Question

a) A bi-convex spherical lens has a focal length of 20 cm. If it is split along a plane AB into two halves to form two plano- convex lenses, then what will be the focal length of each plano- convex lens? (b) What if the lens be cut along CD? ch focal length of each part​

Answer 1

Explanation:

We consider refraction from the three spherical surfaces,

v

μ

2

−

u

μ

1

=

R

μ

2

−μ

1

For refraction from first surface,

v

1

1.5

−

−40

1

=

+14

1.5−1

For refraction from second surface,

v

2

1.2

−

v

1

1.5

=

∞

1.2−1.5

For refraction from third surface,

v

3

1

−

v

2

1.2

=

−14

1−1.2

Adding the three equations gives,

v

3

1

=

40

−1

+

20

1

=

40

1

Hence, v

3

=40cm