- 3223 - PHOTOELECTRIC EFFECT - how it was discovered? The “work function” of the metal is the minimum amount of energy required to induce photoemission of electrons from a specific metal surface. The energy of the incident photon must be equal to the sum of the work function and the “kinetic energy of a photoelectron“. Your camera does that for you.
---------- 3223 - PHOTOELECTRIC EFFECT - how it was discovered?
- The first experiments on the photoelectric effect led to the idea of light behaving as a particle of energy called a “photon“.
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- Based on the “wave model” of light, physicists predicted that increasing light amplitude would increase the kinetic energy of emitted photoelectrons, while increasing the frequency would increase measured current.
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- Contrary to these predictions, experiments showed that increasing the light frequency increased the kinetic energy of the photoelectrons, and “increasing the light amplitude increased the current“. The opposite of what they predicted.
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- To explain this result Einstein proposed that light behaved like a stream of particles called “photons” with an energy of E = h*f.
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------------------------ “h” is Planck’s constant and “f” is the frequency.
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- The “work function” in this model required a minimum amount of energy required to induce photoemission of electrons from a metal surface, and the value of the work function depended on the metal used in the experiment.
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- The “photoelectric effect“ occurs when light shines on a metal and electrons can be ejected from the surface of the metal.
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- This process is also often referred to as “photoemission“, and the electrons that are ejected from the metal are called photoelectrons. In terms of their behavior and their properties, photoelectrons are no different from other electrons.
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- 19th century physicists attempted (but failed!) to explain the photoelectric effect using classical physics. This ultimately led to the development of the modern description of electromagnetic radiation, which has both wave-like and particle-like properties.
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- To explain the photoelectric effect, 19th-century physicists theorized that the oscillating electric field of the incoming light wave was heating the electrons and causing them to vibrate, eventually freeing them from the metal surface.
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- This hypothesis was based on the assumption that light traveled purely as a wave through space. These scientists also believed that the energy of the light wave was proportional to its “brightness“, which is related to the “wave's amplitude“.
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- In order to test their hypotheses, they performed experiments to look at the effect of light amplitude and frequency on the rate of electron ejection, as well as the kinetic energy of the photoelectrons.
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- Based on the classical description of light as a wave, they made the following predictions:
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------------------- The kinetic energy of emitted photoelectrons should increase with the light amplitude.
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------------------ The rate of electron emission, which is proportional to the measured electric current, should increase as the light frequency is increased.
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------------------- If the light intensity was increased. Light amplitude was expected to be proportional to the light energy, so higher amplitude light was predicted to result in photoelectrons with more kinetic energy.
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------------------- Classical physicists also predicted that increasing the frequency of light waves at a constant amplitude would increase the rate of electrons being ejected, and thus increase the measured electric current.
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- When their intuition failed, “photons” were defined to come up with a “new theory“. When experiments were performed to look at the effect of light amplitude and frequency, the following results were observed:
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---------------------- The kinetic energy of photoelectrons increases with light frequency.
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---------------------- Electric current remains constant as light frequency increases.
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---------------------- Electric current increases with light amplitude.
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---------------------- The kinetic energy of photoelectrons remains constant as light amplitude increases.
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- These results were completely at odds with the predictions based on the classical description of light as a wave! In order to explain what was happening, it turned out that an entirely new model of light was needed. That model was developed by Albert Einstein, who proposed that light sometimes behaved as particles of electromagnetic energy which we now call “photons“.
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- The energy of a photon could be calculated using Planck's equation. According to Planck's equation, the energy of a photon is proportional to the “frequency” of the light, not the amplitude. The amplitude of the light is then proportional to the number of photons with a given frequency.
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Because the light amplitude was kept constant as the light frequency increased, the number of photons being absorbed by the metal remained constant. Thus, the rate at which electrons were ejected from the metal (or the electric current) remained constant as well.
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Higher amplitude light means more photons hitting the metal surface. This results in more electrons ejected over a given time period. As long as the light frequency is greater than the threshold. Increasing the light amplitude will cause the electron current to increase proportionally.
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- Since increasing the light amplitude has no effect on the energy of the incoming photon, the photoelectron kinetic energy remains constant as the light amplitude is increased.
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- In order to eject electrons, we need the energy of the photons to be greater than the work function of the metal. We can use “Planck's equation” to calculate the energy of the photon.
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- Based on the wave model of light, physicists predicted that increasing light amplitude would increase the kinetic energy of emitted photoelectrons, while increasing the frequency would increase measured current.
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- Experiments showed that increasing the light frequency did increase the kinetic energy of the photoelectrons, and increasing the light amplitude increased the current.
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- The “work function of the metal” is the minimum amount of energy required to induce photoemission of electrons from a specific metal surface. The energy of the incident photon must be equal to the sum of the work function and the kinetic energy of a photoelectron.
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- And, there you have it, that is the photoelectric effect!
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- July 15, 2021 PHOTOELECTRIC EFFECT 3223
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--------------------- --- Wednesday, July 21, 2021 ---------------------------
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