Why does a convex lens of glass `mu = 1.5` behave as a diverging lens when immersed in carbon disulphide of `mu = 1.65` ?

Explanation

This is because `mu = (mu_g)/(mu_c)=(1.5)/(1.65) lt 1`
From `(1)/(f) = (mu - 1)((1)/(R_1) - (1)/(R_2))`,
`f` becomes negative. Therefore, the lens behaves as a diverging lens.