Question

What is the focal length of double concave lens kept in air with two
les spherical surfaces of radii R, = 30cm and R, =60cm. Take refractive index
of lens as n = 1.5.​

Answer 1

Answer:

-60cm

Explanation:

1/f=(mu-1)(1/r1-1/r2)

=0.5(-1/30-1/60)

=0.5(-3/60)

=-1/40

=> f=-60 cm

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Answer 2

Answer:120cm

Explanation:By lens maker formula,

1/f=(n-1)(1/R1-1/R2)

Given, n=1.5

R1=30cm

R2=60cm

1/f=(1.5-1)(1/30-1/60)

1/f=0.5({2-1}/60)

1/f=0.5×1/60

1/f=5/10×1/60

1/f=5/600

1/f=1/120

f=120cm

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