A planoconvex lens when silvered on the plane side behaves like a concave mirror of focal length 60 cm. However. r.l,hen silvered on the convex side it behaves like a concave mirror of focal length 20 cm. Then the refractive index of the lens is

1F=1f1+1f1+1fm1F=2f1+1fmor    1F=2μ−11R+1∞or   F=R2(μ−1)Now,      −60=−R2μ−1or  120μ−1=R  .  .  .  .1Again,     1F=1f1+1f1+1fmor  1F=2f1+1R/2or  1F=2μ−11R+2Ror  1F=2Fμ−1+1or   F=R2μNow,   −20=−R2μ  or  40μ=R   .   .   .2Dividing1  by  2,  we  get120μ−140μ=RR=1or   120μ−1=40μor   120μ−40μ=120or     80μ=120or   μ=12080=32=1.5