Question

In lens maker formula \( \frac{1}{f} = (n-1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \), which focal length is used?

• A. Always first
• B. Always second
• C. First or second any one
• D. None of these

Hint:

Always consider the lens maker's formula with proper sign convention and determine the focal length based on whether the lens is convex or concave.

Answer 1

The Correct Option is B

Step 1: The lens maker's formula is given as: \[ \frac{1}{f} = (n-1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \] 

Step 2: Here, \( R_1 \) and \( R_2 \) are the radii of curvature of the two lens surfaces. The sign convention is used to determine which radius corresponds to the first and second surfaces. 

Step 3: Generally, the formula considers the second focal length in practical applications as it corresponds to the image formation side. \[ \therefore \text{The correct answer is } \text{Always second.} \] \[ \boxed{\text{Always second}} \]