Question

The focal length of a convex lens is minimum for:

• A. red light
• B. violet light
• C. yellow light
• D. green light

Hint:

$f_{red}>f_{violet}$ because $\mu_{violet}>\mu_{red}$.

Answer 1

The Correct Option is B

Step 1: Concept
According to Lens Maker's Formula, $1/f = (\mu - 1)(1/R_{1} - 1/R_{2})$.
Step 2: Analysis
Refractive index ($\mu$) is higher for shorter wavelengths (Cauchy's equation). Violet light has the shortest wavelength in the visible spectrum, so it has the highest $\mu$.
Step 3: Conclusion
Since $f$ is inversely proportional to $(\mu - 1)$, the focal length is minimum for violet light.
Final Answer: (B)