A ray of light in

Here, we have a case of minimum deviation.

We know, the prism angle,

Refractive index,

Let the angle of incidence be and that of refraction be

Therefore,

OR

Given, Refractive index of lens,

Refractive index of medium,

Focal length of lens,

Let the focal length in the medium be . Let the two radii of curvature be and .

We know

Now, the focal length in medium would be

For lens to act as a plane sheet of glass, the refractive index of the medium should be same as that of the lens, that is 1.5.