CBSE Class 12 Sample Paper for 2022 Boards [Term 2] - Physics
Question 10 - CBSE Class 12 Sample Paper for 2022 Boards [Term 2] - Physics - Solutions to CBSE Sample Paper - Physics Class 12
Last updated at March 29, 2022 by Teachoo
The focal length of a convex lens made of glass of refractive index (1.5) is 20 cm. What will be its new focal length when placed in a medium of refractive index 1.25 ? Is focal length positive or negative? What does it signify?Â
Answer
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Question 10
The focal length of a convex lens made of glass of refractive index (1.5) is 20 cm. What will be its new focal length when placed in a medium of refractive index 1.25 ? Is focal length positive or negative? What does it signify?
Given đ_(đ_đ )=1.5
Focal length of the given convex lens when it is placed in air is
đ=+20 đđ
Refractive index of the given medium with respect to air is
đ_(đ_đ )=1.25
New focal length of the given convex lens when placed in a medium is fâ
1/đ=(đ_(đ_đ )â1) [(1/đ _1 )+(1/đ ^2 )]
Given đ_(đ_đ )=1.5
Focal length of the given convex lens when it is placed in air is
đ=+20 đđ
Refractive index of the given medium with respect to air is
đ_(đ_đ )=1.25
New focal length of the given convex lens when placed in a medium is fâ
1/đ=(đ_(đ_đ )â1) [(1/đ _1 )+(1/đ ^2 )]
1/đ=(đ_(đ_đ )â1) [(1/đ _1 )+(1/đ ^2 )]
Dividing (A) by (B), we get
đâ˛/đ=((đ_(đ_đ ) â 1))/((đ_(đ_đ )â 1))=((1.5 â 1))/((1.2 â 1))=0.5/0.2=5/2=2.5
đ^â˛=2.5đ=(2.5 Ă20)đđ= +50 đđ
as đ_(đ_đ )= đ_đ/đ_đ =1.5/1.25=1.2
New focal length is positive.
The significance of the positive sign of the focal length is that given
Convex lenses still converging in the given medium.
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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 12 years. He provides courses for Maths and Science at Teachoo.