Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space. Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.
- Simply stated, light is energy. Light is the way energy travels through space.
- Light and colour are entirely different phenomena. Whilst light is electromagnetic radiation, the experience of colour is a feature of human vision that depends first of all on the construction of our eyes and the wavelength, frequency and amplitude of visible light that strikes the retina.
- The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
- Whilst the term light can be used to refer to the whole of the electromagnetic spectrum, visible light refers to the small range of wavelengths that our eyes are tuned to.
- The term light can be used in three different ways:
- Light can be used to mean the whole of the electromagnetic spectrum from radio waves, through visible light to gamma rays. When this meaning is intended, the terms radiant energy or photon energy are placed in brackets after the term light in this resource.
- Light can be used to mean the range of wavelengths and frequencies that can be detected by the human eye. A better term is visible light which refers to the wavelengths that correspond with the colours between red and violet, the visible spectrum.
- Light can also be used to mean the range of wavelengths and frequencies between infra-red and ultra-violet. This usage is sometimes useful because the outer limits of the visible spectrum can differ under different lighting conditions and for different individuals.
- Remember that the precise experience of visible light is not exactly the same for all individual humans and is not the same for all living things.
- Light travels through a vacuum at 299,792,458 metres per second but propagates more slowly through other media.
- When light interacts with matter it results in optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.