A double convex lens of refractive index 1.5 has a radius of curvature R each. The focal length of the lens is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( R \)
The focal length of a thin lens follows from the lens maker's formula.
\( \displaystyle \frac{1}{f} = (\mu - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
For a double convex lens with equal radii, using the Cartesian sign convention (light from left to right), \( R_1 = +R \) and \( R_2 = -R \).
\( \displaystyle \frac{1}{f} = (1.5 - 1)\left(\frac{1}{R} + \frac{1}{R}\right) = 0.5 \times \frac{2}{R} = \frac{1}{R} \)
\( \displaystyle f = R \)