    In 1896, Bequerel, a French physicist discovered that crystals of Uranium salts emitted
     penetrating rays similar to X-rays which could fog photographic plates. Two years after this
     Pierre and Marie Currie discovered other elements: Polonium and Radium which had this
     property. The emission was known as Radioactivity.
    Protons and Netrons are held together in the nucleus of an atom by the strong-force. This force
     acts over a very short distance of about ~1 fm, (10-15m) and over this short distance it can
     overcome the electromagnetic repulsion between the positively charged protons.
    The elements with atomic number greater than 82 are radioactive element.
Radioactive Decay                                      Fluorescence & Phosphorescence
    It is the process by which the nucleus of         Fluorescence is the emission of light by a
     an unstable atom loses energy by emitting         substance that has absorbed light or other
     radiation, including alpha particles, beta        electromagnetic radiation. It is a form of
     particles, gamma rays and conversion              photoluminescence. In most cases, the emitted
     electrons.                                        light has a longer wavelength, and therefore
    Its unit is the becquerel (Bq), named in          lower energy, than the absorbed radiation. The
     honour of the scientist Henri Becquerel.          emitted radiation may also be of the same
     One Bq is defined as one transformation           wavelength as the absorbed radiation, termed
     (or decay or disintegration) per second.          "resonance fluorescence".
Einstein’s Mass-Energy Relation                        Phosphorescence is a specific type of
                                                       photoluminescence related to fluorescence.
It states that the universal proportionality           Unlike fluorescence, a phosphorescent
factor between equivalent amounts of energy            material does not immediately re-emit the
and mass is equal to the speed of light squared.       radiation it absorbs. Excitation of electrons to
                                                       a higher state is accompanied with the change
It is the relationship between mass (m) and
                                                       of a spin state. Once in a different spin state,
energy (E) in the special theory of relativity of
                                                       electrons cannot relax into the ground state
Albert Einstein.
                                                       quickly because the re-emission involves
E  mc 2                                               quantum mechanically forbidden energy state
Where E is energy, m is mass, and c is the
speed of light (3 × 108 ms–1).