Question

Two equiconvex lens of radii 30 cm and 20 cm have same refractive indices of 4/3
Then find the
focal length of the combination when they are in contact​

Answer 1

PHYSICS

There is an equiconvex lens of focal length of 20cm. If the lens is cut into tow equal parts perpendicular to the optical axis, the focal length of each part will be

December 27, 2019avatar

Praveen Champ

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ANSWER

The focal length of equiconvex lens is 20 cm

f

1

=(μ−1)(

R

1

1

−

R

2

1

)

forequiconvexlensR

1

=−R

2

=R

thus,

20

1

=(μ−1)(

R

1

+

R

1

)

⇒R=40(μ−1)

when lens is cut into two equal parts perpendicular to optical axis

then,

R

1

=∞,R

2

=R

thus

f

′

1

=(μ−1)(

R

1

1

−

R

2

1

)

f

′

1

=(μ−1)(

R

1

)

by putting value of R we get,

f

′

1

=(μ−1)(

40(μ−1)

1

)

⇒f

′

=40cm

new focal length of each part is 40 cm , becomes double