## Question

Two glasses have dispersive powers in the ratio of 2 : 3. These glasses are used in the manufacture of an achromatic objective of focal length 20 cm. What are the focal lengths of the two lenses of the objective?

### Solution

6.67 cm, –10 cm

For an achromatic combination

…(i)

The focal length of the combination is

Where *F* = +20 cm. Therefore

…(ii)

From Eqs (i) and (ii) we get

**. **

#### SIMILAR QUESTIONS

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