Question

A converging lens has a focal length in air = 20 cm.When it is immersed in a liquid of refractive index less than the refractive index of the material of
the lens the focal length of lens in liquid

a =20cm
b >20cm
c <20cm
d change the sign

Answer 1

@rajathsbhat

Answer 2

i knew it's b but i can't explain good could u tell me why it is b ?

Answer 3

sure. good job though.

Answer 4

you see, in situations like this, you must look at something called 'relative refractive index'.
relative refractive index n=n(glass)/n(water).
Now you know that this 'n' in this situation is less than n(glass).

Coming to the second part of determining the focus, you know that lesser the 'n', greater is the focal length right?
So, the arrangement we have now will give you the exact same result as would a lens of refractive index n=n(glass)/n(water) placed in air. You know what I mean? Since this refractive index is less than the original refractive index, the focus length will be greater.

Answer 5

thanks that really explains good

Answer 6

:)

Answer 7

do u know much in ray and light optics ?

Answer 8

i good at high school optics, yeah.

Answer 9

ok i have many problem but i give myself chance to solve alone so can i revise my questions with later ?

Answer 10

sure. Just mention me in your question & I'll be there :)

Answer 11

ok thanks again :)