[2] In 1849, Foucault experimentally demonstrated that absorption and emission lines appearing at the same wavelength are both due to the same material, with the difference between the two originating from the temperature of the light source.

Foucault achieved the demonstration by showing the rotation of the plane of oscillation of a long and heavy pendulum suspended from the roof of the Panthéon, Paris.

The experiment caused a sensation in both the learned and popular worlds, and "Foucault pendulums" were suspended in major cities across Europe and America and attracted crowds.

In 1857 Foucault invented the polarizer which bears his name,[7] and in the succeeding year devised a method of testing the mirror of a reflecting telescope to determine its shape.

The Foucault test is in use to this date, most notably by amateur and smaller commercial telescope makers as it is inexpensive and uses simple, easily made equipment.

In 1865 he published his papers on a modification of James Watt's centrifugal governor; he had for some time been experimenting with a view to making its period of revolution constant and developing a new apparatus for regulating the electric light.

Foucault showed how, by the deposition of a transparently thin film of silver on the outer side of the object glass of a telescope, the sun could be viewed without injuring the eye.