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In this installment on the history of atom theory, physics professor (and my dad) Dean Zollman discusses Albert Einstein’s explanation of why a light’s color matters more than its brightness in increasing kinetic energy from electrons ejected from atoms. Not everyone welcomed Einstein’s idea about the nature of light. – Kim

By Dean Zollman

Dean ZollmanIn this post, I will look at one of Albert Einstein’s (1879-1955) contributions to our understanding of atoms and matter. Of his many contributions to physics and our understanding of nature, this is the only one for which Einstein used the word revolutionary. It was, but so were many of his other ideas.

Before our discussion, it is a good time to mention that November 2015 is the 100th anniversary of Einstein’s publication of his General Theory of Relativity. At the time of its publication, the General Theory was a sophisticated mathematical description of much of the universe and provided a theoretical description of gravity – an academic exercise that improved humankind’s understanding of the universe but seemed to have little or no practical application. However, among other things the General Theory gave us an understanding about how light and radio waves interact with gravity.

Today, the understanding that comes from Einstein’s mathematical description of gravity is a critical component in making our GPSs as accurate as they are. So, the next time you hear complaints about university professors doing research that seems to have no value, think about your GPS “recalculating.” Without research that 100 years ago seemed of interest to only some physicists, the recalculation would not be accurate.

Now, back to earlier in the 20th century. As we discussed last time, Max Planck had addressed the issue of the spectrum of light coming from a so-called blackbody radiator. He had come to the conclusion that light energy came in packets in which the energy was proportional to the frequency of light. He did not have a strong theoretical reason for this conclusion, but it worked.

The blackbody radiator was not a practical device but many common emitters of light approximated it. However, there was another phenomenon – the photoelectric effect – that did not have a good explanation. Einstein would tackle that problem.

UV Rays and Electrons

The photoelectric effect was discovered by Heinrich Hertz (1857-1894), a pioneer in investigating radio waves. Much of the waves he generated were created by causing a spark to jump between two electrodes.

In 1887, Hertz discovered that he could get a stronger spark if he shined ultraviolet light on the electrodes. Later, scientist discovered that this spark was a result of electrons being emitted by the electrodes. Thus, the ultraviolet light was causing the emission of a greater number of electrons than would occur if the UV light were not there. The phenomenon was called the photoelectric effect.

Artist’s conception of Hertz’s discovery of radio waves

An artist’s conception of Hertz’s discovery of radio waves. The device under the window has two small spherical electrodes in its middle. The wavy line between the spheres represent the spark while wavy lines above and below the spark represent the emission of radio waves. Shining UV light on these spheres increased the intensity of the spark. Hertz is holding a receiver which has a spark between two electrodes. This spark occurs when the radio waves reach the receiver (public domain via Wikimedia Commons).

The basic process of the photoelectric effect is shown in the figure below. Shine UV light on certain metals and electrons are emitted. The number and energies of the electrons could be measured and compared with the properties of the light that was shining on the metal.


Modern view of the photoelectric process

A modern view of the photoelectric process. UV light strikes a metal and electrons are emitted from the metal (public domain via Wikimedia Commons).

To investigate the photoelectric effect with a simulation visit PhET. There you can vary the parameters and “see” how the electrons are emitted.

Measurements Don’t Match Hypothesis

James Clerk Maxwell (1831-1879) had established that light was a form of electromagnetic wave. That is, light waves consist of vibrating electric and magnetic fields. As the light wave enters the metal, electrons begin to vibrate in response to these changing electric and magnetic fields. The electrical bonds holding the electrons to their atoms are a little like rubber bands. Small vibrations do not enable the electron to escape – the rubber band always pulls it back. Larger vibrations, however, could eventually stretch the rubber band far enough that it breaks. The electron escapes from the atom with whatever energy is left over after the bond has been broken. This extra energy becomes the kinetic energy of the ejected electron.

James Clerk Maxwell with his wife, Katherine

James Clerk Maxwell with his wife, Katherine, and the family pet (public domain via Wikimedia Commons)

This idea was used to predict what might happen in the photoelectric effect when the intensity (brightness) or frequency (color) of the light source was varied. The intensity of the light source describes the amplitude of the individual waves. Dim light consists of waves with small amplitudes; bright light consists of waves with large amplitudes.

We could imagine metal surfaces for which the amplitude of the waves in dim light is too small to break the electrical bonds holding the electrons. Increasing the intensity of this light should increase the vibration of the electrons, eventually to the point that the electrons break free. Increasing the intensity even further should increase the kinetic energy of the ejected electrons.

