Kramers' law is a formula, derived in 1923 by the Dutch physicist Henrik Kramers (1894–1952), for the opacity of material inside a star. It states that, at high temperature, stellar opacity is proportional to the density divided by the temperature to the power of 3.5. Opacity that follows Kramers law is mostly due to free-free transitions of electrons and is the main source of opacity inside stars up to about 1 solar mass. In more massive stars, the rule is only applicable to the upper layers, electron scattering opacity being more important at greater depths.