It is a matter of common knowledge that an ordinary gold leaf apperars green by transmitted light while silver leaf appears blue. Faraday found that the gold leaf lost all its colour if heated on glass. T. Turner found that this change occurs at about 550° C. in the case of gold and about 240° C. in the case of silver. Faraday obtained thinner films from the "deflagration" of gold wire by the discharge of a Leyden jar battery. These were red and violet in places and green in others. They turned red on heating, but the green colour could be brought back by rubbing with a rounded piece of agate. The gold films used by Beilby, obtained from paints used for ceramic gilding, behaved in a similar manner. One of his thin purple films turned rose-pink on annealing, and his thicker green films became transparent at a temperature above 400°C. R. W. Wood obtained purple, blue and green films of gold by sputtering. He found that films of all other colours could be turned green by heating, as opposed to Faraday and Beilby. Maxwell Garnett has explained the colours observed by Beilby, Faraday and R. W. Wood by considering the films as made up of minute spherical particles of metal. He finds that the transmission-coefficient T, of films for which πd/λ is very small, is given by T = 1 - 4π
dn
2
k
/λ, where
d
is the thickness of the film,
k
the absorption coefficient,
n
the refractive index, and λ the wave-length of the light used.
Optical constants of sputtered beryllium thin films determined from photoabsorption measurements in the spectral range 20.4–250 eV
