Question

The radii of curvature of a double convex lens are 30 cm and 20 cm, respectively. If the refractive index of the lens material is 1.52, what is its focal length?) *​

Answer 1

Answer:

Correct option is

D

−120cm

From lens maker's formula,

f

1

=(μ−1)(

R

1

1

−

R

2

1

)

We have,

Refractive index, μ=1.5 and R

1

=20 cm and R

2

=30 cm

f

1

=(1.5−1)(

R

1

1

−

R

2

1

)

f

1

=(1.5−1)(

−20

1

−

−30

1

)

f

1

=(1.5−1)

20×30

−30+20

f=−120cm