Question

What is the failure of classical mechanics?

Answer 1

Even though most phenomena that we can in everyday life be explained using classical mechanics, there are some cases where classical mechanics fails to give a proper/correct explanation. Three prominent examples showcasing the failure of classical mechanics are:

1. Blackbody radiation: According to classical mechanics, electromagnetic rays radiated from a blackbody can have any wavelength. This is actually not true as the range of wavelength is dependent on the frequency (wavelength) of the electromagnetic wave and they are emitted as discrete packets of radiation known as photons.
2. Photoelectric effect: In this phenomenon, light incident on metal causes electron emission. According to classical mechanics, the energy in electromagnetic waves is continuous and depends on the intensity of light, hence, a dim light should take some time to eject electrons. But this is in fact not the case as light has discrete energies dependent on its frequency.
3. Hydrogen atom: Electrons revolving around the hydrogen nucleus are constantly changing direction and hence accelerating. According to classical mechanics, an accelerating charged particle should emit electromagnetic radiation and hence lose energy. Hence, the electrons would eventually lose all energy and crash into the nucleus. But, we know that this is not the case.