# Wave theory of light

1. Jan 27, 2007

### kkmans

For wave theory of light, it said that energy of light is depend only on its brightness and independent of its frequency.
I would like to ask which equation shows that wave energy is independent of frequency..??

thank you.

2. Jan 27, 2007

### ZapperZ

Staff Emeritus
You should cite this source. That is the one thing we try to make people do when they talk about something they read or heard.

Actually, it DOES! Maybe the energy PER CYCLE or per period isn't dependent on frequency in classical wave theory of light, but energy in a unit time does! Think about it. The higher the frequency, the more cycles in that unit time that you measure, the more energy you will get. So yes, even in classical wave theory of light, frequency does matter IF you are looking at it per unit time.

Zz.

3. Jan 27, 2007

### americanforest

Then energy of light actually is dependant on frequency by the relation $$E=hf$$ where h is Planck's constant.

4. Jan 27, 2007

### ranger

That can be restated:

The energy of a photon is dependant on the frequency. I think its incorrect to say the energy of light with respect to that equation.

5. Jan 27, 2007

### americanforest

But the light is made up of photons. The only way this dependancy couldn't be extended to light frequencies in general is if there were less photons in high frequency light than in low frequency light. Is this true?

6. Jan 27, 2007

### Ahmed Ismail

the equation of light intensity

the equation of average intensity
I = .5 nceεo sqr(E)
n refractive index of medium
c velocity of light
εo permativity of medium
E max value of Electric filed
as we know that light is an electromagnetic wave has 2 perpendicular fields electric and magnetic

7. Jan 27, 2007

### ranger

It doesnt mean less or more photons, it means photons with lower/higher energy levels.

8. Jan 27, 2007

### Sojourner01

ranger, that wasn't what he said. He's saying that the only way kkmans' scenario would work is if the numbers of photons were intrinsically related to the frequency of the light 'beam'/ray/whatever. This is quite obviously not true.

9. Jan 27, 2007

### ranger

Sojourner01, I thought thats what I said in my previous reply.

10. Jan 27, 2007

### Sojourner01

You said exactly opposite that:

11. Jan 27, 2007

### cesiumfrog

Does one not measure brightness/intensity in units of power per area?

The intensity is proportional to the square of the amplitude, not dependent on the wave frequency.

When one later discovers light quantised in packets of energy proportional to frequency, one must infer (to avoid contradiction) that for a red and blue light source of equal measured brightness there will be a higher rate of photons received from the lower frequency source.

12. Jan 27, 2007

### americanforest

This is what I was trying to say in my earlier post, I'm sorry if I wasn't clear.

13. Jan 28, 2007

### kkmans

>You should cite this source. That is the one thing we try to make people do when they talk about something they read or heard.
oops, I read this from wikipedia.

let's forget about the light, what about other waves like sound and water ?
If we consider wave as the particles performing SHM, their energy is depends on the frequency.
Is there anything wrong with my concept..??

thank you.

14. Jan 28, 2007

### ZapperZ

Staff Emeritus
Why did you quoted one part of my response while ignore the rest that would have answered your question here?

Zz.

15. Jan 28, 2007

### kkmans

hmm..I understand if I consider it per unit time, the intensity is directly proportional to the energy, not the frequecy.
My problem is that is the expression of "energy" include frequency ?
Like other waves, the particles are performing SHM, and their energy is proportional to the square of the frequceny.

16. Jan 28, 2007

### Sojourner01

No. No no no. The energy of a photon is directly proportional to its frequency. Not the square. Linear dependence. No question about it.

Review your ideas of what 'intensity' is. I sense some confusion of concepts creeping in here.

17. Jan 28, 2007

### cesiumfrog

Do you have a reference to support that?

18. Jan 28, 2007

### kkmans

>energy is proportional to the square of the frequceny
If we consider waves as particle performing SHM, then their energy
= 1/2m ω^2 A^2, where ω = 2πf

Can I use this concept in wave theory of light ?

19. Jan 28, 2007

### ZapperZ

Staff Emeritus
No I don't. Rather, look at a mass-spring system. Find the energy per second of the system at a particular amplitude. Now replace the spring so that you have a system that doubles the natural frequency. Now measure again the amount of energy of the system per second with the SAME amplitude. Are you telling me you need a "reference" to figure out that the amount of energy in that unit time has increased?

Zz.

Last edited: Jan 28, 2007
20. Jan 28, 2007

### swain1

The thing I consider confusing in this thread is the way frequency is being thought of. It simply is a number of an event in an amount of time. In this case we are talking about waves. Therefore, the number of oscillations per unit time, be it light waves or mechanical waves the same principals apply. The relation to energy in the quantum mechanical formula E=hf i think is being miss interpreted. Here the frequency is just a multiplier where as Planck's constant is a number related to the most discrete quantisation of energy. Remember where this formula was derived from.