Transcript: In the quantum theory the energy of a photon is given by the product of Planck’s constant and the frequency of the photon. Planck’s constant is a fundamental constant of nature and a very tiny number, 6.6 times 10-34 joule-seconds. What does this mean? This means that radiation is grainy just as matter is grainy at the level of atoms. The graininess of radiation corresponds to individual photons. How much energy is there in a photon? A photon of visible light can be calculated to have an energy of about 4 time 10-19 joules, also a very tiny number. This means that a hundred watt light bulb is emitting photons at a rate of about 2.5 times 1020 per second, or 250 billion-billion photons per second. The huge number of photons in a typical macroscopic situation and their tiny individual energy means the graininess of radiation is invisible to us just as the graininess of matter is invisible to us.