Wave Model Question

$E=hv$

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Wave Model Question

Hi, I was confused about the wave model versus the model that we are usually shown in the photoelectric experiment. I'm having trouble understanding why increasing the intensity of the light doesn't affect the energy of the photons and how we know that this model isn't a wave model. What is the difference between this model used in the photoelectric experiment and a wave model?

Crystal Yu 1D
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Re: Wave Model Question

There are two models used in the lecture. One is the classical model (wave type model) and the other is the quantum model (particle type model).
Light doesn't act ONLY as a particle or ONLY as a wave, but it has the properties of both and the way we model light depends on the scale of the behavior.
(Classic mechanics is used to explain the behavior of LARGE objects, whereas quantum mechanics is used to describe the behavior of SMALL objects.)

I think in the lecture, Professor Lavelle used the example of a watering can where the water that comes out is seen as one continuous unit (classical mechanics), but as you decrease the size of the holes in the nozzle of the watering can, the water that comes out also comes out in smaller and smaller streams. When the holes have been adjusted to a size so small that ONLY ONE DISCRETE molecule of water can come out of the can, we need to shift the perspective from classical mechanics to quantum mechanics.

Light is very similar to this example because it can be observed in terms of wave models, but also in terms of particle models (like in the photoelectric effect experiment where light is absorbed/emitted in discrete units/quanta).

For the particle type model, the intensity of light corresponds to the number of photons of light. On the other hand, for the wave type model, the intensity of light corresponds to the amplitude of the waves. I hope this helps! :)
Last edited by Crystal Yu 1D on Wed Oct 14, 2020 2:10 pm, edited 2 times in total.

Lucy_Balish_3G
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Re: Wave Model Question

The wave model predicts that the amplitude of the wave (the intensity) determines how much energy the wave carries. But this classical model does not apply to light. It can explain its wavelike appearance properties but does not accurately explain its energy properties. This is because the photoelectric effect experiment showed that no matter how much the intensity is increased, if the light did not have enough energy, no electrons would be ejected from the metal. In other words, making the light brighter will not help; instead, you have to find a higher energy and higher frequency light source. The fundamental difference lies in changing the frequency, not the amplitude.

Wil Chai 3D
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Re: Wave Model Question

It could help to think about it in terms of the equation E(light/photon) = hv. In order to eject an electron, E(light/photon) must be high enough to meet the threshold Energy. E = hv tells us that the energy of the light is dependent on the frequency v of the photon, not the amplitude.
Last edited by Wil Chai 3D on Wed Oct 14, 2020 2:10 pm, edited 1 time in total.

Sara_Lim_2C
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Re: Wave Model Question

Disclaimer I'm still figuring this out too, but I think the reason we know it's not the wave model is because in theory, if light did follow the wave model, increasing the amplitude of the wave should increase the energy of the photons. However, when the intensity of the light was increased (increasing the amplitude), there still wasn't enough energy to eject electrons off the metal. So we see that the wave model doesn't quite model how light works in physical practice. Increasing the intensity of the light doesn't increase the energy of the photons, but only makes more photons. But those photons don't have enough energy to remove an electron, so it doesn't matter that there are more of them.

However, once what we call wavelength and frequency (assuming the wave model) of the light was changed (changing to a UV light, for example), the electrons were being ejected off of the metal. What we called increasing the frequency and changing the wavelength was actually just making increasing the energy in the individual photons, which now have enough energy to remove electrons. I think the main thing to know is separating the wave model from the concept of photons.

I don't really know if that answered your question but I hope that sort of helps! And if I'm wrong please correct me!

Rob Tsai 2F
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Re: Wave Model Question

This is a pretty solid question. To put it simply, the classical (wave) model depicts light in a continuous fashion (ie the watering can pouring water analogy), while the quantum (particle) model depicts light acting in discrete units. In the photoelectric experiment, we found that increasing the intensity of light didn't remove electrons out of a metal. According to the wave model, increasing the intensity of light increased the amplitude of the wave which was supposed to have increased the energy the wave carried. However, the experiment contradicted this theory, so scientists created the quantum model as an explanation. Intensity of the light was found only to increase the number of photons, not the energy of the photon itself. No matter how many runners tried to jump the fence, if none had the jumping ability to clear it, then it didn't matter if there were two or twenty runners. Hope this helps!