At point A by Snell's law μ=sinrsin45∘⇒sinr=μ21 At point B, for total internal reflection, sini1=μ1 
From figure, i1=90∘−r ∴(sin90∘−r)=μ1⇒cosr=μ1 Now cosr=1−sin2r=1−2μ21 =2μ22μ2−1 From eqs (ii) and (iii) μ1=2μ22μ2−1 Squaring both sides and then solving, we get μ=23