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.
Takeaways for photography:
A simplified diagram of the photoelectric effect is shown here. In our camera sensors, a similar effect happens in each photosite (pixel) on the silicon semiconductor. Photons generate electrons in each pixel, which are converted to a voltage, which is amplified, digitized, and then recorded as a number on the memory card.