Mutual Inductance is the interaction of one coil magnetic field on other coil as it induces a voltage in adjacent coil. When AC current flow in induction L‌1 , magnetic field is produce. This change in magnetic field cut the other coil and emf is induce according to faraday’s law. E=
dϕ
dt
Emf induce in L‌2 , Oppose the cause according to Lenz’s law.