A photon is a particle that carries electromagnetic radiation. It is the fundamental unit of light.

Photons are:
Photons and mass
  •  The statement above about zero rest mass can be broken down as follows:
    • According to the theory of relativity, any object that has mass requires energy to accelerate.
    • The amount of energy required to accelerate an object increases as the object’s mass increases.
    • Photons are unique in that they have zero rest mass, which means they do not require any energy to be accelerated.
    • As a result, they always move at the speed of light in a vacuum.
    • So photons always travel at approximately 299,792 kilometres per second and never slow down or come to a stop.
Energy of photons, wavelength, frequency and colour
  • The energy of a photon (its photon energy) is intrinsically linked to its wavelength and frequency, and in perceptual terms, to its colour.
    • Photon Energy: The energy of a photon, E, is given by the equation E=hf, where h is Planck’s constant and f is the frequency of the photon. So, a photon with a higher frequency has higher energy.
    • Frequency and Wavelength: Frequency (f) and wavelength (λ) are related by the speed of light (c), through the equation c=fλ. So, photons with a higher frequency have a shorter wavelength, and photons with a lower frequency have a longer wavelength.
    • Colour Perception: In terms of colour perception, different energies (and thus frequencies and wavelengths) of photons are perceived as different colours. For example, photons with high energy (high frequency, short wavelength) are perceived as blue/violet, while photons with lower energy (low frequency, long wavelength) are perceived as red. The range of frequencies (or wavelengths) that human eyes can detect is known as the visible light spectrum.
Photons and the world
  • Interaction with Charged Particles: Photons can interact with charged particles such as electrons and protons. In such events, they can either be absorbed, resulting in the elevation of the particle to a higher energy state or be emitted when a particle transitions from a higher energy state to a lower one.
  • Interaction with Matter: Photons can engage with matter through various processes. They can be scattered, absorbed, or emitted during interactions with atoms and molecules. These processes are crucial for a range of phenomena from the heating of surfaces under sunlight to the transmission of information in fiber optic cables.
  • Polarization: Photons can be polarized. This means their electric and magnetic fields can oscillate in a specific orientation. Polarization is used in various applications, from LCD screens to polarized sunglasses, and is also a significant aspect of certain quantum mechanical phenomena.
  • Energy, Frequency, and Wavelength: The energy of a photon determines its frequency and wavelength. This energy can be transferred during interactions, leading to phenomena like fluorescence or the photoelectric effect. In the visible spectrum, different energies (and therefore frequencies and wavelengths) correspond to different colours of light.