Number of words: 479

You are familiar with Newton’s demonstration of the character of white light by allowing sunlight to pass through a small round hole and letting the pencil of light so isolated fall on the face of a prism. The pencil of light was dispersed by the prism into its constituent rainbow colours. In 1802, it occurred to an English physicist, William Wollaston, to substitute the round hole, used by Newton and his successors to admit the light to be examined with the prism, with an elongated crevice (or slit as we would now say) 1/20th of an inch in width. Wollaston noticed that the spectrum thus formed, of light ‘purified’ (as he stated) by the abolition of overlapping images, was traversed by seven dark lines. These Wollaston took to be the natural boundaries of the various colours. Satisfied with this quasi-explanation, he allowed the subject to drop. The subject was independently taken up in 1814 by the great Munich optician Fraunhofer . In the course of experiments on light, directed towards the perfecting of his achromatic lenses, Fraunhofer, by means of a slit and a telescope, made the surprising discovery that the solar spectrum is crossed not by seven lines but by thousands of obscure streaks. He counted some six hundred and carefully mapped over three hundred of them. Nor did Fraunhofer stop there. He applied the same system of examination to other stars; and he found that the spectra of these stars, while they differ in details from that of the sun, are similar to it in that they are also traversed by dark lines.

The explanation of these dark lines of Fraunhofer was sought widely and earnestly. But convincing evidence as to their true nature came only in the fall of 1859 when the great German physicist Kirchhoff formulated his laws of radiation. His laws in this context consist of two parts. The first part states that each substance emits radiations characteristic of itself and only of itself. And the second part states that if radiation from a higher temperature traverses a gas at a lower temperature, glowing with its own characteristic radiation, in the light which is transmitted the characteristic radiations of the glowing gas will appear as dark lines in a bright background. It is clear that in these two propositions we have the basis for a chemical analysis of the atmospheres of the sun and the stars. By comparisons with the spectral emissions produced by terrestrial substances, Kirchhoff was able to identify the presence of sodium, iron, magnesium, calcium, and a host of other elements in the atmosphere of the sun. The question which had been considered as meaningless only a few years earlier had acquired meaning. The modern age of astrophysics began with Kirchhoff and continues to the present.

Excerpted from page 77-78 of  S. Chandrasekhar ‘Man of Science ’ by A.P.J. Abdul Kalam