Question

Q.1: Find out the focal length of the lens whose refractive index is 2. Also, the radius of curvatures of each surface is 20 cm and -35 cm respectively​

Answer 1

Answer:

The Radius of curvature of two surfaces of a convex lens are 10 cm and 20 cm respectively. The refrective index of glass is 1.5 then determine focal length of the lens.

Explanation:

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Answer 2

Given: The lens is made up of material having refractive index 2. The radius of curvature is 20 cm and -35 cm.

To find: Focal length of the lens

Explanation: Let the focal length be denoted by f, refractive index of lens be ul, first radius of curvature as r1 and second radius of curvature as r2.

ul = 2

r1 = 20 cm

r2= -35 cm

The formula for calculating focal length when radius of curvature and refractive index is given is:

=>$\frac{1}{f} = (ul - 1)( \frac{1}{r1} - \frac{1}{r2} )$

=>$\frac{1}{f} = (2 - 1)( \frac{1}{20} - \frac{1}{ - 35} )$

=>$\frac{1}{f} = \frac{1}{20} + \frac{1}{35}$

=>$\frac{1}{f} = \frac{7 + 4}{140}$

=>$\frac{1}{f} = \frac{11}{140}$

=>$f = \frac{140}{11}$

=>f = 12.73 cm

Therefore, the focal length of the lens whose refractive index is 2 is 12.73 cm.