What does the energy of light depend on? Amplitude of the wave or frequency?

[mahela007]

First of all.. I posted this thread under quantum mechanics because I think it has something to do with Max Planck,s equation.

 

Anyway.. my text book says that the energy of light depends on it's frequency. Now this has me quite puzzled. In a mechanical wave, such as the wave one could make in a piece of string tied at one end to a fixed object, the energy arriving at the fixed end would be dependent on the amplitude of the wave of the string right? So why is this not true for electromagnetic waves?

(I know there are many differences between electromagnetic waves and mechanical waves.. but I just can't figure out what makes the energy of one depend on the frequency and the other depend on amplitude)

[Sisyphus]

Light is not exactly like a classical wave. It is released in discrete packets called photons, that carry energy proportional to their frequency. Increased "amplitude" in normal speech is just a higher number of photons. The difference becomes apparent in things like the photoelectric effect, in which incoming photons cause electrons to be knocked loose. Increasing the frequency of the light increases the energy of the released electrons, and increasing its amplitude only increases the number of released electrons.

 

So in a way they both increase the energy, in the same way that ten thousand tennis balls at 200mph or one cannon ball at 200mph both have more energy than one tennis ball at 200mph.

[mahela007]

so why does Max Planck's equation only mention frequency?

[Klaynos]

so why does Max Planck's equation only mention frequency?

 

E=hf ?

 

Because that is the energy of a single photon.

[mahela007]

thanks for your help

 

yes.. I was talking about E=hf

I don't understand... Don't photons have an amplitude? (I'm very very new to this stuff.. If this was an programming related forum I'd call myself a NOOB.. )

I think I might need a description of what a photon is and why it has a frequency...

[swansont]

thanks for your help

 

yes.. I was talking about E=hf

I don't understand... Don't photons have an amplitude? (I'm very very new to this stuff.. If this was an programming related forum I'd call myself a NOOB.. )

I think I might need a description of what a photon is and why it has a frequency...

 

The E&M fields oscillate at some frequency. However, unlike classical waves, the energy of the system is quantized and it requires no medium in which to propagate. So there is an amplitude of the fields, and there is an intensity which is proportional to the number of these quantized vibrational modes, which we call photons.

[gib65]

thanks for your help

 

yes.. I was talking about E=hf

I don't understand... Don't photons have an amplitude? (I'm very very new to this stuff.. If this was an programming related forum I'd call myself a NOOB.. )

I think I might need a description of what a photon is and why it has a frequency...

 

I'm not sure if this is what swansont said - he's rather cryptic at times (no offense swansont), but from what I understand, you only get amplitude when you have a whole stream of photons - the reason being that amplitude = the number of photons.

 

BTW, I've always wondered how E is said to come only in descrete units when f is a continuous quantity. Couldn't you make E whatever you want by plugging in the right value for f in E = hf? Does the quantization of E assuming that f is fixed (and that we have a constant k standing for the number of photons: E = khf)?

[swansont]

I'm not sure if this is what swansont said - he's rather cryptic at times (no offense swansont), but from what I understand, you only get amplitude when you have a whole stream of photons - the reason being that amplitude = the number of photons.

 

BTW, I've always wondered how E is said to come only in descrete units when f is a continuous quantity. Couldn't you make E whatever you want by plugging in the right value for f in E = hf? Does the quantization of E assuming that f is fixed (and that we have a constant k standing for the number of photons: E = khf)?

 

I was attempting to distinguish between the amplitude of the light signal (intensity) and the amplitude of the electric or magnetic field. "Amplitude" can apply to all three.

 

You're going to see the quantum behavior more when dealing with bound/confined systems — frequency is not a continuous quantity for many of these, e.g. atoms which radiate/absorb at discrete frequencies. For a "photon-in-a-box" the supported cavity modes are discrete as well (standing waves); this tends to a continuous spectrum as the box becomes large. So if your light is coming from an atomic system or a cavity (or both, as in a laser), you have discrete frequencies from which to choose. Light from a blackbody is going to give you a continuous spectrum.

[mahela007]

thanks for the replies... By the way.. when electron jump down to lower energy levels in the atom, do they always liberate only one photon?

[swansont]

thanks for the replies... By the way.. when electron jump down to lower energy levels in the atom, do they always liberate only one photon?

