# I Do photons of all energies exist?

1. Dec 6, 2016

### Vibin Narayanan

If a quantum sistem can vibrate only in discreate energies, can there exist photons of all energies (All frequencies in the real number line)?

2. Dec 6, 2016

### DrChinese

The short version is yes, all frequencies are possible. The energy levels of bound electrons are discrete, true, but light can be generated in other ways.

3. Dec 6, 2016

### ZapperZ

Staff Emeritus
I can take a clump of electrons, and shake it real fast up and down. Voila! I've just generated EM radiation/light of a certain frequency. What is to prevent me from having a large range of frequency of light by doing that?

If you think this is not realistic, look up how the various EM radiation are created at synchrotron light source. You need to expand your understanding of how light is created here. It is NOT just simply via atomic transition.

Zz.

4. Dec 6, 2016

### RockyMarciano

Sure, that's like asking how can the real line be made of discrete points.

5. Dec 6, 2016

Is there a maximum number to the wave frequency in this system?

6. Dec 6, 2016

### Staff: Mentor

The harder you shake it, the hotter it is. The hotter it is, the more energetic the emitted electromagnetic radiation, and the higher the frequency.

7. Dec 6, 2016

### Jilang

Would there be a limit to how fast you could shake it?

8. Dec 6, 2016

### bahamagreen

I think the "hotness" would approach infinity as the "shaking" would approach c.

9. Dec 6, 2016

### Vibin Narayanan

I'm aware that accelerating electric charges produce EM waves. But, how is this acceleration linked with frequency of wave produced?

10. Dec 6, 2016

### Staff: Mentor

It's a bit perplexing that you're asking this question in an I-level thread, as you should have seen the answer around the end of your first undergraduate year when you studied introductory E&M..... And if you haven't already studied introductory E&M you would be marking your threads "B" instead of "I".

But even if you haven't looked at classical E&M yet, there is an intuitive way of thinking about the situation. Consider a charged particle that is oscillating, accelerating back and forth. How does the electric field at some distance from the particle change? Coulonb's law, which you will have seen in high school, says that the field is pointing towards the particle, so if the particle is moving back and forth with a particular frequency, the electric field must be swinging back and forth with the same frequency as it tried to keep up. It's analogous to what happens if you place the palm of your hand flat on the surface of a body of water and then accelerate your hand up and down - there will be waves in the water, and of course their frequency will match the frequency of your hand movements.

WARNING WARNING WARNING The paragraph above is a terrible oversimplification, suitable only for helping someone who is not serious about understanding what's really going on. This picture only works for the particular case of a charged particle oscillating back and forth, and only if you are willing to accept all sorts of oversimplifications. To understand what's really going on, you should work through the last chapter or so of Purcell or other first-year E&M textbook.

11. Dec 6, 2016

### Staff: Mentor

You are starting from a mistaken premise. Some quantum systems have discrete energy levels, but some don't. Briefly (and heuristically), bound systems have discrete energy levels and free systems do not.