AP EAPCET 25-Aug-2021 Shift 1 Paper

Given, refractive index of lens, µg=1.5
Refractive index of medium, µm=1.6
Power of lens, P=−5D
Focal length of lens, ft=

1

−5

m=−20‌cm
From lens maker formula, we have
P=

1

f1

=(

µ2

µa

−1)(

1

R1

−

1

R2

) ...(i) (∵µa= refractive index of air)
P′=

1

fm

=(

µ+2

µm

−1)(

1

R1

−

1

R2

) ...(ii)
Dividing Eq. (ii) by Eq. (i), we get

P′

P

=

( µt µm −1)

( µs µs −1)

=

1.5 1.6 −1

1.5 1 −1

⇒

P′

P

=

−1

16

×

2

1

⇒P′=

−1

8

×(−5)
⇒ P′=0.625D
This is close to oprion (a).,