2.
Radioactivity
At
the end of 1895, Wilhelm Röntgen discovered X-rays.
He published his results before the end of the year,
and word of his discovery spread quickly.
Henri
Becquerel heard about the discovery a few months later,
when Henri Poincaré described the discovery at
a meeting of the French Academy of Science. Röntgen's
X-rays had come from the fluorescent glass walls of
a cathode ray tube, and Becquerel wanted to see if other
luminescent objects could produce the rays.
Becquerel
experimented with uranium in February 1896. He exposed
the mineral to sunlight to make it luminesce, then laid
it on top of a photographic plate wrapped in black paper.
The
discovery of radioactivity came on Sunday 1st March
1896. Becquerel had set up the experiment the previous
Friday, but it had been cloudy so the uranium could
not be made luminescent. He had put the uranium and
the plates in a drawer for the weekend, but decided
to develop the plates anyway. He found the same fogged
images. The uranium salts were radiating even without
'activation' by sunlight.
The
phenomenon was named 'radioactivity' by Pierre and Marie
Curie at the start of the 20th Century. They had begun
studying the phenomenon in 1898 and found that thorium,
polonium and radium also showed the same effect. In
1903 the Curies shared the Nobel Prize with Becquerel
for their work.
At
the same time Rutherford was investigating radioactivity.
In 1898 Rutherford confirmed Becquerel's observation
that there were at least two parts to the uranium rays.
Rutherford named the easily absorbed rays 'alpha' and
the more penetrating rays 'beta'. He went on to discover that
these rays were particles.
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