# When do you use Quantum Mechanics? [duplicate]

Given a problem, how does one know whether to use quantum mechanics or classical mechanics?

Take for example electron scattering from a nucleus. The electrons are given a wavefunction in this case. Why not use the classical approach?

## marked as duplicate by Aaron Stevens, Community♦Feb 11 at 12:05

From the particle's speed $$v$$ you can estimate its de-Broglie wavelength by $$\lambda = \frac{h}{mv}$$. Let $$d$$ be the size of the structures involved.
When $$\lambda$$ turns out to be much smaller than $$d$$, then the particle's wave nature has no big effect and you can safely use classical mechanics as a good approximation.
When $$\lambda \ll d$$, then you can safely use geometric optics (with light rays).