Question

Power of convex lens is $5$ D. Four students measure object and image distances as shown. 

• A. Students A & B are correct
• B. All are correct
• C. Student A is wrong
• D. Students C & D are wrong

Hint:

Always convert focal length into correct units before applying lens formula.

Answer 1

The Correct Option is C

Step 1: Finding focal length of lens.
\[ P = \dfrac{1}{f} \Rightarrow f = \dfrac{1}{5} \text{ m} = 20 \text{ cm} \]
Step 2: Checking each student's data using lens formula.
Lens formula:
\[ \dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u} \]
Step 3: Verifying Student A.
\[ \dfrac{1}{f} = \dfrac{1}{37} + \dfrac{1}{35} \Rightarrow f \neq 20 \text{ cm} \]
Step 4: Verifying Students B, C and D.
\[ \dfrac{1}{60} + \dfrac{1}{30} = \dfrac{1}{20} \]
\[ \dfrac{1}{30} + \dfrac{1}{60} = \dfrac{1}{20} \]
\[ \dfrac{1}{25} + \dfrac{1}{100} = \dfrac{1}{20} \]
Step 5: Conclusion.
Student A's observation is incorrect.