Question

The power of a combination of 2 lenses X Y is 5 Dioptre.If the focal length of X is 15 cm.State the nature and focal length of Y.
Plz give your answer if u know the correct one..
URGENT!!

Answer 1

X is a convex lens.
It gives erect image.
its focal length is always positive.
it gives erect and virtual image.
It is know as converging lens.

and Y is a concave lens.
and its focal length is -10cm.
it gives real and inverted image.
Focal length of concave lens is always negative.
it is known as diverging lens.

Hope it helps you✌✌✌✌........

Answer 2

Given: Combined power of two lenses X & Y, (P) = 5 d

Let the combine focal length of lenses X & Y, be 'F'. 

Let F1 and F2 be the focal length of lens X and Y respectively. 

Here, the focal length of lens X is given as F1 = 15 cm

Relation between power and focal length is given as 

                  F = 1 / P

Since, the lens having power 1 d (diopter) always has focal length 100 cm

So,             F = [1 / ( 5 / 100)] cm

or,              F = 20 cm

Now, we shall calculate the focal length of lens 'Y'.

 Concept: when two lenses are in contact to each other, then the relation between their focal lengths and combined focal length is given as,

                 (1 / F) = (1/F1) + (1 / F2)

Or,           (1 /F2) = (1/F) - (1/ F1)

Or,           ( 1/ F2) = (1 / 20cm) - (1/15 cm)

Or,                  F2 = - 60 cm

Hence, the focal length of second lens 'Y' will be 60 cm and the nature of the lens will concave just because of having negative sign.