In quantum mechanics, we believe that every object has both particle and wave properties. If so, there should be rules for linking the properties of the particles and those of the waves.
De Broglie suggested the following equation, thinking that particles with masses such as protons and electrons can also be regarded as waves.
$$λ=\frac{h}{p}$$
\(h\) is Planck’s constant, \(p\) is the particle momentum. Then the particle has the same properties as wave of wavelength \(λ\). This wavelength \(λ\) is called the de Broglie wavelength.