A photon is an elementary particle, meaning it has no smaller components. We also say that photon is the quantum of light, meaning it is the smallest possible amount of light. Since light is just the visible part of electromagnetic radiation, it is more accurate to say it is the quantum of electromagnetic radiation. All photons in vacuum move at the speed of light.

Photon is also the force carrier for the electromagnetic force. Like all force carriers, photon is a gauge boson, which means it has integer spin that equals 1. Photon has no mass, so it can carry the electromagnetic force over long distances. It has properties of both particles and waves, so we say that it exhibits wave-particle duality. It doesn't have electric charge.

Photons are the most abundant particle in the Universe. As mentioned above, they are quantum of the electromagnetic radiation. Depending on the frequency at which photons vibrate, the radiation can be classified as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays or gamma rays. Radio waves have lowest frequencies and are the weakest form of electromagnetic radiation, gamma rays the strongest, which means that the higher the frequency, the more energy photons transmit.

But if photons simply carry the radiation, what happens at the source?

In atoms, electrons orbit the nucleus at constant distances in a stable system consisting of multiple shells one above another. When an electron absorbs energy from whatever source, it gets into an excited state and moves into a higher orbit, higher level. When it stops receiving energy, it moves back to its natural orbit by emitting this excess energy in the form of a photon. Take a match for example – its head is made of phosphorus which we can light by striking the match against a surface. It receives the energy from the friction that occurs and starts burning. As it burns, it releases the energy via photons. Photons that we can see with our eyes carry the information which we perceive as yellow-orange light.

Frequency of a photon tells us a lot about the source of light since photons carry the information about properties of the matter that emits them. One example are colours of everyday materials that reflect the light. Depending on material, one part of visible light, which is white, is absorbed by the material and the other part is reflected to our eyes. Photons hit electrons in our eyes and transmit the information that the material reflected e.g. green part of the visible light. 

Most of the information we receive about the world around us is by using our eyes i.e. we detect photons. Similarly, in astronomy, the majority of observations are made based on photons. We measure visible light, radio waves, gamma rays, infrared and X-rays to find more about stars, black holes, pulsars etc. It is all based on photons, but we are so used to it, that we don't consciously perceive it all the time.

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