Absolutely Small, Chapter 4: The Photoelectric Effect and Einsteins Explanation

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. 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.

Using our beach ball analogy, we would expect waves hitting the dock more frequently would result in more beach balls being knocked off the dock compared to the same sized waves hitting the dock less often. Now that we know what physicists thought would happen, let's look at what they actually observed experimentally! When experiments were performed to look at the effect of light amplitude and frequency, the following results were observed:. The kinetic energy of photoelectrons increases with light frequency. The kinetic energy of photoelectrons remains constant as light amplitude increases.

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. The energy of a photon could be calculated using Planck's equation:. The amplitude of the light is then proportional to the number of photons with a given frequency. As the wavelength of a photon increases, what happens to the photon's energy?

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That means that increasing the wavelength decreases the light's frequency. Therefore, as the wavelength of a photon increases, its energy decreases. We can think of the incident light as a stream of photons with an energy determined by the light frequency.

When a photon hits the metal surface, the photon's energy is absorbed by an electron in the metal. The graphic below illustrates the relationship between light frequency and the kinetic energy of ejected electrons. The effects of wave frequency on photoemission. The higher energy blue light ejects electrons with higher kinetic energy compared to the green light. Furthermore, the kinetic energy of the photoelectrons was proportional to the light frequency. Thought it would go a little further.

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Nice history of the science though. More info required here. Get to Know Us. Amazon Web Services Goodreads Shopbop. Not Enabled Word Wise: Enabled Average Customer Review: Einstein said that light is not composed of waves, but rather of photons or quanta of light.

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In the photoelectric effect, a photon acts like a particle rather than a wave. So Einstein said that a beam of light is composed of many photons, each of which is a discrete particle. As discussed in detail later, these are not particles in the classical sense of a particle. As shown in Figure 4. The process is in some sense like the cue ball in a game of pool hitting a stationary ball and sending it across the table. The cue ball hitting the stationary ball transfers energy to it in the form of kinetic energy, that is, energy of motion.

The collision causes the cue ball to give up energy and the target ball to gain energy. A light beam is composed of many photons, but one photon ejects one electron from the metal. In the photoelectric effect, one photon hits one electron and knocks it out of the metal. When the intensity of light is increased, the light beam is composed of more photons. Because one photon hits one electron, increasing the intensity of the light beam does not change the speed of the electron that is ejected.

In pool, the speed of a target ball is determined by how fast the cue ball was moving. Imagine two cue balls were simultaneously shot at the same speed at two different target balls. After being hit, the two target balls would move with the same speed. When more photons of a particular color hit the metal, more electrons come out, but all with the same speed. In contrast to the wave picture, increased intensity does not produce a harder hit on an electron; increased intensity only produces more photons hitting more electrons.

Each photon hits an electron with the same impact whether there are many or few. Therefore, electrons come out with the same speed independent of the intensity. An increase in the intensity of a light beam corresponds to the beam being composed of more photons.

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More photons can hit and eject more electrons, so an increase in intensity results in more electrons flying out of the metal. To explain why changing the color of the light to red longer wavelength, lower energy caused electrons to be ejected with a lower speed, Einstein used a formula first presented by Planck Max Karl Ernst Ludwig Planck, Planck first introduced the idea that energy comes in discreet units, called quanta, while he was explaining another phenomenon involving light, called black body radiation.

When a piece of metal or other material is heated to a high temperature it will glow; it is emitting light. If it is quite hot, it will glow red. An example is the heating element of an electric stove or space heater when turned to high. As its temperature is increased, the color shifts toward blue. This is not only true of a piece of metal but also of stars. Red stars are relatively cool. A yellow star, such as our own sun, is hotter. A blue star is very hot. In , classical physics could not explain the amount of light that came out at each color from a hot object.

The energy steps between these frequencies are called quanta. In his description of black body radiation, Planck postulated that the energy E could only change in discreet steps. The recognition that energy changes in discreet quanta at the atomic level marked the beginning of quantum mechanics. Using this formula, Einstein explained the reason that red light generates slower electrons than blue light.