Question

The focal length of the field lens (which is an achromatic combination of two lenses) of a telescope is $90$ cm. The dispersive powers of the two lenses in the combination are $0.024$ and $0.036$.The focal lengths of the two lenses are

• A. $30$ cm and $60$ cm
• B. $30$ cm and $-45$ cm
• C. $45$ cm and $90$ cm
• D. $15$ cm and $45$cm

Answer 1

Answer: (B)

$30$ cm and $-45$ cm

Answer 2

Answer (b) $30$ cm and $-45$ cm