Quantum mechanics was not born because we were asking the question of what is matter made up of. Quantum mechanics was born because we were asking the question, what is light?
In the 17th Century, Newton and Huygens gave their own models of the composition of light. Newton said the light is made up of particles, which he called Corpuscles. In contrast, Huygens argued that light is a wave, a disturbance in the medium, just like the sound waves. Using their own theories of light, Newton and Huygens were able to explain refraction, reflection, shadows, rectilinear motion, and different colors of light.
But since Newton had a greater reputation among the physicists back in the 17th century, it was his model of light that was accepted. However, in the year 1801, one of the most important experiments in the history of mankind was performed - Young’s double-slit experiment, which gave a fatal blow to Newton’s particle theory of light.
The experimental setup of the double-slit experiment with light is very easy to understand. In the double-slit experiment, we have a monochromatic source of light that passes through a couple of slits and falls on a screen behind the slits. When both the slits are open, we observe alternate dark and bright fringes of light - known as the interference pattern. This pattern disappears when we close one of the slits.
Newton’s particle model of light could not explain the results of this experiment. But when we consider light to be a wave, this phenomenon has a natural explanation. Light waves reach the screen and due to superposition. They produce an interference pattern on the screen.