Page 21 - Dream June 2020

ANNULAR S LAR ECLIPSE
REVELATION
until 1950. With the advancements in astrophysics, these measurements are now possible to take in other frequencies of the electromagnetic spectrum, other than the visible range. However, the experiments of Dyson and Eddington during the total solar eclipse in 1919 are unique and very important in establishing the general theory of relativity.
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.
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
Eclipse instruments at Sobral
Image credit: C. Davidson - File: Eddington A. Space-Time and Gravitation. 1920.djvu, Public Domain: https://commons.wikimedia.org/w/index.php?curid=38550909
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,
According to the general theory of relativity, light changes its path while travelling through the warped space-time. The more massive an object, the bigger the distortion, and the more its gravity can bend light. Einstein predicted that light rays from any distant star would bend while passing near any massive object like the Sun.
june2020/dream2047 21