Refractive index of water w.r.t. glass
Refractive index of medium 2 w.r.t. medium 1 is written as
n21 = speed of light in medium 1/ speed of light in medium 2
n21 = v1/v2
Refractive index of glass w.r.t. air nga = 1.65 = va/vg ⇒ vg = va/1.65
Refractive index of water w.r.t. air nwa = 1.33 = va/vw ⇒ vw = va/1.33
Refractive index of water w.r.t. glass nwg = vg/vw = ( va/1.65)/(va/1.33)
= 1.33/1.65
= 0.80