 

Sometimes you can get 2, but only under some circumstances. The 2S-1S transition in Hydrogen for example. Since the angular momentum is unchanged in the atomic states, you can't emit just one photon. Since there is no other decay channel open, you can see two-photon decays (each with opposite spins, so they add to zero), but the lifetime will be much longer than for other excited states because the transition probability is much lower. For a system that can decay with one photon, or a different way with two, the one-photon transitions will be the dominant decay.

[galen]

To return to the thread. Surely amplitude must be restricted to the magnitude of the peak values of the electric and magnetic fields. Intensity is descriptive of the numbers of photons per second.

The amplitude of a photon wave depends on the medium of transmission.

A particular energy photon has a range of possible amplitudes dependant only on the medium of transmission.

We have no way to modulate the amplitude of a known energy photon in a single medium. So we can not vary this parameter in a photo-emission experiment.What has been investigated is the photon energy and the intensity.

It is somewhat confusing that in a given medium,increasing the energy of the photon also increases its amplitude.

[Amr Morsi]

It may be helpful here ......... the power of light passing through unit area (with its direction) is equal to E x H; where E is the electric field intensity and H is the magnetic field intensity.

[Radical Edward]

thanks for your help

 

yes.. I was talking about E=hf

I don't understand... Don't photons have an amplitude? (I'm very very new to this stuff.. If this was an programming related forum I'd call myself a NOOB.. )

I think I might need a description of what a photon is and why it has a frequency...

 

they don't have an amplitude. Photons are excitations of the electromagnetic field. I write about this in detail here:

 

http://www.thelightsideofscience.com/2010/02/what-is-photon.html

 

(yes; a shameless blog plug, but I really don't want to have to type all that out again!)

[Farsight]

Cough. I see E=hf has been mentioned. Oddly enough, people don't really think much about action. Take a look at various pictures of the electromagnetic spectrum and then take a look at Planck's constant. Pay careful attention to the Value section where it says this:

 

The dimensions may also be written as momentum multiplied by distance.

 

It's related to displacement current.

[pioneer]

I would say that the energy of photons is within the frequency. Distance is a static variable, while time/frequency is a dynamic variable. The dynamic variable will contain the energy.

 

We can measure distance with a meter stick. We measure time/frequency with a clock. Meter sticks do not require batteries, since one does not need energy to measure a static variable. However, clocks do require an energy source. It takes energy to measure energy.

 

An interesting effect using the energy within frequency is done in photography. It has to do with shutter speed. If the shutter speed or frequency is too low, one will get motion blur within the picture. This is where aspects of the picture look blurry.

 

The difference in frequency between stopping all the motion and the frequency needed for motion blur (energy difference), will create uncertainty in distance (blur). Since time is stopped in the photo, distance becomes a dynamic variable, which can no longer be measured exactly without adding energy. One has to give kinetic energy to the coordinate system, which takes energy, to simulate the uncertainty created by the frequency difference with time stopped.

 

[mannzzu]

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Merged post follows:

Consecutive posts merged

The E&M fields oscillate at some frequency. However, unlike classical waves, the energy of the system is quantized and it requires no medium in which to propagate. So there is an amplitude of the fields, and there is an intensity which is proportional to the number of these quantized vibrational modes, which we call photons.

 

what do you mean by no medium? no particles in medium?

Edited March 30, 2010 by mannzzu
Consecutive posts merged.

[Sisyphus]

 

 

what do you mean by no medium? no particles in medium?

 

The medium is the material that a wave travels through. For example, when you flick the end of a rope, a wave travels down it. The rope is the medium. No part of the rope is itself traveling down itself. Light is a wave that travels through space, that does not require any medium through which to travel.

[mannzzu]

The medium is the material that a wave travels through. For example, when you flick the end of a rope, a wave travels down it. The rope is the medium. No part of the rope is itself traveling down itself. Light is a wave that travels through space, that does not require any medium through which to travel.

what does vacuum(which is considered empty (empty medium)) in this context referring to classical or quantum theory?

acc to classical vacuum means no particle or no mass or energy is present(with no properties)

but acc to quantum as i found in wiki "The vacuum has, implicitly, all of the properties that a particle may have: spin, or polarization in the case of light, energy, and so on. On average, all of these properties cancel out: the vacuum is, after all, "empty" in this sense".

now which case are we considering.