187
The first X-ray image was taken by
Röntgen of his wife Anna’s hand. The
dark circle is her wedding ring. On
seeing the image, Anna is said to have
exclaimed: “I have seen my own death.”
A CENTURY OF PROGRESS
“rays” must have come from the
cathode, or negative electrode.
They were named cathode rays
by Hittorf’s colleague Eugen
Goldstein, but in 1897, British
physicist J. J. Thomson showed
that they are streams of electrons.
Discovering X-rays
During his experiments, Hittorf
noticed that photographic plates
in the same room were becoming
fogged, but he did not investigate
this effect any further. Others
observed similar effects, but
Wilhelm Röntgen was the first to
investigate their cause—finding
that it was a ray that could pass
right through many opaque
substances. At his request, his
laboratory notes were burned after
his death, so we cannot be sure
exactly how he discovered these
“X-rays,” but he may have first
observed them when he noticed
that a screen near his discharge
tube was glowing even though
the tube was covered in black
cardboard. Röntgen abandoned his
original experiment and spent the
next two months investigating the
properties of these invisible rays,
which are still called Röntgen rays
in many countries. We now know
that X-rays are a form of short-
wavelength electromagnetic
radiation. They have a wavelength
ranging from 0.01–10 nanometers
(billionths of a meter). In contrast,
visible light falls between the range
of 400–700 nanometers.Using X-rays today
Today, X-rays are produced by firing
a stream of electrons at a metal
target. They pass through some
materials better than others, and
can be used to form images of the
insides of the body or to detect
metals in closed containers. In
CT (computed tomography) scans,
a computer combines a series of
X-ray images to form a 3D image
of the inside of the body.
X-rays can also be used to form
images of very small objects, and
X-ray microscopes were developed in
the 1940s. The image resolution that
is possible when using light
microscopes is limited by the
wavelengths of visible light. With
their much shorter wavelengths,
X-rays can be used to form images of
much smaller objects. Diffraction of
X-rays can be used to figure out how
atoms in crystals are arranged—a
technique that proved crucial in
elucidating the structure of DNA. ■See also: Michael Faraday 121 ■ Ernest Rutherford 206–13 ■
James Watson and Francis Crick 276–83
Wilhelm Röntgen
Wilhelm Röntgen was born
in Germany, but lived in
the Netherlands for part of
his childhood. He studied
mechanical engineering
in Zurich before becoming
a lecturer in physics at
Strasbourg University
in 1874, and a professor
two years later. He took
senior positions at several
universities during his career.
Röntgen studied many
different areas of physics,
including gases, heat transfer,
and light. However, he is best
known for his research into
X-rays, and in 1901 he was
awarded the first Nobel Prize
in Physics for this work. He
refused to limit the potential
uses of X-rays by taking out
patents, saying that his
discoveries belonged to
humanity, and gave away his
Nobel Prize money. Unlike
many of his contemporaries,
Röntgen used lead protective
shields in his work with
radiation. He died from
an unrelated cancer at
77 years old.Key works1895 On a New Kind of Rays
1897 Additional Observations
on the Properties of X-rays