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Doubt on the predictions on the photoelectric effect according to the wave theory of classical physics

Physics Asked by Lumbini A Tambat on June 15, 2021

I read in some texts that classical physics predicted the following in the photoelectric effect,

  • KE of electrons ejected is directly proportional to intensity of
    light
  • Increasing the frequency would increase measured current.
    this is the text I am referring to

My question’s straight, why was it that they predicted such an effect of varying frequency for the Photoelectric effect? And also give an example of the effect of frequency on an experiment such as jerking a rope with a high frequency and transmitting the energy onto a beach ball, which is based on the wave-physics of the classical physics.

2 Answers

Presumably you know that experiments demonstrated the falsehood of the Classical model.

The model was based on the simple idea that the more energy you hit the electrode with, the more energy it would give each electron. Light was treated as a wave of such energy.

In that model, it follows that greater wave amplitude (light intensity) would impart more energy to everything it hit.

It is less obvious that a shorter wavelength carries greater energy than a longer wavelength of the same amplitude. It has to do with the rate of change of the wave form (the slope of the curve when you draw it), and is true in both the classical and quantum models.

However I am unclear why shorter waves should be expected to increase the current, as that is the number of electrons not their energy. Perhaps it is because there are more peaks per second, which would supposedly therefore knock more electrons out.

Of course, we all know that experimental observations gave the lie to all that. Einstein explained it as energy thresholds, by treating the light as discrete particles or packets, and in doing so co-founded quantum physics and earned himself a Nobel prize.

Correct answer by Guy Inchbald on June 15, 2021

Beach ball A and beach ball B are in a depression on sand. Ropes are connected to the balls. A low frequency is sent through the rope to ball A. A high frequency is sent through the rope to ball B.

Ball A sways back and forth, then leaves the depression. The kinetic energy of ball A is random value between zero and the energy of the last wave on the rope.

Ball B sways back and forth, then leaves the depression. The kinetic energy of ball B is random value between zero and the energy of the last wave on the rope.

The energy of the aforementioned last wave is proportional to the amplitude and the length of the wave. Or was it amplitude squared? Anyway, we have decided that the amplitudes are the same, right? So the maximum kinetic energy of ball A is 3 times larger if the last wave absorbed by ball A is three times longer than the last wave absorbed by ball B.

Here "last wave" means the wave-crest that causes the ball to leave the depression.

Energy available for dislodging balls = Total available energy - kinetic energy of free balls

Answered by stuffu on June 15, 2021

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