# How is light related to photons? [duplicate]

This question already has an answer here:

This may seem like a duplicate but I do not understand other explanations.

I have read that light is an electromagnetic wave (a fluctuation or disturbance in the electric and magnetic field). How does this have anything related to a packet of energy described as a photon. All I understand about photons is that they are packets of energy absorbed and released by the movement of electrons in different energy levels.

Please explain as best as you can for a high school student.

## marked as duplicate by Sebastian Riese, Gert, Kyle Kanos, ACuriousMind♦, user36790 Dec 11 '15 at 2:56

• What about the question What is the relation between electromagnetic wave and photon? is not a duplicate of this? Please don't say "this may seem like a duplicate" but actually explain why it is not! – ACuriousMind Dec 10 '15 at 16:51
• I meant that this may be a duplicate but I would still like simpler answers because I do not understand other articles. – Person Dec 11 '15 at 13:17

Great question to be asking. Light is an electromagnetic wave, and think of the wave being made up of a large number of photons $N$ (even though technically it's a little more complicated than that). The wave has a frequency $f$. The energy of a single photon is $hf$, which might lead you to believe that the total energy is $E_{tot}=Nhf$, but in fact it's slightly different:

$E_{tot}=(N+1/2)hf$

The extra $(1/2)hf$ is sometimes called "zero-point energy", and it's interesting because it means that even when you have zero photons there is still some electromagnetic energy there.

Now to answer your question: every time we add a photon, the total energy $E_{tot}$ still increases by $hf$. Since there's no such thing as a fraction of a photon, the possible values of $E_{tot}$ are discretized, meaning only discrete values are possible. The jumps between these values are addition/subtraction of a photon.

Let me try a slightly different approach.

How do we know light is an electromagnetic wave? Well, first, is there any reason to think it's a wave? Yup. See descriptions of interference patterns and polarization. These phenomena behave exactly as if light is a wave. So, why is it electromagnetic? This connection actually came about from the other end. James Clerk Maxwell produced his famous equations and realized that, if you plug in some experimentally-determined constants (relative permeability and magnetic susceptibility), his equations can produce a propagating wave with a velocity exactly equal to that of light. This, of course, suggested that light is an electromagnetic wave - unless you are a big believer in coincidence.

For about 40 years, Maxwell's equations ruled. Then, those pesky experimental types started noticing that some aspects of light just don't behave as a wave should. The photoelectric effect is a good example. Various folk realized that something was wrong, and in 1905 a young patent clerk named Einstein (does the name ring a bell?) won the Nobel Prize for his paper which concluded that, at a small enough scale, light can also be thought of as particles. You'll note that his Special Relativity paper, also published in 1905, did not get the Nobel.

So, is light a wave or a particle? The answer is either a resounding yes or a thunderous no. It all depends on how you measure light. If you have a lot of energy being transmitted, waves are the way to describe it. If you're talking about really small amounts, quantization effects start to show up and it's better to think in terms of particles. And, just to mess with your mind, there are situations where, in effect, both are true. The classic experiment that deals with this is the double slit experiment and at very low light levels, you can detect single photons - but they appear at different places as if they were also waves. If this all seems pretty weird, don't worry. The physics community as a whole struggled for nearly 50 years with wave particle duality, and the problem is made worse by the fact that, at some level, it appears to apply to pretty much everything. Certainly you can get interference patterns from electrons, and they're particles, right?

But back to photons. The process you mention, emission by electrons changing energy levels, occurs at very, very small scales, and when looking at this level the particle description is the one that is most obvious and useful. There are various other ways to produce light (or at least EM radiation) which do not involve electrons changing energy levels. These include, for instance, electron/positron annihilation and nuclear fission and fusion. But producing light (or heat - infrared radiation) is common in our experience, so that's a useful source to consider. If the electrons are just bouncing around, you get essentially random wavelengths and this is seen in blackbody radiation. But if you can somehow constrain the way electrons can gain or lose energy, you can get light at very specific wavelengths, such as lasers. Explaining the specificity of these wavelengths is basically impossible with classic wave theory, but simple enough under quantum mechanics, which is the ruling model at atomic levels. Then, of course, if you've produced a whole lot of photons you get a beam of light which may well be usefully described (for lens design, for instance) as an electromagnetic wave.

The thing to keep in mind is an old saying (well, not that old) in science: All models are wrong, but some are useful

• This is very detailed and very useful in understand. I see what you mean by light can be measured by both a particle and wave? You see I don't get how the particle version of light makes sense. I still do not understand what on earth a photon is and how it makes up light. Light it just a disturbance. Where do photons come in? Thanks – Person Dec 11 '15 at 13:24
• @Person - I'm sorry, but I've simplified things as much as I can (even to the point that I got a downvote). Follow the links, and try to forget what you think you already know. Light is not "just a disturbance". Try talking to a teacher, although many high school teachers won't be able to help you. – WhatRoughBeast Dec 12 '15 at 15:56