Question

calculate the effective power of two lences of 5 D and -5D in a contact. what the nature of the resultant lens formed after the combination.
​

Answer 1

Three lenses in contact have a combined focal length of 12cm. When the third lens is removed the combined focal length becomes

7

60

cm. What is the focal length of the third lens?

Let f

1

, f

2

and f

3

be three focal lengths of the lenses.

Combined Focal length: f=12 cm

So,

f

1

=

f

1

1

+

f

2

1

+

f

3

1

When the third lens is removed,

f

′′

1

=

f

1

1

+

f

2

1

, where f

′′

=

7

60

This means,

12

1

=

7

60

1

+

f

3

1

⇒ f

3

=−30 cm

Since f

3

is negative, it is a diverging lens having focal length 30 cm.