January, 1979

The Photoacoustic Effect

"I have devised a method of producing sounds by the action of an intermittent beam of light from substances that cannot be obtained in the shape of thin diaphragms or in the tubular form ; indeed, the method is specially adapted to testing the generality of the phenomenon we have discovered, as it can bo adapted to solids, liquids, and gases."

1880 to the Present

During recent years the words photoacoustic and optoacoustic have been used somewhat interchangeably to describe basically the same phenomena. In this article the word photoacoustic will be used exclusively.

Einstein's Researches on the Nature of Light

The fundamental contributions made by Albert Einstein toward our present-day understanding of the nature of light are reviewed. In order to place this work into proper perspective, the theories of Hooke, Huygens, Newton, Fresnel, and Maxwell are first summarized, and a brief account is given of the researches of Kirchhoff, Wien, Rayleigh, Jeans, and Planck on equilibrium radiation laws. The main publications of Einstein that led to the formulation of the quantum theory of radiation are then discussed. These include his investigations on the particle aspects of radiation, on the wave-particle duality, on the elementary processes of energy exchange between gas molecules and a radiation field, and on photon statistics.

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