Browse by Stream
QnA
Home
QnA
Go To Your Study Dashboard

Get Answers to all your Questions

header-bg qa

To find the focal length of a convex mirror, a student records the following data :

Object Pin

 Convex Lens Convex Mirror

Image Pin
22.2 cm 32.2 cm 45.8 cm 71.2 cm

The focal length of the convex lens is f1 and that of mirror is f2. Then taking index correction to be negligibly small,  f1 and f2 are close to :

  • Option 1)

    f_{1} = 12.7\, cm\;\; \; \; \; \; \; f_{2}=7.8\, cm

  • Option 2)

    f_{1} = 7.8\, cm\;\; \; \; \; \; \; f_{2}=12.7\, cm

  • Option 3)

    f_{1} = 7.8\, cm\;\; \; \; \; \; \; f_{2}=25.4\, cm

  • Option 4)

    f_{1} = 15.6\, cm\;\; \; \; \; \; \; f_{2}=25.4\, cm

 

Answers (1)

As we learnt in

Thin lens formula -

\frac{1}{v}-\frac{1}{u}= \frac{1}{f}

 

- wherein

u \, and \, v are object and image distance from lens.

 

 Taking f2 = 12.07

USe mirror formula

\frac{1}{f}=\frac{1}{v}+\frac{1}{u}=\frac{1}{12.7}=\frac{1}{25.4}+\frac{1}{u}=\frac{1}{12.7}-\frac{1}{25.4}=\frac{1}{u}

u = 25.4 = v'

Now using len's formula

\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{f_{1}}=\frac{1}{24.4+13.6}+\frac{1}{10}

\frac{1}{f_{1}}=\frac{1}{39}+\frac{1}{10}\ \; \Rightarrow\ \;f_{1}=\frac{390}{49}=7.96

Correct option is 2.

 


Option 1)

f_{1} = 12.7\, cm\;\; \; \; \; \; \; f_{2}=7.8\, cm

This is an incorrect option.

Option 2)

f_{1} = 7.8\, cm\;\; \; \; \; \; \; f_{2}=12.7\, cm

This is the correct option.

Option 3)

f_{1} = 7.8\, cm\;\; \; \; \; \; \; f_{2}=25.4\, cm

This is an incorrect option.

Option 4)

f_{1} = 15.6\, cm\;\; \; \; \; \; \; f_{2}=25.4\, cm

This is an incorrect option.

Posted by

Vakul

View full answer

JEE Main high-scoring chapters and topics

Study 40% syllabus and score up to 100% marks in JEE

Similar Questions

Trending Articles/News

anam-khan

JEE Main 2025: Hardwired to excel? Know BTech electrical engineering cut-offs at NITs
anam-khan

JEE Main 2025 chemistry needs conceptual understanding; 5 tips to score high
anam-khan

JEE Main 2025 exam city intimation slip for session 1 soon; paper pattern, exams from January 22

Latest Question