Light is so familiar to us that we rarely think about what light really is. Physicists had been wondering for centuries over the question whether light is wave or a particle? But just over two centuries ago, a ground-breaking experiment answered this question that had occupied the physicist for such a long time.Often regarded as the base of quantum mechanics, this experiment was carried out by British physicist and polymath, Thomas Young, in 1801.
In this experiment, a beam of monochromatic light (light of same wavelength) is passed through two long and narrow opening (double-slit) and observed on the screen. After doing this, Young noticed that light was forming light and dark bands on the screen.Since, only wave shows such nature because of its constructive and destructive interference,light was proven as a wave. This proved the Huygens Wave theory correct and disapproved Newton who believed that light was a particle.
Later on, same experiment was performed by the scientists using electron gun instead of monochromatic light. They shot electrons and the observations shocked everyone. Electrons were forming bands like a wave. But wait, how was that possible? To make this happen electrons had to move through both the slits simultaneously. So, scientists added a measuring machine that would track the motion of the electrons and performed the experiment again. This time the results were even more shocking. Electrons behaved as particles and didn't form bands as before. This experiment was repeated time and again but the result was same.
Then, scientists concluded that reality only exists when it is observed and this experiment became the foundation of the quantum mechanics.