Astronomy Principles and Practice Fourth Edition.pdf

210 The radiation laws
Figure 15.1. The positions of the chief absorption features in the solar spectrum as designated by Fraunhofer.
real clue to the relationships and identities of the spectral lines he observed. In the paper he published
in 1802, describing his laboratory measurements of the refractive indices of various materials, he also
makes comment on the spectrum of the Sun which he obtained. His description is vague but it appears
that he saw dark bands which he supposed to be natural boundaries separating the zones of pure colour.
He appears not to have attached much significance to this observation and dispenses with it in a few
sentences. It may have been that he expected there to be natural divisions between the seven zones of
the spectrum which had been designated by Newton and was, therefore, not surprised to see the dark
bands. Newton himself made no mention of what we know as the solar absorption features, probably
because of the poor quality of his glass—a point on which he himself complained.
Some years later in 1817, Fraunhofer, who had been experimenting in ways of defining the colours
which are used to determine the refractive index of glasses, published the results of his observations of
the solar spectrum made by using an improved spectroscope. He noted that the spectrum was crossed
by many dark lines. By repeating his observations with a range of optical elements he proved that the
lines were a real feature of the spectrum and were contained in the sunlight. He made a map of several
hundred of the lines and designated the prominent ones with the letters A, B, C, ...,bywhichtheyare
still known (see figure 15.1). Although Fraunhofer was unable to offer an explanation for their cause,
the positions of these lines provided the first standards of colour for use in measuring the refractive
index and dispersion of different materials and in comparing the spectra from other luminous sources.
Fraunhofer also was the first to make visual observations of the spectra of the planets and of the
brighter stars including Sirius by using an objective prism. He observed that all the spectra exhibited
dark lines—those of the planets were similar to the Sun, those of the stars were different. Thus, it can
be said that he was the first pioneer stellar spectroscopist. His work did not rest here, however, and he
continued by inventing and manufacturing many diffraction gratings. With the aid of these devices he
succeeded in measuring accurately the wavelengths, in terms of laboratory standards of length, of the
spectral features and providing a basis whereby the results of different observers could be compared
directly. In other words he was the instigator of observational spectrometry.
15.2 The velocity of light
It will already have been appreciated that the velocity of light in vacuo and indeed the velocity of
all electromagnetic waves is a fundamental constant; its value appears in the description of and in
formulas relating to many different physical processes. This fundamental constant of nature is one of
the starting points for our understanding of the behaviour of matter from the extremes of processes
involving individual atoms to those involving the whole conglomeration of matter within the universe.
Indeed, it was because of experiments connected with the velocity of light that these new physical
theories, in particular, Einstein’s theories of relativity, arose.
We have already seen (section 11.3) that light has a finite velocity—this was first discovered by
the astronomer Roemer. Confirmation that light had a finite velocity came from Bradley’s discovery of
aberration (see section 11.4).
Because of the magnitude of the velocity involved, laboratory measurements of the velocity of
light proved to be very difficult. The first laboratory determination was achieved by Fizeau in 1849 and