What Is a Photon?

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A photon is a little packet of energy, an elementary particle that is the basic unit of light and electromagnetic radiation. Find out how people came to understand photons with help from a science teacher in this free video on physical science.

Part of the Video Series: Physical & Life Science
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Video Transcript

Hi, I'm Steve Jones, I'm going to explain what a photon is. Now early in the 20th century, in the 1920's, people started to understand the structure of the atom and that there were electrons which surrounded a nucleus and these electrons could only have specific energies. That meant that if the electron, if the substance was activated in some way or heated up in some way, the electrons would occupy these levels here. They couldn't occupy a level in between, it wasn't allowed. And therefore when the electrons lost or gained energy, they always did it by going from one energy level to the other. This transfer of energy, this changing energy, is related by this formula, the changing energy from energy level one to energy level zero is equal to H times V, this is the Greek letter nu and nu is the frequency of the radiation which was emitted. So if the energy changes were different, the frequencies were different. That is the colors were different and the colors could be kind of x-ray colors that is not visible to us but x-rays, or they could be even radio waves depending on the energy. This Hv was described as a photon. Now the word photon makes you think, well photon it's like proton, neutron, so why photon? Well photon is, comes from photos which is Greek for light so it's related to light and these are electromagnetic radiations. These photons are electromagnetic radiations and they have some properties which make it seem like a wave and some properties which make it seem like a particle. So basically, you've got little packets of energy and we call them photons, they really are waves and they have some kind of properties that make them like particles too. So basically that is what a photon is.


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