Question

what is the effect on newtons ring if the plano convex lens of the small radius of curvature is used​

Answer 1

At the point of contact of lens and the plate thickness is zero, so optical path difference becomes only λ/2 which is condition for destructive interference. ... iv)When a planoconvex lens of large radius of curvature is placed on a flat glass plate, a thin film of air is trapped between lens and plate

Answer 2

At the point of contact of lens and the plate thickness is zero, so optical path difference becomes only λ/2 which is condition for destructive interference. ... iv)When a planoconvex lens of large radius of curvature is placed on a flat glass plate, a thin film of air is trapped between lens and plate