Complex sentence: Both centrosymmetric and non-centrosymmetric media will exhibit an effect similar to Pockels, the Kerr effect, but in that, the refractive index change will be proportional to the square of the electric field strength and will therefore be much weaker than the Pockels effect.
Split into simpler sentences: Both centrosymmetric and non-centrosymmetric media will exhibit an effect similar to Pockels, the Kerr effect. The refractive index change will be proportional to the square of the electric field strength and will therefore be much weaker than the Pockels effect.