The focal length of a convex lens made of glass of refractive index (1.5) is 20 cm. What will be its new focal length when placed in a medium of refractive index 1.25 ? Is focal length positive or negative? What does it signify?

The focal length of a convex lens made of glass of refractive index (1.5) is 20 cm. What will be its new focal length when placed in a medium of refractive index 1.25 ?
Is focal length positive or negative? What does it signify?

ANS 10 Given aஜౝ = 1.5
Focal length of the given convex lens when it is placed in air is
f = + 20 cm Refractive index of the given medium with respect to air is
aஜౣ = 1.25 New focal length of the given convex lens when placed in a
medium is f
ᇱ

New focal length is positive. The significance of the positive sign of the focal length is that given convex lens is still converging in the given medium.