# B EM waves and photons

1. Mar 10, 2016

### Karan Punjabi

Guys, I am studying about electromagnetic waves and I'm not able to differentiate between a photon and a em wave or I'm not understanding it. Please help me to solve it out. Thanks

2. Mar 10, 2016

### drvrm

An electromagnetic wave is a wave generated by an oscillating charge as in Hertz's experiment.
the range of electromagnetic waves spreads from far radio waves to the X-rays
the visible region of the spectrum we see called visible light.

And a photon is a particle representing a "quantum" of electromagnetic waves- these photons have dual character or dual signature in various experiments conducted by the people who wish to identify the nature of photons,
If one looks up the history of development of "quantum" nature of physical systems one finds several instances when light which can be represented by E.M. waves does provide evidence of its photonic nature as well as wave -nature.
when light knocks up an electron from metal by transferring its energy the phenomena of photoelectrons coming out with certain kinetic energy can be explained on 'particle' type nature- or in compton scattering experiment it interacts with electron like a particle being scattered.
whereas the interference/diffraction //polarization effects do point to the wave nature of photons in terms of EM waves.
one can construct a packet of electromagnetic waves by superposition of waves and the wave packet can represent a photon of definite energy and momentum.

Last edited: Mar 10, 2016
3. Mar 10, 2016

### Staff: Mentor

Try to forget that you ever heard the word "photon" until you're done studying electromagnetic waves. Photons are part of the theory of quantum electrodynamics (quantum mechanics meets relativity meets electrodynamics) which you won't get to for several years to come.

4. Mar 10, 2016

In studying EM waves, you also will learn that the energy density of the electromagnetic waves (and also the power/unit area incident onto a surface) is proportional to the second power of the electric field amplitude. This energy is proportional to the number of photons. (Photon energy Ep=h*f). By computing the energy, you can calculate the number of photons. If you are studying diffraction and interference theory, it helps to have at least a partial understanding of what the photons represent. There is a wave-particle duality, but the energy and photon count is conserved in an interference pattern.

5. Mar 10, 2016

### Sturk200

They are two different ways of modeling the same phenomenon, namely light. One is continuous (waves), the other is quantized (photons). I think it's right to say that photon theory is more complete than wave theory, as it is able to account for things like the photoelectric effect. But wave theory is still "illuminating," if you will.

6. Mar 10, 2016

The wave theory of light=diffraction/interference theory is very important for doing any kind of modern physics where things like diffraction grating based spectrometers and thin film optical (bandpass) filters are in widespread use as research tools. The theory behind these is for the most part classical electromagnetic wave theory.

7. Mar 10, 2016

### davenn

it's not just related to light .... it is for the whole EM spectrum !!

then you don't quite understand what photons are

8. Mar 11, 2016

### Karan Punjabi

Ohk that means I shouldn't go for photons right now.But how should I imagine a EM waves, just as a wave? So in photoelectric effect can i study it without the term photon?

9. Mar 11, 2016

### Karan Punjabi

So at this level what should i do? Can i understand photons?

10. Mar 11, 2016

### Karan Punjabi

But now i simply want to differentiate or to understand EM wave or photon

11. Mar 11, 2016

### blue_leaf77

The study of photon is much more elaborate and richer than what you may think it is. For your current stage of study, I suggest that you hold onto the understanding that photons are the entity that is responsible to the energy unit of an electromagnetic radiation. Namely, given the frequency of the radiation, its total energy can be expressed as an integer multiple of the one photon energy corresponding to this frequency.
I would also like to warn you in advance about a common misconception in this matter so that you won't fall for it, is that people with limited mathematical knowledge of photons tend to easily associate the wavefunction of photons with the electric (or magnetic) field disturbances. That's not true, because in the quantum description of light, electric and magnetic fields are operators, hence they are a different and separate object as photon's wavefunction.
If you dive deeper in this direction, you will also learn that the energy of an electromagnetic radiation, despite expressible as an integer multiple of a photon's energy, can also have some uncertainty due to the uncertain nature of the number of photons contained in that radiation.
Lastly, if you feel like the second and third paragraph of my post is taking too long or too abstract to grasp, then you do need to hold for the time being your complete description about the difference between the EM wave and photons to the basic level, for which the first paragraph of this post shall be representative.

Last edited: Mar 11, 2016
12. Mar 11, 2016

### Sturk200

Some folks say "light" and mean electromagnetic radiation. I'm one of those folks.

Come on, you can't just say something like that without saying why you're saying it.

Probably to fully understand photons you have to study quantum electrodynamics directly, and to study quantum electrodynamics you have to study quantum mechanics, and probably before you study quantum mechanics you should study classical mechanics and electrodynamics, so probably you won't be able to understand photons for a little while. I know I don't fully understand them yet and I'm on my second semester of QM. But you can still learn the basics, like that the energy is frequency dependent, and that their behavior is probabilisitic, and all the other stuff they write about photons in intro physics books.

Plus you can watch them shoot through a bottle!

13. Mar 11, 2016

### Karan Punjabi

Yeah I'm understanding it. Will try to grasp more

14. Mar 11, 2016

### Karan Punjabi

I didn't understand what the video is all about?

15. Mar 11, 2016

### Sturk200

They made a camera that is so fast that it can make an image of light in motion. That's all...

16. Mar 11, 2016

Okay...
Great