# 1.4. Light and energy

Light and energy are fundamental to the behavior of matter. In this section, we will outline how light and energy are related to mass, momentum, velocity, wavelength, and frequency. We will also introduce electronic transitions of the hydrogen atom.

## Planck constant and relation

In 1900, German physicist Max Planck explained the spectral-energy distribution of radiation emitted by a black body by assuming that the radiant energy exists only in discrete quanta that are proportional to the frequency. The Planck relation states that the energy () of a photon is proportional to its frequency () and inversely proportional to the wavelength (): . is the Planck constant, 6.62607015×10-34 Joule x Hertz-1 (J x Hz-1), with Hertz being defined as inverse seconds (Hz = s-1), and is the speed of light, which is equal to 299,792,458 meters per second (ms-1).

## The de Broglie wavelength

The de Broglie wavelength formula relates the mass (), momentum (), and velocity...