20th February 1873. Willoughby Smith?
Today the magazine Nature reported on the discovery by WILLOUGHBY SMITH (1828-91) of the electrical resistance of Selenium which varied dramatically with the amount of light falling on it.
Later this discovery of the effect of light on Selenium, a non conductor of electricity, resulted in the Photo Electric or Voltaic Cell, which we now know relies on electrons being ejected from the element into an electrical circuit.
It was whilst working for the Gutta Percha Company in London that Smith developed the use of Selenium to test the insulation on copper and iron cables laid for underwater use to transmit telegraph messages from Dover to Calais.
SINGLE FUNCTION SOLAR CELL WHERE PHOTONS OF SUNLIGHT DISLODGE ELECTRONS FROM LOWER P-TYPE SELENIUM OR SILICON UP VIA N-TYPE THEN TO FLOW INTO CIRCUIT.
The element Selenium (34 Se) was much used in photo electric cells, which included Baird in his early television work, but has been supplanted by Silicon in modern devices.
It might seem magical that light, which in Classical Theory was regarded as a electromagnetic wave, can be transferred into electricity.
Then came Einstein, who won his Nobel Prize not from his work on Relativity, but for work on the Photo-Electric Effect.
It was from his time that an alternative theory of light was adopted from his notion of light as a series of discrete packages described as Quanta or Photons rather than continuous waves.
It is these which then strike the electrons on the Selenium thus imparting enough energy to release them into a circuit.(SEE DIAGRAM).
Willoughby Smith had noticed in his work on insulation that it was affected by light intensity on the Selenium. However later it was found in work on photo electric cells, that the important factor is light wavelength.(1)
EXPLANATION OF DIAGRAM NUMBERS:
- Photons bombard upper surface.
- Photons (in yellow) carries Energy to Cells.
- Photons give up Energy to Electrons (green) in low p-Type.
- Electrons use Energy to jump across barrier to upper n-type which escapes to form current.
- Electrons flow around circuit to light lamp.
(1) So strong beams of light towards the red-end of the Spectrum might produce no electrical potential, whilst weak beams from towards the violet end would provide higher voltages.
The lowest frequency that cause electrons to be emitted is the Threshold Frequency. However this varies according to which metal is being used.
This is at odds with the Classical Theory of Electromagnetism which predicts that an electron’s energy should be proportionate to the intensity of Radiation.