What did Planck propose about light in 1900 to explain experimental results?

In 1900, Max Planck proposed the groundbreaking idea that light is emitted and absorbed in discrete packets of energy known as "quanta" or "photons." This concept was revolutionary because it contradicted the classical wave theory of light, which treated light as a continuous wave. Planck introduced the idea that the energy of these quanta is proportional to the frequency of the light, leading to the formulation of Planck's law of black-body radiation. This proposal effectively explained the observed spectral distribution of radiation emitted by black bodies, which could not be adequately described using classical physics.

The concept of light coming in bundles of energy laid the foundation for quantum mechanics and established the basis for understanding phenomena such as the photoelectric effect and atomic structure. It ultimately highlighted the dual nature of light, which exhibits both wave-like and particle-like properties, but Planck's initial contribution specifically focused on the idea of quantization of energy in the form of bundles.