A photon is an elementary particle that is the quantum of electromagnetic waves, including light.

Photons are described by quantum mechanics and exhibit wave-particle duality — sometimes behaving like a particle and sometimes like a wave.

Max Planck first postulated (1900) that electromagnetic energy could only be emitted in “chunks”, or quanta.

Albert Einstein later (1905) explained the photoelectric effect, where electrons could be ejected from a surface only if the frequency (color) of the incident light was above a threshold and did not depend on the intensity (number of photons) of the light — this meant that the energy of light was quantized by photons.

Planck and Einstein’s discovery of photons and the photoelectric effect⁽¹⁾Camera sensors are made from silicon semiconductors where the photons generate electron-hole pairs.  This effect is virtually the photoelectric effect occurring in a semiconductor. underlies how image sensors in digital cameras work.

• Photons are elementary particles with zero mass, travel at the speed of light, and have an energy proportional to the frequency of the light. Blue photons have a higher frequency and hence more energy than red photons.
• Brighter, more intense light has a higher flux of photons than dim light. There is no concept of a brighter or darker photon, only the number of photons arriving per unit time.
• Photons arrive at your camera sensor in a random, irregular manner. When you are photographing in average daylight, the rate of photons incident on your sensor is very high, and the fluctuations in the number of photons relative to the average are small. This is not the case when you are photographing in low light when photons arrive at varying times, causing a dominant noise source (photon noise) in your images.

Takeaways for photography:

• In low light, the largest source of noise in our images is inherent photon noise due to the irregular arrival of photons.  Photon noise does not depend on or camera or sensor.