Light is of fundamental significance as an energy and information carrier. Light rays enable us to see, and sunlight is the Earth’s most important source of heat. Light is also the basis for almost all life on Earth through the photosynthesis of plants, in which sunlight is used as a source of energy when plants covert carbon dioxide and water to biomass. We utilise the energy from the sun to grow plants and we are also starting to make use of the physical energy from sunlight directly through solar cells.
Light as a physical phenomenon
Light is an electromagnetic radiation sensitive to the eyes at a wavelength range of 390–770 nano metres. Also nearby wavelengths are usually included in the concept of light, although our eyes cannot register them, including infrared light which has longer wavelengths, and ultraviolet light which has shorter wavelengths. Synchrotron radiation can also be included even though its wavelengths extend far beyond the visible spectrum.
Light is emitted when excited atoms dispose of their excess energy, through several physical, chemical and biological processes. Some objects, such as the sun, lamps and candles, produce their own light, but most objects are only visible when light from elsewhere hits them and is reflected from them.
A physicist can describe light as both a wave and a particle, a photon. We decide which description we favour. In quantum mechanics the two distinctions are united: Light is a stream of particles and a wave.
In vacuum, the light propagation speed is 300 000 kilometres per second. The speed is constant and independent of whether or not the light source is moving. The speed changes when the light passes through a material. Gas will have no significant impact on the light speed while glass will reduce it up to 50 %. The changing of the light speed when passing through various materials causes light refraction.
Physical phenomena such as refraction, diffraction and interference are explained by the wave model of light, but when explaining photoelectric, photochemical or photobiological processes, we use the photon model. In these processes, the individual photon’s interaction with an atom or molecule is described, and the energy of the photon is therefore crucial.
Properties of light
The characteristics of light as a wave are frequency, wavelength, oscillation direction (polarisation), amplitude and speed of propagation. Of these, only frequency is independent of which medium the light travels in, and frequency also determines the colour.
Candela is the unit used for luminance, lumen for light output, and lux for illumination. A candle has a brightness of approximately 1 candela, the sun approximately 100 billions and billions of billions (1028) candela.
Text: Pia Romare
Photo: Catrin Jakobsson
Facts retrieved from: Flervetenskaplig Ljusforskning (Multidisciplinary light research), Pufendorf Institute Report Series No 1
An atom is excited when it receives/stores energy by shifting its electrons to a position with higher energy (generally farther from the atomic core).
The behaviour of a wave motion at a sharp edge or gap.
A phenomenon that occurs in the interaction between two or more waves, that is, when waves meet, they amplify, attenuate or cancel each other out.