metric system, equal to one billionth of a metre - or equivalently, one millionth of a millimetre - making it one of the smallest units of measurement in common scientific use. The prefix nano derives from the Greek word for dwarf, and is used in the metric system to denote a factor of one thousand millionth, reflecting the vanishingly small scale at which the nanometer operates. To appreciate the scale involved, a single human hair is approximately 80,000 to 100,000 nanometres in diameter, and a typical atom measures just a fraction of a nanometre across.
In photography and optical science, the nanometer is the standard unit used to measure and express the wavelengths of electromagnetic radiation, including visible light, ultraviolet radiation, and infrared radiation - all of which have direct relevance to photographic practice. The visible spectrum, comprising the narrow band of electromagnetic radiation to which the human eye is sensitive, spans a range of approximately 400 to 720 nanometres, with the shortest visible wavelengths at the violet end of the spectrum measuring around 400nm and the longest visible wavelengths at the deep red end measuring around 700 to 720nm. Between these limits, the full range of colours visible to the human eye - from violet through blue, cyan, green, yellow, and orange to red - is distributed across this span of approximately 320 nanometres.
Understanding wavelengths expressed in nanometres is practically relevant to photographers in several contexts. The spectral sensitivity of photographic films and digital image sensors - describing which wavelengths of light they respond to and with what relative sensitivity - is expressed in nanometres, as are the transmission characteristics of optical filters, which describe the wavelength ranges they pass or block in terms of nanometre cut-off or peak transmission values. The emission spectra of artificial light sources - describing the wavelengths of light they produce and in what proportions - are similarly expressed in nanometres, providing the physical basis for understanding why different light sources have different colour temperatures and why they render colours differently in photographic images.
The nanometer is also relevant in the context of the anti-reflective coatings applied to photographic lens elements, which are engineered to a precise thickness of a fraction of a wavelength - often in the range of 100 to 200 nanometres - to produce the destructive interference that suppresses surface reflections across the visible spectrum. Similarly, the pixel pitch of digital image sensors - the centre to centre distance between adjacent photosites - is measured in micrometres rather than nanometres, but the physical dimensions of the semiconductor structures within each photosite operate at the nanometre scale, reflecting the extraordinary precision of modern semiconductor manufacturing technology.