The Photon Model of Light

In 1905, Albert Einstein proposed his photon model of light to explain the photoelectric effect. In this theory, light consists of individual packets of energy called photons. A beam of light can then be thought of as many separate photons each carrying a specific amount of energy, rather than a continuous stream of energy. When a photon is absorbed it transfers all of its energy at once.

Photons are massless and travel at a speed of 300,000,000 m/s in a vacuum. They have wave-particle duality, which means they show both wave-like and particle-like behavior. They act as individual particles when interacting with matter - for example, during the photoelectric effect - but act as electromagnetic waves - capable of diffraction, interference, etc. - when propagating through space. This means that photons have a frequency and a wavelength. The energy of each photon actually depends on its frequency, with higher frequency photons being more energetic.

In this model, increasing the intensity of light illuminating a surface means increasing the number of photons being delivered to that surface per unit time per unit area.