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.
 
atomic-bomb-2026117_1280.png
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.
 
little-boy-atomic-bom-01.png
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.
 
3.png
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_Nuclear_Explosion.jpg
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.
 
Spare_Little_Boy_atomic_bomb_casing_at_the_Imperial_War_Museum_in_London_in_November_2015.jpg
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.
 
fat man.jpg
Model of 'Fat man'
Reference:
https://live.staticflickr.com/2823/11374698284_c24bb8d056_b.jpg
https://upload.wikimedia.org/wikipedia/commons/7/74/Atom_Bomb_Nuclear_Explosion.jpg
https://upload.wikimedia.org/wikipedia/commons/0/04/Spare_Little_Boy_atomic_bomb_casing_at_the_Imperial_War_Museum_in_London_in_November_2015.jpg