Doubt from optics

Question 17 i got option C but answer says option A how

What's the answer of first one ?

I° is net intensity so intensity of each slit will be I°/4

17.5 cm

For this question , you need to used formula of refraction at curved surface for lens to find focal length as medium on both side of lens are different.

use two times refraction at curved surface for lens and one for slab
1)by using formula first time at surface 1 of lens you will get v=30cm
2)on surface two you will get v=20 cm
3) for plane surface of slab,you can use slab shift formula or refraction at curved surface formula where u=10cm and R= Infinity .you will get v=7.5 cm

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