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# The objective lens of an optical Instrument is an achromat combination with a focal length of 90 cm. The two lenses possess dispersive powers 0.024 and 0.036 respectively and are in contact with each other. Then their focal lengths are

$\begin {array} {1 1} (1)\;-30\: cm, \: 45\: cm & \quad (2)\;45\: cm, 30\: cm \\ (3)\;30\: cm, -45\: cm & \quad (4)\;30\: cm, -30 \: cm \end {array}$

(3) 30 cm, -45 cm