How did 'Quantum Mechanics' develop ?

Explanation

`to` When the existing theory is simply unable to explain new observations. This causes a major upheaval in science.
`to` In the beginning of the twentieth century, it was realized that Newtonian mechanics, till then a very successful theory could not explain some of the most basic features of atomic phenomena.
`to` Similarly, then accepted wave picture of light failed to explain the photoelectric effect properly.
`to` This lead to the development of a radically new theory (Quantum Mechanics) to deal with atomic and molecular phenomena.