# How light waves explain Photo-electric effect (not particles).

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Apparently the photo-electric effect is used as a means to help prove that light exibits characteristics of particles.

Wouldn't it make more sense that waves cause this effect?

If a wave is merely a disturbance to the electromagnetic field, then based upon how strong the wave is determines if the electrons get knocked off right?

Consider the field as a type of material, and a certain strength makes it feel more or less abbrassive (also explaining the more coarse appearance of the "photon clusters"), this would suggest it is not particles causing it, but that it is actually waves.

Let me know how I am wrong, if I am.

Edited by prominent
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The obvious trouble with thinking of light as just waves in this situation is the experimental fact that the energy of the emitted electrons does not depend on the intensity of the incoming light, but rather just the frequency.

Explaing this leads to the particle like nature of light.

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I don't see how that is the case because the frequency is the aspect related to the disturbance of the field (the actual shaking/moving of the field). Think of a rope being shook, and this shows that whether the rope is thick or thin, you can adjust the frequency to make it either rub slower or quicker. The intensity of the wave isn't forced forwards because the electromagnetic field isn't being moved forwards, just disturbed as a wave that translates the shape of the disturbance through so that things get shook at the end like they did at the beginning. So regardless of the intensity of the wave (the thickness/thinness), if the electromagnetic field is being disturbed and shook, it will move the electrons more when the frequency is raised to therefor add more disturbance to them, therefor causing them to get knocked off. In this way you don't have to explain light as a particle.

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I don't see how that is the case because the frequency is the aspect related to the disturbance of the field (the actual shaking/moving of the field). Think of a rope being shook, and this shows that whether the rope is thick or thin, you can adjust the frequency to make it either rub slower or quicker. The intensity of the wave isn't forced forwards because the electromagnetic field isn't being moved forwards, just disturbed as a wave that translates the shape of the disturbance through so that things get shook at the end like they did at the beginning. So regardless of the intensity of the wave (the thickness/thinness), if the electromagnetic field is being disturbed and shook, it will move the electrons more when the frequency is raised to therefor add more disturbance to them, therefor causing them to get knocked off. In this way you don't have to explain light as a particle.

But as you make the field stronger, you have more energy. Why doesn't the light interact with atoms, except at specific frequencies? That's not wave behavior.

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But as you make the field stronger, you have more energy. Why doesn't the light interact with atoms, except at specific frequencies? That's not wave behavior.

Exactly.

When photon in absorbed by particle, particle is accelerated. Photon is giving it, its kinetic energy.

And reverse, if photon is emitted by particle, it's slowed down.

If absorbed photon has too small energy to eject it, electron is simply going to higher orbit around nucleus. And we see this as spectral lines in element spectrum.

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I don't understand why that has to be considered particle-like. The way I see it, light acts like a pressure against space, and the disturbance is the tension/vibration of this pressure. Just because you have a force against an electron doesn't mean it'll move unless there is a disturbance to the space allowing it to become displaced. If the frequency isn't high enough then it won't escape regardless of the pressure.

In the same fashion, if I press my foot down on some sand, the harder I push down doesn't determine how far my foot sinks into the sand. I have to shift my foot around to displace the sand to get my foot further down. Note that the foot isn't being thrown into the sand, because like a wave, only the distortion gets propagated.

If you look at light functioning this way, then it doesn't make sense to explain photoelectric effect with particles.

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I don't understand why that has to be considered particle-like.

Imagine we have light source with radius 1m emitting 1 million photons.

It has sphere area = 4*PI*r^2 = 4*PI*1=~12.56 m^2

1 mln photons/12.56 = ~79,618 photons per m^2 of sphere in the all directions.

We can measure that 2 meter from the center we will have

4*PI*2^2=~50.26 m^2 sphere area

1 mln photons / 50.26 = ~19,894 photons per m^2 at distance 2m from center.

4x less.

4 meters from center:

4*PI*4^2=~201.06 m^2 sphere area

1 mln photons / 201.06 = ~4974 photons per m^2
16x less.
And so on so on, with higher distances.
Each of these photons have exactly the same energy/frequency/wavelength as it had at source.
Just their quantity per area unit dropped with distance.
Repeat this thought experiment with waves on water:
If you drop stone to water, you will see waves in the all directions, and amplitude constantly going lower and lower with distance from center and suddenly disappearing.
Wave on water is continuous.
If photons would be just waves, we could not see stars million or billion light years from here. Amplitude of wave-only light would be ~0.
Edited by Przemyslaw.Gruchala
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Atoms don't absorb their quantum of energy from waves. If they did, then it would not depend on the frequency of the light.

For example:

We know a particlular transition requires 1eV of energy, which corresponds to a 1.24 micron wavelength for light. If we send in light with a wavelength of 1 micron, we have more than enough energy. But regardless of the intensity, the atom isn't going to absorb the light. If we send in light with 1 eV of total energy at 1.24 microns, all of the light will be absorbed.

Waves don't behave like that.

Each of these photons have exactly the same energy/frequency/wavelength as it had at source.

