The refractive index of medium a with respect to medium b is given by, bμa=sinisinr⇒aμb=sinrsini InΔOAB,sini=ABOB;InΔO′AB,sinr=ABO′B ∴aμb=ABO′BABOB=ABO′B×OBAB=OBO′B As the point A and B are very close, OB≈OA (real depth) and O′B≈O′A (apparent depth). ∴aμb=RealdepthApparentdepth