JEE Advanced 2020 Full Test 5 Paper 1

‌

1

F

=‌

(µ−1)

R

=‌

1

20

⇒20=‌

R

(µ−1)

..............(1)
If equiconvex lens of f=20
Then ‌

1

20

=(µ−1)‌

2

R

From equation (1) and (2) we conclude
∴R=40(µ−1)
That option (A) is correct. −‌

1

fL

=(µ−1)[‌

1

R

−‌

1

∞

]=‌

(µ−1)

R

for refraction at the convex surface
∴ power PL=‌

1

fL

=−‌

(µ−1)

R

For reflection at the silvered plane surface
FM=α
∴ power PM=0
For refraction at the convex surface again
PL=‌

(µ−1)

R

Hence power of the system
P=PL+PM+PL
=2PL+PM=−2‌

(µ−1)

R

∴ focal length of the system
F=‌

1

P

=−‌

R

2(µ−1)

=−10cm
u=−15cm