- Online Text
- Unit Guide (PDF)
Section 8: Photoelectric Effect
At the end of the 19th century, a variety of physical processes couldn't be explained using the science and physics that had been used since the time of Isaac Newton. This includes nearly all of the ways that light and matter interact already discussed in this unit, such as the emission spectra of the atoms. It also includes two others that are discussed in this section: the photoelectric effect and blackbody radiation.
The photoelectric effect is the name given to the process when light shines on an object made out of an element, and then electrons are emitted from that object, usually in the form of electricity. In a classic photoelectric cell, there is a surface coated with a material such as cesium. When light strikes the surface, some of the electrons are ejected from the atoms and travel through to an electrode, where they are collected. The photoelectric effect is one of the principles exploited in many modern broad-ranging technologies—from the digital imaging devices in cameras and smartphones to motion sensors.
Figure 3-15. Photovoltaic Cell
A demonstration of the conversion of light energy to electric energy in photovoltaic cells by a process similar to the photoelectric effect. When light falls on the silicon material in the cell, electrons are ejected from the silicon atoms, which flow to the metal electrode, causing a measurable current in the meter.
© Science Media Group.
In the late 19th century, the photoelectric effect proved a dilemma for scientists. They had observed that when blue or ultraviolet light strikes the photosensitive surface, a current is created, and the brighter the light, the larger the current. But if red light is used, no matter how bright the light, there is no current. However, as shorter and shorter wavelengths of light are used to strike the surface, at one point, a threshold is reached (in the case of cesium, in the yellow part of the visible spectrum) where the current starts to be detected. Why?
The seed to the solution was planted by Max Planck (1858–1947), a German theoretical physicist whose specialty of study was the field of thermodynamics, a topic that is taken up in more detail in Unit 9. In 1894, when he was a professor at the University of Berlin, he was commissioned by the local electric companies to try to find a way to make more efficient light bulbs. Others had studied the radiation coming off of hot objects, and had found, as any blacksmith knows, that the hotter the object, the shorter the average wavelength of light coming off the object.
Until Planck, no one had been able to come up with a theoretical description to quantify this phenomenon, called "blackbody radiation" which is the term for the characteristic continuous spectrum of light that comes from objects at any given temperature. By 1905, it was clear that the existing science, based on classical mechanics predicted an ideal blackbody would radiate infinite power. This clearly doesn't happen; if it did, every time a person turned on a heating coil on a stove, she would get an instantaneous sunburn. Plank found a solution to this anomaly in classical physics, named the "ultraviolet catastrophe." He proposed that if light is emitted not continuously, but in discrete packets of energy called "photons," he could explain blackbody radiation. In particular, Planck calculated that the energy of electromagnetic radiation is emitted in packets that are multiples of a very tiny discrete packet of energy, 6.63 x 10-34 J•s, which is now known as Planck's constant (h).
Figure 3-16. Blacksmith Working Iron at Different Temperatures
Three photographs of iron from a blacksmith shop show how the radiation coming off hot objects varies by temperature. On the left, an object is glowing mainly in the infrared (which we can't see) and in the long wavelengths of red light. In the center, we see a hotter object that is glowing orange. Finally, on the right is an object which is much hotter, and it glows bright yellow. Eventually, if the metal were heated to a high enough temperature, it would look like and be called white hot because it would be giving off a full continuous spectrum of light covering the whole visible range. Like the coil on a stove, these pieces of metal are examples blackbodies. Blackbody radiation is the continuous spectrum of energy that is produced by objects that are related to their temperature.
© Science Media Group.
A common analogy compares quantized energy with the rungs of a ladder. In order to climb the ladder, one must put in enough energy to raise one's foot above the rung. Putting in too little energy will result in no net climb; intermediate levels are not allowed—one can only go up or down in increments of one or more whole rungs. Planck's theory of quantized energy was first presented in 1900 in Berlin at a conference. Planck himself was unsure of the validity of his calculations, but he was proven correct and his work was instrumental for Albert Einstein's (1879–1955) future work on a phenomenon on the photoelectric effect. Planck received the Nobel Prize in 1918.
This discovery—that amounts of energy are quantized, or can only occur in set intervals—revolutionized chemistry and physics, launched the field of quantum mechanics, transformed the classical view of the atom, and led directly to the modern model of the atom which we still use today. Whether or not Planck's discovery led to more efficient light bulbs, his theory of the quantum nature of energy has had huge implications.
Quantum mechanics provides the solution to the problem of the photoelectric effect. In 1905, Albert Einstein connected the dots when he published a paper that linked the quantum energy packets proposed by Planck with the photoelectric effect—in particular, the observation that only light of a certain minimal energy level could eject electrons. If light itself were discrete packets of energy (photons), then Einstein could explain mathematically what was happening. In the end, the take-home message of Einstein's work is this: These photons needed to have enough energy (a low enough wavelength) to create the photoelectric current. The number of photons that strike the surface does not matter if the photons don't have enough energy.