In 1896, a French physicist accidentally discovered radioactivity after leaving uranium crystals on photographic plates wrapped in black paper inside a dark drawer

In 1896, a French physicist accidentally discovered radioactivity after leaving uranium crystals on photographic plates wrapped in black paper inside a dark drawer. Henri Becquerel was studying uranium compounds and wanted to know whether phosphorescent materials could produce X rays after exposure to sunlight. A period of overcast weather changed the experiment. The plates remained in darkness for several days. When Becquerel developed them, he found that the plates had been exposed. The result showed that uranium could produce penetrating radiation without sunlight. This finding became the starting point for the study of radioactivity and later influenced nuclear science, medicine and physics.

The experiment that led to the discovery

In 1895, Wilhelm Röntgen discovered X rays. Becquerel learned that X rays came from a glass vacuum tube that became fluorescent when struck by cathode rays. Becquerel wanted to investigate whether there was a connection between visible light, phosphorescence and X rays. He asked whether materials that produced light after being stimulated could also produce X rays. His knowledge of uranium compounds and phosphorescent crystals made him suited to investigate the question.


He placed phosphorescent crystals on photographic plates. He wrapped the plates in opaque black paper so that ordinary light could not reach the photographic material. The setup was exposed to sunlight for several hours. The sunlight was expected to stimulate the crystals. When the plates were developed, they showed images of the mineral samples.

Becquerel also placed objects such as a coin or a metal cutout between the crystal and the paper. The objects produced images on the photographic plate. This showed that radiation from the material could pass through the covering and affect the photographic plate. On February 24, 1896, Becquerel reported his findings to the Académie des Sciences. He noted that uranium salts were particularly active.

In 1896, a French physicist accidentally discovered radioactivity

The key development came soon after the first experiments. Becquerel had planned to continue exposing his uranium compounds to sunlight. However, the weather in Paris was overcast. He placed the experimental materials in a drawer because there was not enough sunlight.

The uranium crystals remained with photographic plates wrapped in black paper. The setup stayed in darkness for several days. Becquerel later decided to develop the photographic plates even though the uranium crystals had not received the sunlight he expected to be necessary. The result surprised him.

The plates had been exposed. The images were clear and strong. Sunlight had not reached the photographic material, and the uranium compounds had not been stimulated by sunlight during the period.

This observation changed the direction of his research. The radiation did not depend on sunlight or visible phosphorescence. Becquerel concluded that uranium compounds were producing penetrating radiation on their own.

He initially described the process in terms related to phosphorescence. He later traced the activity to uranium itself. The radiation came from the uranium element. The finding became known as Becquerel radiation. Marie Curie later used the term radioactivity for the phenomenon.

Why the discovery was different from X rays?

Becquerel first believed that the radiation from uranium was related to X rays. Further research showed that the source was different. X rays were being studied in connection with equipment such as vacuum tubes. Uranium produced radiation without needing such a process.

The discovery showed that matter could release radiation from within itself. This changed scientific thinking about atoms. At the time, scientists did not yet understand that atoms could change and release particles and energy. The discovery therefore became part of the work that later led to the study of atomic structure and nuclear transformations.

Becquerel's education and scientific background

Henri Becquerel was born Antoine-Henri Becquerel on December 15, 1852, in Paris, France. He died on August 25, 1908, in Le Croisic, France, at the age of 55. He came from a family connected to science. His grandfather was Antoine-César Becquerel. His father was Alexandre-Edmond Becquerel, and his son was Jean Becquerel.

Henri studied at the Lycée Louis-le-Grand before entering the École Polytechnique from 1872 to 1874. He then received engineering training at the École des Ponts et Chaussées from 1874 to 1877. He became an assistant teacher at the École Polytechnique in 1876. In 1895, he became its professor of physics.

He also worked with his father at the museum and later took over the physics professorship there after his father's death. Becquerel worked as an engineer in the Department of Bridges and Highways and became chief engineer in 1894.

Before studying radioactivity, he researched several areas of physics. These included electricity, magnetism, optical phenomena, infrared radiation and phosphorescence.

He also studied how uranium compounds absorbed light and produced phosphorescence. By 1896, he had been a member of the Académie des Sciences since 1889.

Research after the discovery

Becquerel published seven papers on radioactivity during 1896. He published two more in 1897 and none in 1898. At first, the scientific community did not give the discovery the same attention given to X rays. X rays could produce photographs with sharper shadows and were easier to demonstrate.

Interest in radioactivity changed in 1898. Gerhard Carl Schmidt and Marie Curie independently found radioactivity in thorium. Marie Curie and Pierre Curie, working with Gustave Bémont, later discovered polonium and radium. These discoveries showed that radioactivity was not limited to uranium.

Becquerel's work on beta particles

Becquerel continued studying radiation after his first discovery. In 1899 and 1900, he studied the deflection of beta particles in electric and magnetic fields.

His measurements showed that beta particles had the same charge-to-mass relationship as the electron identified by J.J. Thomson. This connected radioactivity with the study of subatomic particles.

Becquerel also studied uranium X, a substance associated with uranium radiation. Uranium X gradually lost its ability to emit radiation. Uranium itself, after being inactive when freshly prepared, later regained radioactivity.

Similar observations were later made by Ernest Rutherford and Frederick Soddy while studying thorium X and thorium.

Their work contributed to the transformation theory of radioactivity. The theory explained radioactive decay as a subatomic process in which one element could change into another.

Radiation and its effect on the body

Becquerel also investigated the effect of radiation on the human body. In 1901, he reported a burn after carrying an active sample of radium produced by the Curies in his vest pocket.

His report helped encourage doctors and scientists to investigate the effects of radiation. This work later contributed to the medical use of radiation.

The Nobel Prize and lasting legacy

In 1903, Henri Becquerel shared the Nobel Prize in Physics with Marie Curie and Pierre Curie for their work on radioactivity. He also received medals and memberships from scientific societies in other countries. The Académie des Sciences later elected him president and one of its permanent secretaries.

The SI unit of radioactivity is the becquerel, written as Bq. It is named after Henri Becquerel. The discovery that began with uranium crystals, photographic plates, black paper and a dark drawer became part of the foundation of nuclear science.

It showed that radiation could come from an element without sunlight or another external source. The work by Becquerel, followed by research from Marie Curie, Pierre Curie, Rutherford, Soddy and other scientists, helped establish radioactivity as a field of scientific study.