An achromatic convergent lens of equivalent focal length +20 cm is made of two lenses (in contact) of materials having dispersive powers in the ratio of 1:2 and having focal lengths $f_{1}$ and $f_{2}$ . The correct option is
$f_{1} =$ 10cm, $f_{2} =$ 20cm
$f_{1} =$ 20cm, $f_{2} =$ -10cm
$f_{1} =$ 20cm, $f_{2} =$ 10cm
$f_{1} =$ 10cm, $f_{2} =$ -20cm

Question

Toppr Admin · Accepted Answer

Using- the additive property of powers of a lens-xA0-P-P1-P21f-1f1-1f2Also- the focal length is directly proportional to the dispersive power-

An achromatic convergent lens of equivalent focal length +20 cm is made of two lenses (in contact) of materials having dispersive powers in the ratio of 1:2 and having focal lengths f1 and f2. The correct option isf1= 10cm, f2= 20cmf1= 20cm,f2= -10cmf1= 20cm, f2= 10cmf1=10cm, f2= -20cm