AP EAMCET Engineering 2013 Solved Paper

From lens maker’s formula

1

f

=(gmµ−1)(

1

R1

−

1

R2

)
Now,
‌gmµ=

gµ

mµ

=

1.5

1.75

For concave lens as shown in the figure, in this case
R1=−R and R2=+R

1

f

=(

1.5

1.75

−1)(−

1

R

−

1

R

) =+

0.25×2

1.75R

⇒ f=+3.5R
The positive sign shows that the lens behaves as a convergent lens.