TS EAMCET 6-Aug-2021 Shift 2 Question Paper

Focal length of the biconvex lens is f. Radii are equal, i.e. R1=R2=R.
‌aµg=‌

3

2

‌aµw=‌

4

3

According to lens Maker's formula,
∴‌‌‌

1

f

=(µ−1)(‌

1

R1

−‌

1

R2

)
Here, R1=R and R2=−R.
∴‌‌‌

1

f

=(‌

3

2

−1)(‌

1

R

+‌

1

R

)=‌

1

2

×‌

2

R

=‌

1

R

⇒‌‌f=R
When the lens is dipped in the water, then
‌

1

fw

‌‌=(‌

‌aµg

‌aµw

−1)(‌

1

R

+‌

1

R

)
‌

1

fw

‌‌=(‌

‌ 3 2

4 3

−1)×(‌

1

R

+‌

1

R

)
‌‌=(‌

3

2

×‌

3

4

−1)×‌

2

R

=(‌

9

8

−1)×‌

2

R

⇒‌‌‌

1

fw

‌‌=‌

1

8

×‌

2

R

=‌

1

4R

=‌

1

4f

⇒fw=4f