Class 12 Physics Nuclei Radioactivity


  • Radioactivity is a nuclear phenomenon in which an unstable nucleus undergoes decay to form stable nuclei.
  • Radioactivity was discovered accidentally by a scientist Henry Becquerel (1896).

Henry Becquerel


  • Only those nuclei which are unstable will show the phenomenon of radioactivity.
  • Fluorescence is a property shown by fluorescent objects; when visible light falls on a fluorescent object then these objects will also start emitting light.
  • Henry Becquerel performed experiments on some fluorescent objectsand he gave a hypothesis that fluorescent objects along with the visible light also emit some kind of radiation as well.
  • Experimental set-up:-
    • He took a photographic plate and placed inside a box.He placed a fluorescent object on the top of the box and allowed the sun light to fall on it.
    • As the box is opaque it won’t allow the rays of light to pass through it.
    • When the photographic plate will be developed there should be no traces of image on the plate.
    • He repeated the experiment by taking different compounds and observed there was no image formed on the photographic film.
    • This showed that sunlight is the only ray which is emitted in the above case and as the plate was covered with opaque object so the sunlight is not able to reach the plate.


  • He then took the compound of uranium known as uranium potassium sulphate.
  • When the photographic plate was developed there was a black spot on the photographic plate.
  • This shows that some radiation was emitted that reached the photographic plate.


  • In order to know from where the radiation was emitted, he kept a blade on top of a box and uranium potassium sulphate compound was placed above the blade.
  • After performing the experiment, when the photographic plate was developed, the photo of blade was seen on the plate.
  • This shows that the radiation was emitted by the uranium potassium sulphate compound.


  • Accidentally while he was performing the experiment it was not sunny that day.
  • The result which he got was same i.e. when the photographic plate was developed there was the same image of the blade on the plate.


  • The radiation was emitted by the uranium potassium sulphate compound and sunlight did not play any role in emitting the radiations.
  • After series of experiments he proved that it was uranium alone which was responsible for emitting the radiations.
  • He concluded that there are certain elements which emit radiation on their own.
  • This phenomenon was named as radioactivity.

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