Just their quantity per area unit dropped with distance.

Repeat this thought experiment with waves on water:

If you drop stone to water, you will see waves in the all directions, and amplitude constantly going lower and lower with distance from center and suddenly disappearing.

Wave on water is continuous.

If photons would be just waves, we could not see stars million or billion light years from here. Amplitude of wave-only light would be ~0.

I flight were purely waves, the intensity drop-off would be exactly the same until you reach the point where you either have a photon or didn't. Waves intensity would not "suddenly disappear". Until you get to the point where you are detecting single photons, this is not a good example.
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If photons would be just waves, we could not see stars million or

billion light years from here. Amplitude of wave-only light would be ~0.

How is that proof that a light wave dissipates over distance?

When you throw a rock into water, you're only throwing one. If you kept throwing rocks at key intervals so that pressure directed is constant, you'd eventually get the waves to be continuous because the whole body of water would be oscilating together. This is similar to how a small vibration on a bridge can build until the whole bridge is oscilating.

Edited by prominent
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ahh, i remember when myth busters did the frequency oscillation thought, like the bridge example above.

if i remember correctly they said it's a myth..

but watching myth busters,IMO , is like calling an apple an orange in most cases.

I find it hard to take seriously on most cases of such.

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Atoms don't absorb their quantum of energy from waves. If they did, then it would not depend on the frequency of the light.

For example:

We know a particlular transition requires 1eV of energy, which corresponds to a 1.24 micron wavelength for light. If we send in light with a wavelength of 1 micron, we have more than enough energy. But regardless of the intensity, the atom isn't going to absorb the light. If we send in light with 1 eV of total energy at 1.24 microns, all of the light will be absorbed.

Waves don't behave like that.

I flight were purely waves, the intensity drop-off would be exactly the same until you reach the point where you either have a photon or didn't. Waves intensity would not "suddenly disappear". Until you get to the point where you are detecting single photons, this is not a good example.

I still don't see the reason for having to assume it is a particle. The intensity of the wave wouldn't enter the atom if there is not enough displacement occuring to allow the translation. See, if the displacement isn't happening frequently enough, then no translation can occur. It's like having a heavy object sit ontop of sand inside a jar, it won't sink to the bottom until you begin to shake it with enough frequency.

Another way to put it is that the lower frequency light propagates more directly, yet with less distortion to space, hence when "traveling" through a medium can propagate more quickly in the direction it is going. The higher frequency light creates more distortion to space and thus in a medium its direction becomes more bent.

You can see with this then that the higher frequency is bending more of space, causing the translation of energy to occur.

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It's like having a heavy object sit ontop of sand inside a jar, it won't sink to the bottom until you begin to shake it with enough frequency.

That's completely different story..

By shaking you're making space - matter-sand cannot overlap taking the same space as other.

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That's completely different story..

By shaking you're making space - matter-sand cannot overlap taking the same space as other.

You're not making space by shaking it, you're displacing the sand and allowing the space that is already there to be reorganized. It's basically a distortion to the sand "field".

Edited by prominent
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I still don't see the reason for having to assume it is a particle. The intensity of the wave wouldn't enter the atom if there is not enough displacement occuring to allow the translation. See, if the displacement isn't happening frequently enough, then no translation can occur. It's like having a heavy object sit ontop of sand inside a jar, it won't sink to the bottom until you begin to shake it with enough frequency.

I don't think that analogy works, because I'm shaking at at an even higher frequency and the system doesn't absorb the energy. Further, this becomes a model of the atom, not of light.

The bottom line is that the energy of the absorbed light is quantized, as well as the angular momentum, and it's been shown experimentally that the energy is frequency dependent. You need to build a mathematical model of waves that accounts for this, if you want to treat the system as purely a wave. Analogies don't take you very far.

Another way to put it is that the lower frequency light propagates more directly, yet with less distortion to space, hence when "traveling" through a medium can propagate more quickly in the direction it is going. The higher frequency light creates more distortion to space and thus in a medium its direction becomes more bent.

You can see with this then that the higher frequency is bending more of space, causing the translation of energy to occur.

How could one measure this "distortion of space"? Plus, we already know that the speed of propagation in a vacuum does not depend on frequency.

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I still don't see the reason for having to assume it is a particle.

1 micron wavelength photon has E=h * 3*10^8 / 1*10^-6 = h * 3*10^14

1.24 micron photon has E=h * 3*10^8 / 1.24*10^-6 = h * 2.42*10^14

1 micron wavelength photon is not causing effect, even though it has 24% more energy than 1.24 micron wavelength photon.

Another way to put it is that the lower frequency light propagates more directly, yet with less distortion to space, hence when "traveling" through a medium can propagate more quickly in the direction it is going. The higher frequency light creates more distortion to space and thus in a medium its direction becomes more bent.

You can see with this then that the higher frequency is bending more of space, causing the translation of energy to occur.

Frequency must match. Not just being higher.

Edited by Przemyslaw.Gruchala

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