[This article is under construction.]
A photon is a type of quantum. So, it’s like the difference between a blue jay (photon) and a bird (quantum). If this answers your question, you can stop reading. If you want to know more about what a photon and a quantum really are, read on.
A Photon Is a Bit of Light
A photon is a bit of light. To see how light can be divided into bits, photons, it’s necessary to understand some things about light. Light travels as a wave, an electromagnetic wave. An electromagnetic wave is an electrical wave and a magnetic wave traveling together.
As the electrical wave rises and falls, it creates a magnetic wave and then, the magnetic wave rises and falls, creating an electrical wave, and on and on. This video shows how electrical and magnetic waves create each other: https://www.youtube.com/watch?v=1SQV9kBN_b4
When physicists speak of “light,” they mean any kind of electromagnetic wave. This includes visible light, the kind that we see with our eyes. But electromagnetic waves also include X-rays, ultraviolet rays, infrared, microwaves, radio waves, and many others. Any type of electromagnetic wave can also be called “radiation.” The difference between the various types of electromagnetic radiation or waves is their wavelength (see image above).
The red “spring” at the top of the accompanying image shows the spectrum of electromagnetic waves from radio waves on the left to gamma rays on the right.
What about photons?
When light, that is, an electromagnetic wave, strikes an object, it immediately collapses into tiny bits of energy. Each of these bits is a photon. It’s as if an ocean wave hits a rock, and shatters into a gazillion tiny droplets. Each “droplet” of the light wave is a photon, and each carries a bit of the energy of the light wave. If the light wave were to hit a piece of film, we would be able to see all the tiny spots of light. Each photon creates a bit of the photo, usually a small fraction of a pixel. Together, they form the image.
So, what is a quantum?
Subatomic particles travel as waves, as we have seen for electromagnetism. When these waves strike an object, they collapse into bits. In the case of electromagnetism, the bits are called photons. But the more general term is “quanta,” or in the singular, “quantum.” There are other types of waves at the subatomic level besides electromagnetic waves. For example, electrons travel as waves as do neutrinos, and other subatomic particles. Quanta are the bits that various subatomic waves create when they interact with objects.
I should clarify that the waves don’t physically shatter when they hit objects. It’s more like they interact with the objects and due to the laws of quantum physics, the waves transform into tiny energy-bearing particles, that is, quanta. And, if I really wanted to be accurate, waves at the subatomic level are not like ocean waves or sound waves. Their physical nature is still under debate, with one possibility being that they are no more a mathematical description of a wave. But this is diving deeper into quantum physics than is useful here.