The existence of quantized energy levels in an atom can be inferred from

This concept is inferred from quantum mechanics. According to quantum mechanics, electrons can absorb or emit energy by a discrete amount of energy called quanta. This follows from the two contradicting theory of wave-particle theory of light. It was widely accepted already that light behaves like a wave. However, questions are still unanswered on why electromagnetic waves emit or absorb light in specific frequencies. Until Planck proposed his equation: E = hν, where E is the energy, h is called the Planck's constant and ν is the frequency.

In this equation, it signifies that increase energy is progressing at discrete amounts of frequencies and this constant is denoted by h.