Question

a convex lens of glass of refractive index u is immersed in a medium of refractive index u'.how will the lens behave when u < u'

Answer 1

we know
according to lens maker formula
1/f=(y-1){1/R1-1/R2)

if biconvex exist
f=R/2(y-1) {for convex lens f>0

here y is relative refractive index

a/c to question,
u<u'
u/u'<1

hence,
you see focal length will be negative when medium refractive is greater then lens refractive index.
hence nature of convex lens convert concave lens