TS EAMCET 20-July-2022 Shift 2 Solved Paper

Given, focal length of convex lens, f=25‌cm

Refractive index, µg=1.5
By using lens Maker's formula,
‌

1

f

=(µg−1)(‌

1

R1

−‌

1

R2

).....(i)
When cover lens is immersed in water, then focal length (fw) is given as
‌

1

fw

=µ(‌

µg

µw

−1)(‌

1

R1

−‌

1

R2

)......(iii)
Dividing Eq. (i) by Eq. (ii), we have
‌‌

1 f

1 f

=‌

(µg−1)( 1 R1 − 1 R2 )

(‌ µg µw −1)(‌ 1 R1 −‌ 1 R2 )

‌⇒‌‌‌

fw

f

=‌

(µg−1)µw

(µg−µw)

=‌

(1.5−1) 4 3

(1.5−‌ 4 3 )

=4
‌⇒fw=4f=4×25=100‌cm
⇒fw=4f=4×25=100‌cm
∴ Change in focal length =fw−f=100−25=75‌cm