The focal length of a convex lens made from a material of refractive index ( n₁ ) is ( f ) when placed in air. If it is immersed in carbon disulphide of refractive index ( n₂ ), then its focal length and nature will be:
a) ( f ⋅ n₁n₂ ), still convex
b) ( f ⋅ n₁n₂ ), concave
c) ( f ⋅ n₂n₁ ), still convex
d) ( f ⋅ n₂n₁ ), concave
