Explain Photoelectric effect .
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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.
Contrary to the predictions, experiments showed that increasing the light frequency increased the kinetic energy of the photoelectrons, and increasing the light amplitude increased the current.
Based on these findings, Einstein proposed that light behaved like a stream of particles called photons with an energy of \text{E}=h\nuE=hν.
The work function, \PhiΦ, is the minimum amount of energy required to induce photoemission of electrons from a metal surface, and the value of \PhiΦ depends on the metal.
The energy of the incident photon must be equal to the sum of the metal's work function and the photoelectron kinetic energy: \text{E}_\text{photon}=\text{KE}_\text{electron}+\PhiE
photon
=KE
electron
+Φ
Introduction: What is the photoelectric effect?
When light shines on a metal, electrons can be ejected from the surface of the metal in a phenomenon known as the photoelectric effect. 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. The prefix, photo-, simply tells us that the electrons have been ejected from a metal surface by incident light.
[Who discovered the photoelectric effect?]
18871887
The photoelectric effect.
The photoelectric effect.
In the photoelectric effect, light waves (red wavy lines) hitting a metal surface cause electrons to be ejected from the metal. Image from Wikimedia Commons, CC BY-SA 3.0.
In this article, we will discuss how 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.
Predictions based on light as a wave
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. This hypothesis was based on the assumption that light traveled purely as a wave through space. (See this article for more information about the basic properties of light.) Scientists also believed that the energy of the light wave was proportional to its brightness, which is related to the wave's amplitude. 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.
Based on the classical description of light as a wave, they made the following predictions:
The kinetic energy of emitted photoelectrons should increase with the light amplitude.
The rate of electron emission, which is proportional to the measured electric current, should increase as the light frequency is increased.
To help us understand why they made these predictions, we can compare a light wave to a water wave. Imagine some beach balls sitting on a dock that extends out into the ocean. The dock represents a metal surface, the beach balls represent electrons, and the ocean waves represent light waves.
If a single large wave were to shake the dock, we would expect the energy from the big wave would send the beach balls flying off the dock with much more kinetic energy compared to a single, small wave. This is also what physicists believed would happen 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.
Explanation:
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