### Theory:

The atom bomb works on the principle of uncontrolled chain reaction. In an uncontrolled chain reaction, the number of neutrons and fission processes multiply in a nearly geometrical progression. This chain reaction causes an explosion by releasing a large amount of energy in a short period.

Atom bomb

Structure of an atom bomb:
An atom bomb is made of a piece of fissile material with a subcritical mass. This piece has a cylindrical void into which a cylindrical fissile material of subcritical mass can be inserted.

Inside view of an atom bomb

The cylinder is injected into the void with a conventional explosive when the bomb has to be detonated. The two pieces of fissile material are combined to produce a supercritical mass for the explosion.

Structure of an atom bomb

The explosion of an atom bomb:
During the explosion, a massive amount of energy is released in the form of heat, light, and radiation.

Atom bomb explosion

A region of extremely high temperature and pressure is generated in a fraction of a second, accompanied by the emission of dangerous radiation like gamma rays that harm living creatures.

In $$1945$$, during World War II, atomic bombs of this type were dropped on Hiroshima and Nagasaki in Japan. The nuclear bomb dropped on Hiroshima was called 'Little boy'. It was a gun-shaped bomb with a uranium core.

Model of 'Little boy'

The bomb that was dropped over Nagasaki was named 'Fat man'. It was an explosion type bomb with a plutonium core.

Model of 'Fat man'
Reference:
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