When monocromatic orange light is used instead of violet light in a convex lens, its focal length will:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → increase
The focal length of a lens depends on the refractive index of the glass through the lens maker’s formula.
\( \displaystyle \frac{1}{f} = (\mu-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right) \)
The refractive index is larger for shorter wavelengths, so \( \mu_{\mathrm{violet}} \gt \mu_{\mathrm{orange}} \). Orange light has the longer wavelength, hence smaller \( \mu \), smaller \( 1/f \) and therefore a larger focal length.
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Ray Optics and Optical Instruments – …
