Page 21 - Dream 2047 June 2020

ANNULAR S LAR ECLIPSE
REVELATION
Discovery of helium
In mid-1800, many
scientists were thrilled with
a new instrument called a
spectroscope. Similar in
design to a telescope, the
spectroscope worked like
a prism, dispersing visible
light into measurable
wavelengths. It was observed
that heating any element
produced bright light; for
example, sodium burns with
a yellow flame. When one
looks at the hot glowing gas
through a spectroscope, a few
discrete bright ‘lines’, called
line emission spectrum, are
observed. The same gas, when cooled and passed through background white light, absorbs light of precisely those colours that it would have itself radiated when hot. This time, one would see dark lines, called line absorption spectrum in a rainbow-coloured background. Every chemical element produces a unique spectrum, a sort of 'fingerprint', that confirms the presence of an element.
Pierre Janssen
Researchers started analysing the emission and absorption spectra of specific elements in the lab. Then they observed stars through spectroscopes and tried matching the spectra with known elements. Thus, it became possible to make out the chemical composition of stars across the galaxy.
French astronomer Pierre Janssen was very interested in spectroscopic analysis of visible light. He travelled across Europe and Asia to observe the night sky. He chased after eclipses, visiting Italy in February 1867 and then to Guntur, India, for the total solar eclipse of 18 August 1868.
He camped in Guntur to watch the solar corona visible during a total solar eclipse. From the spectroscopic analysis, Janssen observed that the prominences were mostly made of super- hot hydrogen gas. But he also noticed that a yellow line in the spectrum did not match the wavelength of any known element.
Around the same time, one English amateur astronomer, Norman Lockyer, made a similar observation. His observation of Sun was, however, without the eclipse, using a special instrument called coronagraph that blocked out the light of the Sun so that researchers can glimpse the burning star's hot, thin, corona. Lockyer and English chemist Edward Frankland named the unknown element helios, after the Greek word for the Sun. Helium is the first and only element to be discovered and identified
outside Earth.
For some time, helium
was believed to exist only
on the Sun and other stars. However, in 1882, Italian physicist Luigi Palmieri recorded helium’s yellow spectral line in his data while analysing lava from Mount Vesuvius. Helium is probably best known today as the gas that fills birthday balloons. Helium is used in medical equipment (Magnetic Resonance Imaging) as well as in spacecraft and radiation monitors. It is also used in microscopes, airbags in cars, and in many physics experiments.
The author is Scientist ‘F’ in Vigyan Prasar. Email: rnath@vigyanprasar.gov.in
Spectroscope used in the nineteenth century
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