The device created by Lebedev for measuring the pressure of light was a very sensitive torsional dynamometer (torsion balance). Its movable part was a light frame suspended on a thin quark thread with wings fixed to it – light and black discs up to 0.01 mm thick. The wings were made from metal foil. The frame was suspended inside a vessel from which air was evacuated.
The light falling on the wings exerted different pressure on the light and black discs. As a result, a torque acted on the frame, which twisted the suspension thread. The angle of twisting of the thread was used to determine the light pressure
The light pressure for ordinary light sources on Earth, even the brightest (except lasers), is very small, so it is very difficult to detect it. Lebedev’s experiments were a great achievement for his time. The famous English physicist JJ Thomson assessed these experiments as follows: “You may know that I fought with Maxwell all my life, not recognizing his light pressure, and now your Lebedev made me surrender to his experiments.”
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