Question

A convex lens is dipped in a liquid whose reflective index is equal to refrective index of material of lens then it's focal length

Answer 1

There can be 2 two ways to answer this ,

Since the refractive index of material of lens and medium is same :thinking:
how will I be able to identify the lens now ?!
Hence it's focal length is INFINITE

Other proper way =_= ( the lengthy one ) ...

Use Lens maker formula ,
When the lens is dipped in the liquid ,

$\frac{1}{f} = (n - 1)( \frac{1}{r1} - \frac{1}{r2} )$

Where n is refractive index of glass with respect to the liquid ,

Hence n = 1

$\frac{1}{f} = (1 - 1)( \frac{1}{r1} - \frac{1}{r2} ) \\ \\ \frac{1}{f} = 0 \\ \\ f = \frac{1}{0}$

Hence focal length becomes Infinite hehe..
[ Calling 1/0 infinite sounds a bit weird to me so can't help with the chuckle ]