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Tunnelling
Quantum tunnelling describes the small probability that an incoming particle will not be reflected but will pass through a potential barrier.
Explanation
The cause of tunnelling is the fact that the wave function is not zero within the energy barrier, but merely becomes (very) small.
Example 1
Only in this way can alpha particles (two neutrons and two protons) escape the strong nuclear force. This explains alpha radiation in radioactive substances (alpha decay).