## Question

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.

### Solution

36.0 cm

If the focal lengths of a lens for yellow and red colours be *f _{Y}* and

*f*respectively and the refractive indices be

_{R}*n*and

_{Y}*n*respectively, then by the lens formula, we have

_{R}

Dividing eq. (i) by eq. (ii), we have

Here *f _{Y}* = 34.0 cm,

*n*= 1.54 and

_{Y}*n*= 1.51.

_{R}

#### SIMILAR QUESTIONS

The radius of curvature of both the surfaces of a double convex lens is *R*. The refractive index of the material of the lens is 1.5. When a point-source of light is placed on the principal axis of the lens at a distance of 20 cm in-front of the one surface of the lens, then rays emerge from the other surface normally. Determine the radius of curvature *R* of each surface of the lens.

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