Einstein and photovoltaics, the sixth entry in our CleanTechnica miniseries launching the new year continues by celebrating the UN’s 2015 Year of Light. Here, physicist John Perlin, author of Let It Shine: The 6000-Year Story of Solar Energy, reveals that Albert Einstein is the father of modern photovoltaics.
Perlin’s expertise on solar energy is considerable. Let It Shine is the only book that has thoroughly chronicled the development and application of solar throughout time, focusing on key themes, people, and events that have laid the foundation for an enduring Solar Age.
We have reported on Willoughby Smith and his startling experiments proving the photosensitivity of selenium. The year: 1872. It was this discovery that stirred keen interest in two British scientists, William Grylls Adams and Richard Evans Day. (Other previous posts in this miniseries: Author John Perlin Celebrates the Coming Year of Light, Author John Perlin & the Solar Cell, The Pathway to Today’s Solar Revolution: Discovering the Photosensitivity of Selenium, Photovoltaics Discovered in 1875: Interview with Author John Perlin; and Photovoltaic Dreaming: First Attempts at Commercializing PV.)
Our fifth post concerned what happened after news of the photovoltaic effect had reached other scientists, inspiring a Charles Fritts, an American inventor, who built the first solar panels in 1881. Of his work, Fritts reported a current “that is continuous, constant and of considerable force not only by exposure to sunlight but also to dim, diffused daylight.”
In our last episode on English scientist George Minchin, Minchin suggested that the energy of light might differ with wave length and therefore might provide the scientific explanation for photovoltaics. Perlin pointed out that such a viewpoint was nothing short of heresy, as the wave theory of light, a basic precept of 19th century science, required the notion that light’s intensity depended on the incoming amount, not the variation, of its length. Here is our interview with Mr. Perlin:
CleanTechnica: Please elaborate on why Minchin’s concept might be regarded as scientific heresy.
Perlin: A newly discovered phenomenon – light causing materials to shed electrons – called the photoelectric effect – brought into question the wave theory.
Physicist Philip Lenard, for example, discovered in 1902 that increasing the amount of light striking a material did not intensify the energy of the electrons it shed in the process; but by increasing light’s frequency, i.e., decreasing the wave length, it did.
CleanTechnica: Were there other experiments like this?
Perlin: Yes. In another experiment, no matter how much light of long wave lengths poured onto an object no reaction occurred while just the faintest exposure to shorter wave light a photoelectric effect could be measured. These observations did not jibe with wave theory where merely adding more light should have sufficed.
CleanTechnica: How does Albert Einstein become related to such heretical theories?
Perlin: Einstein, among all his contemporaries, came up with the simplest yet the boldest and most revolutionary explanation. In one short but elegant equation, Einstein demonstrated what Minchin had earlier alluded to, that science had hitherto failed to account for all the energy flowing from the sun.
CleanTechnica: Can you share his equation, and elaborate?
Perlin: E=hv, where E is energy, h is Plank’s constant, and v is the frequency of light. In his simple equation, containing only two variables, he showed that light waves carry packets of energy he called light quanta and presently referred to as photons. Einstein went on to show that intensity of light quanta, as Minchin had suspected, vary according to the wave length – the shorter, the more energy they pack.
CleanTechnica: Obviously, Einstein’s work was not initially embraced?
Perlin: Absolutely. Einstein’s colleagues made no rush to embrace his radical revision of light. Seeing light as both waves and energetic particles stunned even his strongest supporters. His closest colleagues charged that he had “missed the target in his speculations [concerning] light quanta.” Like everyone else in the field, they saw it as an attack on one of the pillars that held up the edifice of 19th century science.
CleanTechnica: Did the list of naysayers include the renowned American physicist, Milliken?
Perlin: Yes. Even after proving the validity of Einstein’s light quanta equation in explaining the photoelectric effect, Milliken dismissed the physicality of the photon as “untenable.” In 1923, 18 years after Einstein published his light quanta piece, American physicist, Arthur Hailey Compton, created a collision between a stationary electron and a short wave of light. Just as in billiards, when the cue ball strikes its target, the light wave transferred some of its energy and momentum to the electron. This is exactly what happened in Adams and Day’s experiment: sufficiently energized photons radiating from the candle’s flame struck electrons in the selenium. They transferred energy and momentum to the electrons. Science defines electricity as the movement of energized electrons. After the Compton Experiment, the reality of Einstein’s photon and the photovoltaic effect gained universal acceptance in the scientific community.
In the next episode, we show the role that Einstein’s most famous equation, E = mc2 , explains how the sun generates solar PV’s fuel.