## Question

An achromatic objective lens of a telescope is made of two lenses of crown and flint glasses. If the dispersive powers of crown and flint glasses are 0.02 and 0.03, and the focal length of the lens made from crown glass is 16.0 cm, find the focal length of the lens made of flint glass. Also find the focal length of the combination.

### Solution

Let *f* be the focal length of the crown lens, and *f’* that of the flint lens. Then, for ‘achromatic’ objective lens, we have

Where ω and ω*’* are dispersive powers of crown and flint glasses respectively. Putting the given values:

The focal length of the lens combination is given by

#### SIMILAR QUESTIONS

A real image is formed by a lens at a distance of 20 cm from the lens. The image shifts towards the combination by 10 cm when a second lens is brought in contact with the first lens. Determine the power of the second lens.

Two lenses of powers +2.50 D and –3.75 D are combined to form a lens combination. Determine the focal-length of this combination.

Two lenses of powers +12 D and –2D are placed in contact. This combination is placed between an object and a screen which are 60 cm apart. If this combination is kept in two positions between object and screen, a real image of the object is formed on the screen. Calculate the distance between the two positions of the combination.

The refractive indices of the material of a lens for yellow and red colours are 1.54 and 1.51 respectively. The focal length of the lens for yellow light is 34.0 cm. Calculate the focal length of the lens for red light.

The refractive indices of the material of a lens for violet and red colours are 1.56 and 1.54 respectively. If the focal length of the lens for violet light is 40 cm, then calculate (i) focal length of the lens for red light. (ii) Longitudinal chromatic aberration of the lens and (iii) Dispersive power of the material of the lens.

The refractive indices of the material of a lens for violet, yellow and red colours of light are respectively 1.66, 1.64 and 1.62. The mean focal length of the lens is 10 cm. Determine the chromatic aberration of the lens between the violet and the red colours.

An achromatic convergent lens combination is made by keeping in contact a convex lens of focal length 20 cm and a concave lens of focal length 30 cm. Dispersive power of the material of convex lens is 0.18. Find the dispersive power of the material of the concave lens, and the focal length of the combination.

The dispersive powers of crown and flint glasses are 0.02 and 0.04 respectively. Find the focal length of the convex lens of crown glass which forms achromatic doublet with a concave lens of flint glass of focal length 80 cm.

For an achromatic combination of lenses, the ratio of dispersive powers of materials used in lenses is 2:3. One lens has focal length equal to 10 cm. Find focal length, power and nature of second lens.

The focal length of an achromatic combination of two lenses is 60 cm. If the ratio of the dispersive powers of the two lenses is 5/3, then find out the focal length of the two lenses.