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The radii of curvature of the two surfaces of a lens are 20 cm and 30 cm and the refractive index of the material of the lens is 1.5. If the lens is concave -convex, then the focal length of the lens is

A

24 cm

B

10 cm

C

15 cm

D

120 cm

Text Solution

Verified by Experts

The correct Answer is:
D
(d) The focal length of the lens,
`1/f=(mu-1)(1/R_(1)-1/R_(2))`
`1/f=(1.5-1)(1/20-1/30)rArr1/f=0.5((30-20)/600)`
`or1/f=1/2xx10/600=1/120`
or The focal length, f = 120 cm
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Knowledge Check

  • The radii of curvature of the two surface of a lens are 20 cm and 30 cm and the refractive index of the material of the lens is 1.5 . If the lens is concave - convex, then the focal length of the lens is :

    A
    24 cm
    B
    10 cm
    C
    15 cm
    D
    120 cm

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    A
    24 cm
    B
    10 cm
    C
    20 cm
    D
    24cm

  • The radii of curvature of the two surfaces of a lens are 20cm and 30 cm and the refractive index of the material of the lens is 1.5 . If the lens is concave - convex, then the focal length of lens is

    A
    `24 cm`
    B
    `10 cm`
    C
    `15 cm`
    D
    `120 cm`

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