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
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).