What constant is denoted by the letter h in quantum mechanics?

The constant denoted by the letter h in quantum mechanics is the Planck constant. It sets the scale at which quantum effects become important and connects a photon’s energy with its frequency through E = hf.

Max Planck introduced the constant in 1900 while studying black-body radiation. Classical physics could not correctly explain why hot objects emit the observed spectrum of light. Planck’s mathematical solution required energy to be exchanged in discrete amounts, or quanta, rather than continuously.

Albert Einstein used the same idea in 1905 to explain the photoelectric effect, proposing that light consists of energy packets now called photons. The constant also appears in the de Broglie relation, λ = h/p, which connects a particle’s momentum with its matter-wave wavelength.

The symbol h should not be confused with the reduced Planck constant, written ħ or “h-bar,” which equals h divided by 2π and commonly appears in quantum equations involving angular frequency and momentum. Since 2019, h has an exact SI value of 6.62607015 × 10⁻³⁴ joule-seconds and helps define the kilogram.