The Photoelectric effect and classical physics

In summary: If classical waves have a certain energy, then why doesn't the kinetic energy of the electrons depend on the intensity of the light?
  • #1
erty
26
0
The Photoelectric effect and "classical physics"

The kinetic energy of the ejected electrons predicted by the classical physics should be related to the intensity of the light.
According to experimental results, the kinetic energy of the electrons is proportional to the frequencies of the light, and not the intensity.

What is "classical physics" in this case, and which equations do I use in order to calculate this?
Why is the kinetic energy of the electrons depended on the intensity (according to "classical physics"), and not the frequency?
 
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  • #2
erty said:
The kinetic energy of the ejected electrons predicted by the classical physics should be related to the intensity of the light.
According to experimental results, the kinetic energy of the electrons is proportional to the frequencies of the light, and not the intensity.

What is "classical physics" in this case, and which equations do I use in order to calculate this?
Why is the kinetic energy of the electrons depended on the intensity (according to "classical physics"), and not the frequency?

Look at the energy of a typical classical wave. What characteristic of that wave do you use to calculate its energy?

Still confused? Look at a mass-spring system. How would the energy change if you change the amplitude of oscillation?

Zz.
 
  • #3
ZapperZ said:
Look at the energy of a typical classical wave. What characteristic of that wave do you use to calculate its energy?

Still confused? Look at a mass-spring system. How would the energy change if you change the amplitude of oscillation?

Zz.

Hmm, I don't know anyting about it. Could you list a couple of words, then I'll try to look them up or google them
 
  • #4
Simple Harmonic Motion. You will find all the information you need pertaining to energy, intensity, amplitude, power, etc.
 
  • #5
Classical physics refers more to the concept of light the Young proposed--that it is a wave and can be thought of much like ripples coming from water (the ripples being the spread of light). Einstein however explained that light should also be thought of as a bunch of particles. So in the ripple (classical) approach, if the ripples are bigger (that is, the light is brighter) then it should hit the metal harder and therefore electrons should eject with more energy. Because experiments disproved this, Einstein came up with the theory that light is a bunch of particles that also have wave properties which are called photons. Now the energy of each individual photon is proportional to its frequency. That explains half of it. The reason that brighter light does not eject electrons is because all that is is more photons (not stronger ones) . Think of shooting a billion ping pong balls at the Great Wall of China--not going to do too much.
 
  • #6
erty said:
Hmm, I don't know anyting about it. Could you list a couple of words, then I'll try to look them up or google them

What exactly don't you know? You have learned about classical waves, haven't you? Figure out how energy in a wave is determined.

Zz.
 
  • #7
Ja4Coltrane said:
Classical physics refers more to the concept of light the Young proposed--that it is a wave and can be thought of much like ripples coming from water (the ripples being the spread of light). Einstein however explained that light should also be thought of as a bunch of particles. So in the ripple (classical) approach, if the ripples are bigger (that is, the light is brighter) then it should hit the metal harder and therefore electrons should eject with more energy. Because experiments disproved this, Einstein came up with the theory that light is a bunch of particles that also have wave properties which are called photons. Now the energy of each individual photon is proportional to its frequency. That explains half of it. The reason that brighter light does not eject electrons is because all that is is more photons (not stronger ones) . Think of shooting a billion ping pong balls at the Great Wall of China--not going to do too much.

Very good explanation. I'll dig into the physics stuff now that turdferguson and ZapperZ gave me some words to look up.

ZapperZ said:
What exactly don't you know? You have learned about classical waves, haven't you? Figure out how energy in a wave is determined.
Actually... no. I'm still in high school (or rather, the European equivalent) and this is my first year with physics. We're learning about waves at the moment, but this is _very_ fundamental (and probably simplified to the umpteenth power). I'm doing a assignment on the wave/particle duality (still on a very low level).
But there is plenty to write about.
 
  • #8
Why can't waves have discrete values (E = hf applies to particles?)?
 
  • #9
What makes you think that the energy distribution of photons is discrete?
 
  • #10
Hootenanny said:
What makes you think that the energy distribution of photons is discrete?

I figured it out.

Thanks, everybody!
 

What is the photoelectric effect?

The photoelectric effect is a phenomenon in which electrons are emitted from a material when it is exposed to light. This was first observed by Heinrich Hertz in 1887 and later explained by Albert Einstein in 1905.

What is the difference between classical and quantum physics?

Classical physics is the study of the behavior of matter and energy at the macroscopic level, while quantum physics deals with the behavior of matter and energy at the microscopic level. Classical physics follows the laws of Newtonian mechanics, while quantum physics follows the laws of quantum mechanics.

How does the photoelectric effect support quantum physics?

The photoelectric effect provided evidence for the particle nature of light, which was a key concept in quantum physics. It showed that light behaves as both a wave and a particle, and that the energy of each photon is directly proportional to its frequency, rather than its intensity as predicted by classical physics.

What is the work function in the photoelectric effect?

The work function is the minimum amount of energy required to remove an electron from a material. It is a characteristic property of a material and depends on the type of material and its surface properties. In the photoelectric effect, the energy of the incident photons must be greater than the work function in order to emit electrons.

What is the significance of the photoelectric effect?

The photoelectric effect has had a significant impact on our understanding of the behavior of light and matter. It provided evidence for the existence of photons and led to the development of quantum mechanics. It also has practical applications, such as in solar cells and photomultiplier tubes.

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