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Every solar panel turns light directly into electric current. One of the first people to record that effect was a French physicist working with metal plates in a liquid in 1839. This post covers what Edmond Becquerel did, using his own reports to the French Academy of Sciences, works through why the colour of light mattered, and ends with what a student can learn from his careful way of separating causes.
Who was Edmond Becquerel?
He came from a family of physicists. The Nobel Prize biography of his son Henri describes the father, Alexandre Edmond Becquerel, as a professor of applied physics who had done research on solar radiation and on phosphorescence. Edmond's father, Antoine César, had been a Fellow of the Royal Society. The American Physical Society's history of the solar cell records that in 1839 Edmond, working with metal electrodes in an electrolyte solution, noticed small electric currents when the metals were exposed to light, though he could not explain the effect.
Henri later discovered radioactivity, a story told in our post on Henri Becquerel. This post is about the father's work, half a century earlier.
What did he do in 1839?
His reports are printed in the Academy's Comptes rendus for 1839, volume 9, which you can read on the Internet Archive. At the session of 29 July 1839 he described using electric currents to detect chemical reactions caused by sunlight where two liquids met. He dipped two platinum plates, one in each liquid, and connected them to a very sensitive galvanometer.
He then noticed that the plates themselves responded to light. In a memoir read on 4 November 1839, "on the electrical effects produced under the influence of the sun's rays", he set out to separate that effect. He used a wooden box blackened inside and divided into two compartments by a thin membrane, with a platinum plate in each, and uncovered one side at a time.
Was it light or heat?
This was the key question, and his test was neat. A yellow glass screen let a great deal of heat through, yet it completely stopped the effect on the platinum plates. When he spread the sun's spectrum across a plate, he got a measurable current only in the violet or blue rays. He concluded that the cause was not heat but rays that accompany the most refrangible part of the light, the blue and violet end.
He then coated platinum plates with silver chloride and silver bromide, each 4 square centimetres in area. In diffuse daylight the needle moved 15 degrees with the chloride and 26 degrees with the bromide; a direct sunbeam on the bromide sent it to 55 degrees. Silver chloride kept giving a steady current, still detectable after two hours in sunlight, while the bromide lost most of its effect within ten minutes. With chloride, red, orange and yellow light gave no current, green only a trace, and the strongest effects came from violet and from rays beyond the violet.
Silver chloride turning grey in sunlight is the same reaction in the NCERT Class 10 chapter on chemical reactions: 2AgCl → 2Ag + Cl₂.
Worked example: why did violet work and red not?
Becquerel had no explanation. Today we use the photon picture in the NCERT Class 12 chapter on dual nature of radiation: light comes in photons of energy E = hc/λ, and this energy does not depend on how bright the light is. Use h = 6.626 × 10^−34 J s, c = 3.00 × 10^8 m/s and 1 eV = 1.602 × 10^−19 J.
Red light, λ = 700 nm = 7.00 × 10^−7 m: E = (6.626 × 10^−34 × 3.00 × 10^8) ÷ 7.00 × 10^−7 = 2.84 × 10^−19 J = 1.77 eV.
Violet light, λ = 400 nm: E = 4.97 × 10^−19 J = 3.10 eV.
Ratio: 3.10 ÷ 1.77 = 1.75. Each violet photon carries 75% more energy than a red one.
If the reaction on the plate needs more energy per photon than red light provides, no amount of red light will drive it, while dim violet light will. That fits Becquerel's spectrum readings.
Common mistakes:
Thinking brighter light means more energetic photons. Brightness changes the number of photons, not the energy of each.
Mixing up nanometres and metres. 400 nm = 4.00 × 10^−7 m.
Confusing the photovoltaic and photoelectric effects. In the photoelectric effect light knocks electrons out of a metal; in the photovoltaic effect light drives a current inside a cell. Our photoelectric effect problems cover the first.
To practise E = hc/λ with feedback on each step, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
How did we get from 1839 to solar panels?
APS traces the path: selenium cells made by Charles Fritts in 1883 were under one percent efficient; Russell Ohl found the silicon p-n junction in 1940; and Bell Labs built the first practical silicon cell in 1954. Read the next two steps in our posts on Russell Ohl and the Bell Labs silicon solar cell.
What can a student take from his way of working?
Notice the side effect. He was studying liquids and spotted that the plates were reacting too.
Design a test that splits two causes. Yellow glass separated heat from light.
Record numbers, even rough ones. His needle readings let others compare materials and colours.
Try it now: work out the photon energy of green light at 530 nm and decide where it falls between Becquerel's red and violet.
