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The Photoelectric Effect: Definition, History, Application & Equation

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  • 0:00 What is the…
  • 0:25 History of the…
  • 1:40 How Einstein Explained It
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson, you will learn what the photoelectric effect is, how it was discovered, how it applies to everyday life, and the equation associated with it. A short quiz will follow.

What Is the Photoelectric Effect?

The photoelectric effect occurs when electrons are released after light is shone onto a metal. Electrons released in this way are called photoelectrons, which leads to the name photoelectric effect. According to classical physics, this happens because energy is transferred to the electrons from the light. This theory turned out to be true, although the way it happens is quite different than we originally expected.

History of the Photoelectric Effect

Heinrich Hertz was the first to observe and publish observations about the photoelectric effect in 1887. He employed a spark gap, with an ultraviolet light source. He realized that if the light was partially blocked, sparks didn't occur as often. This turned out to be because electrons were being given energy by the ultraviolet light, which released them from their atoms and allowed them to jump across the gap. Sparks, and electricity in general, are caused by the movement of electrons.

J. J. Thomson studied this further using cathode ray tubes. He found that the particles released in the photoelectric effect were the same ones observed previously in so called 'cathode rays.' At the time these were called corpuscles, but later were named electrons.

Until this point, the observations were not especially difficult to explain. But in 1902, Philipp Lenard noticed that the energy electrons produced increased as the frequency, or color, of the light increased. This was very unexpected.

Something called Maxwell's wave theory of light said that brighter (higher intensity) light would lead to higher energy electrons. But this isn't what Lenard observed. Only a higher frequency light produced higher-energy electrons. A brighter light only produced more numerous electrons.

How Einstein Explained It

Albert Einstein explained this problem in 1905. He realized that light must contain packets, or quanta, which are quantities, of energy called photons. He also noted that the amount of energy of those packets depended on the frequency, or color, of the light by the following equation:

E = hf where h = 6.63 X 10^-34

In this equation, E is the energy in the packet in Joules, h is Planck's constant (just a number), and f is the frequency of the light in Hertz.

Based on this equation, low frequency light has packets that have less energy. So, if you shine a light of low enough frequency, the electrons can't get enough energy to escape from the metal. And as you increase the frequency above the minimum level needed (called the 'threshold frequency'), the electrons leave the metal with more and more energy (they leave at a faster speed).

The brightness, or intensity, of the light only determines how many packets there are, and therefore how many electrons are able to escape, NOT how much energy those electrons have. This fit perfectly with the observations made by Lenard and others.

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