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Q & A: What exactly is a photon?

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Most recent answer: 05/08/2013
So, what exactly is a photon? I just read a bunch about how they don't have charges but have electrons around them. But I would like to understand this better. In light, when it is behaving like a photon, what exactly does that mean? Also, can you give an example of when this happens in the world, or is it just experimental things you can't see?
- Elizabeth (age 17)

Hi Elizabeth,

I'm not sure exactly what you were reading about photons, but I can assure you that they don't have electrons around them (in general). They exist as single packets of electromagnetic energy, independent of any other form of particles, even in vacuum.

Each photon behaves statistically just like the light beam emitted from the source. The light beams that you see are usually made of billions of billions of tiny photons, each traveling at the speed of light. So, how do we know that light is made of photons?

In fact, there are many, many reasons. I'll give you a simple example or two, and then point you towards more interesting and difficult to understand examples.

If you take a flashlight and try to shine it through a lot of dark glass, it gets dimmer and dimmer. If you keep attenuating the beam, and then take a picture of the beam with a camera, you will see single, tiny flashes of light, limited only by the resolution of your camera's pixels.

So, you really can detect single photons with a camera. In addition, frogs can see single photons, and experiments are currently underway to determine if humans can see single photons!

The fact that light is made of single photons is very interesting and useful... we can use these photons to encode information, test our theories about the universe, and more.

The quantization of light is also necessary to make sense of many experimental results in quantum mechanics. Try googling "photoelectric effect."

Let us know if you have specific questions. It's a big subject!
David Schmid


(published on 05/08/2013)

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