Question

An achromatic telescope objective is to be made by combining the lenses of flint and crown glasses. The proper choice is-

• A. Convergent of crown and divergent of flint
• B. Divergent of crown and convergent of flint
• C. Both divergent
• D. Both convergent

Answer 1

The correct option is A Convergent of crown and divergent of flint

An achromatic objective lens is a compound lens of two glasses, one is a concave made of flint glass which has a higher dispersion & the other is convex made of crown glass which is of lower dispersion.

For best combination, ${\mu }_{\text{crown}}<{\mu }_{\text{flint}}$

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(A\right)$ is the correct answer.