An achromaic convergent system of focal length `+20cm` is made of two lenses (in contact) of materials having dispersive powers in the ratio `1:2`.Their focal lenths must be respectively

An achromatic doublet of focal length 90 cm is to be made of two lenses, the material one having 1.5 times the dispersive power of the other. The doublet is converging type. Find the focal length of each of lens.

A achromatic combination is made with a lens of focal length f and dispersive power omega with a lens having dispersive power of 2omega . The focal length of second will be

A convergent lens of focal length 20cm and made of a material with refractive index 1.1 is immersed in water. The lens will behave as a

Two lenses of focal lengths -20cm and +10cm are put in contact.Find power of the combination

Two thin lenses of focal lengths 20 cm and 25 cm are placed in contact. The effective power of the combinations is

Three lenses lenses of focal length 20 cm each are kept in contact with each other . Find the power of their combination.

Two thin lenses of focal length +10 cm -5 cm are kept in contact. What is the (i) focal length and (ii) power of the combination ?