In Wikipedia (https://en.wikipedia.org/wiki/Fundamental_theorem_of_Galois_theory), it is stated that:
The field $E^H$ is a normal extension of $F$ if and only if $H$ is a normal subgroup of $Gal(E/F)$.
"In this case, the restriction of the elements of $Gal(E/F)$ to $E^H$ induces an isomorphism between $Gal(E^H/F)$ and the quotient group $Gal(E/F)/H$".
My question is how exactly does the restriction induce the isomorphism?
Thanks for any help. A brief explanation (or direction to a reference) will suffice.