Question

what is focal length of symmetric concave lens of radius of curvature 30cm kept in air?Take n=1.5 n=refractive index ​

Answer 1

Answer:

Correct option is

A

20 cm

If the curved surface of a plano-convex lens is silvered, the focal length of the lens is given by

F

1

=

R

2μ

⟹F=

2μ

R

=

2×1.5

30

=10cm

The real image will be equal to the size of the object if the object distance u=2F=2×10=20cm

Answer 2

Explanation:

Using lens makers formula

f

1

=(μ−1)[

R

1

1

−

R

2

1

] where R

1

=−ve

f

1

=(1.5−1)[

(−30)

1

−

60

1

]

f

1

=(0.5)[

60

3

]=−0.5×

20

1

f=−40cm