UPSEE 2019 Solved Paper

Given, focal length of convex lens, f=10cm,
µlens =1.5 and µliquid =1.75
As, lens maker's formula

1

f

=(µlens −1)[

1

R1

−

1

R2

]
∵ For a equiconvex lens R1=R and R2=−R

1

f

=(µlens )[

1

R

−

1

(−R)

]
⇒

1

f

=(µlens −1)

2

R

⇒

1

10

=(1.5−1)

2

R

⇒R=10cm
If lens is dropped in liquid, then

1

f′

=[

µlens

µliq

−1]

2

R

⇒

1

f′

=[

1.5

1.75

−1]

2

10

⇒f′=−35.71cm
Hence, the lens behaves like a concave (negative focal length) lens of focal length 35cm.