A particle's de Broglie wavelength, , is defined as Planck's constant divided by the particle's momentum, *p*: = *h/p*. The de Broglie wavelength is named after Louis de Broglie, the French physicist who first suggested that it might be useful to describe particles as waves. A relativistic electron has a de Broglie wavelength of around a nanometer, while a car driving down the highway has a de Broglie wavelength of around 10^{-38} meters. Quantum mechanical effects tend to be important at the scale of an object's de Broglie wavelength; thus we need to describe electrons quantum mechanically, but classical physics is adequate for cars and most other macroscopic objects.