On the other hand, the frequency of the light should have little effect on either the ejection of electrons or their kinetic energy. Changing the frequency of the light source simply changes the rate at which wave disturbances reach the metal surface. If the amplitudes of the waves are not sufficient to help the electrons break the bonds, changing the rate at which these waves reach the metal surface should not make much difference. Similarly, changing the frequency of the incident light should not affect the kinetic energy the ejected electrons have.

The problem was that the measurements showed something quite different. The frequency of the light determined the energy of the electrons – the higher the frequency the greater the energy. The amplitude determined the number of electrons that were emitted but had no effect on the energy. The experimenters could shine as bright a red light (low frequency) as they possibly could on a metal and no electrons would be emitted. But shine a very dim violet light on the same metal and electrons would immediately pop out. Maxwell’s wave theory of light could not explain the observations of the photoelectric effect.

Following Planck’s ideas, Albert Einstein proposed that in its interaction with electrons in the photoelectric effect, light could be best described as a stream of small particles, or “energy packets.” Each packet, called a quantum of light or a photon, acts as a unit when it interacts with an electron. Again using Planck’s idea, Einstein suggested that the energy carried by each photon is defined by:

Energy = (Planck’s constant) x (frequency)

Einstein’s concept of the photon provides a solution to the problems posed by the photoelectric effect. Each photon acts as a unit when it interacts with matter. When a photon encounters an electron, it does one of two things: (1) transfers all its energy to the electron and ceases to exist or (2) does nothing and goes on its way. By associating energy with frequency, the photon explains why electrons are ejected by light of one frequency, such as high frequency UV light, but not of another, such as low frequency red light. It also explains why the kinetic energy of the released electrons depends only on the frequency of the incident light.

Albert Einstein in 1905

Albert Einstein in 1905, at the time he proposed a solution to the photoelectric effect (By Lucien Chavan / ETH Zürich, CC BY-SA 3.0, via Wikimedia Commons)

In a letter to Conrad Habicht, Einstein called the photoelectric explanation “Very Revolutionary.”

Acceptance Took Years

Many textbooks, including ours (The Fascination of Physics), imply that once Einstein had published his ideas about connecting Planck’s approach with the photoelectric effect, everything fell into place, and it was quickly accepted by the physics community.

Unfortunately, that did not happen.

Philipp Lenard (1862-1947) proposed some alternative ideas. He saw the effect as a triggering phenomenon. In Lenard’s model, light did not contribute energy to electrons. Instead, the light just selected those electrons which were ready to be emitted by an atom anyway. To continue with the rubber band analogy, the rubber band connecting the nucleus and electron was already stretched. The light just provided the mechanism to release the band and shoot the electrons out. Ideas such as Lenard’s allowed people to hold on to the wave theory of light. So, they ignored or rejected Einstein’s ideas.

The best example to show that it took a long time for Einstein’s idea to be accepted is Planck’s letter supporting Einstein’s nomination to the Prussian Academy of Science. In 1913 (eight years after publication of the photoelectric explanation), Planck wrote, “That [Einstein] may occasionally have missed the mark in his speculations, as for example with his hypothesis of light quanta, ought not be held too much against him …”

Eventually, experiments showed that Einstein was correct. Today, his explanation of the photoelectric effect is well accepted. In 1921, Albert Einstein received the Nobel Prize “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect.”

Further, the work of Rutherford, Einstein, and Planck set the stage for a model of the atom that persists even today. More about that next time.

A political footnote: The major players – Einstein, Hertz and Lenard – became caught up in the insanity of Nazi Germany. Hertz’s family background was Jewish, but his grandparents had converted to Christianity when Hertz’s father was 7 years old. Heinrich Hertz was a Lutheran. However, that did not stop the Nazis from declaring Hertz as undesirable. His portrait was removed from the Hamburg City Hall because of his Jewish background. (It has long since been restored.) Einstein was Jewish and fled Germany to avoid the persecution. Lenard was on the other side. He was a German Nationalist and joined the Nazi party. He championed “German Science” and worked with Hitler to rid the country of “Jewish fallacies” such as Einstein’s theories. Today, it is difficult to understand how an intelligent person could have held such views.

(The paragraphs about the wave model and the photoelectric effect are adapted from The Fascination of Physics, copyright by J.D. Spears and D. Zollman. They are used by permission of the authors.)

Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.


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