Until the late 19th century, Newtonian physics dominated the scientific worldview. However, by the early 20th century, physicists discovered that the laws of classical mechanics do not apply at the atomic scale. Classical Physics (Statistical Thermodynamics) cannot explain the experimentally observed spectra of blackbody radiation.
Classical physics assumed that radiation is emitted continuously by the matter with smooth continuous spectrum of all possible energy levels. In 1900, Max Planck postulated that the electromagnetic energy is emitted not continuously (like by vibrating oscillators), but by discrete portions or quants. Quantum mechanics was born. Planck’s Law states that the energy of light is proportional to the frequency, and the constant that relates them is known as Planck’s constant (h). His work, led Einstein in determining that light exists in discrete quanta of energy, and can be considered not only as a wave-like entity but also as a particle, or photon, with the energy given by the Planck-Einstein relation
Where ‘h’ is Planck’s constant (h = 6.626 × 10-34 J.s) And v, λ and c are the frequency, wavelength and speed of light wave, respectively.