Here, we have a case of minimum deviation.
We know, the prism angle,
Let the angle of incidence be and that of refraction be
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 .
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.
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