KEAM 2019 Physics and Chemistry Paper

Given, power of lens =+2.5D,µ‌water ‌=1.3 and µ‌lens ‌=1.5
Lense maker's formula for equiconvex lense
‌

1

f

=(µrel−1)‌

2

R

Since, power of lens, P=‌

1

f

So, for air, lense medium
25=(‌

1.5

1

−1)‌

2

R

‌‌(∵µ‌air ‌=1)
⇒‌‌25=‌

0.5×2

R

⇒‌‌R=‌

1

25

m
(where, R= radius of curvature )
New focal length, when lens in water
‌

1

f′

=(‌

µ‌lens ‌

µ‌water ‌

−1)‌

2

R

⇒‌‌‌

1

f′

=(‌

1.5

1.3

−1)2×25
So, near by option is (b).