Light is a form of energy that behaves as a wave and as a particle (photon). It can travel through space and transparent media like air, water, or glass. Light doesn’t need a medium to propagate through and travels through empty space at a speed of 299,792 kilometres per second.

light in classical physics
light in quantum mechanics
  • In the field of quantum mechanics, light is described as a stream of particles called photons, which are the quanta of the electromagnetic field. According to this theory, photons are massless particles of light that have no electric charge but have momentum and each photon constitutes a single packet of electromagnetic energy.
  • One of the most famous experiments that demonstrate the particle-like nature of light is the photoelectric effect, in which electrons are emitted from a metal surface when exposed to certain wavelengths of light. The photoelectric effect can not be explained by the wave theory of light but is explained by Einstein’s theory of the photoelectric effect, which proposes that photons transfer their energy to electrons in the metal.
  • The wave model and the quantum mechanical model of light are not mutually exclusive and can be used to develop different perspectives on the same phenomena.
    • The wave model is useful for understanding light in situations where it behaves like a wave but largely ignores the way it interacts with matter at a sub-atomic scale.
    • The quantum mechanical model of light is useful in understanding interactions between light and matter at a sub-atomic scale, particularly interactions involving single photons and other quantum particles such as electrons.
About light and colour
light & colour
  • Light and colour are related but distinct concepts. Light is a form of electromagnetic radiation, while colour is a perception that results from how the human eye and brain respond to different wavelengths of visible light.
  • The human eye can perceive only a small part of the electromagnetic spectrum, known as visible light, which includes wavelengths between about 400 and 700 nanometres.
  • The perception of colour depends on the wavelengths of light that stimulate the cones in the retina.
  • The perception of colour can vary among individuals and living organisms.
  • 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.
  • Colour perception in humans primarily depends on the design of our eyes and the wavelength, frequency, and energy of the visible light that strikes the retina at the back of our eyes.
  • Colour is a visual experience unique to each of us at any given moment because of our different points of view and perspectives on the world. So we share our experiences of colour using language to share our experiences of colour.
About light, radiation, radiant energy & electromagnetic energy

There is a difference in meaning between the terms light, electromagnetic radiation, radiant energy and electromagnetic energy in physics.

Electromagnetic radiation
Radiant energy
    • Radiant energy is most commonly used to refer to electromagnetic radiation carried by electromagnetic waves and photons.
    • Radiant energy can be measured using instruments such as photometers, which detect the intensity of light or other forms of electromagnetic radiation.
Electromagnetic energy
  • Electromagnetic energy is a more general term that refers to any form of energy that is carried by electromagnetic waves, including both radiant energy and other types of energy that are not radiant (e.g., static electric fields).
  • The type of energy associated with electromagnetic radiation is a measurable quantity in physics, and its measurement is essential for understanding and analyzing physical systems and processes.
  • The unit of measurement for electromagnetic energy in the International System of Units (SI) is the joule (J), which is defined as the amount of energy required to perform one joule of work
  • The electronvolt (eV) is another unit of energy commonly used in atomic and subatomic physics.

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.
  • 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.