Question

If a convex lens has a radius of curvature 10 cm and 20 cm and refractive index 1 5 then find its
focal length.

Answer 1

Answer:

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Explanation:

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Class 12

>>Physics

>>Ray Optics and Optical Instruments

>>Combination of Mirror and Lenses

>>A plano - convex lens with a radius of c

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A plano-convex lens with a radius of curvature 10 cm and refractive index 1.5 is pressed with another identical plano-convex lens so that a double convex lens is formed. The focal length of the resultant lens is :

Medium

Solution

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Correct option is B)

Given : R=10cm

μ=1.5

Focal length of plano-convex lens,

f

1

=(μ

−

1)(

R

1

)

f

1

=(1.5−1)(

10

1

)

⇒f=20cm

Now two lenses of focal length f=20 cm are placed in contact with each other.

Equivalent focal length of two lenses :

f

eq

=

f

1

+f

2

f

1

f

2

=

40

20×20

=10 cm