Optical materials in lenses – properties and pairing of flint glass with crown glass Which statement best describes flint glass in relation to crown glass and its use in achromatic doublets for chromatic correction?

Introduction / Context:
Surveying and optical instruments rely on compound lenses designed to minimize chromatic aberration. Two classic optical glasses—crown and flint—are paired to form achromatic doublets. Understanding their relative refractive indices and dispersions helps explain why one element is made of crown and the other of flint. This question asks you to identify the correct statement about flint glass and its pairing with crown glass.

Given Data / Assumptions:

• Crown glass: relatively lower refractive index and lower dispersion.
• Flint glass: relatively higher refractive index and significantly higher dispersion.
• Achromatic doublet: convex crown combined with concave flint to cancel secondary spectrum while achieving desired focal length.

Concept / Approach:

Chromatic aberration occurs because refractive index varies with wavelength. By pairing a positive (convex) element of crown glass with a negative (concave) element of flint glass, designers balance focal powers so that color dispersion introduced by one element is counteracted by the other. The success of this scheme relies on flint’s higher dispersion relative to crown. However, the refractive index of flint glass is not “double” that of crown; it is only moderately higher, so statements exaggerating the difference are incorrect.

Step-by-Step Solution:

Verification / Alternative check:

Optical design references list typical n_D values (e.g., crown ~1.52; flint ~1.62) and Abbe numbers showing flint’s higher dispersion, corroborating the achromat pairing.

Why Other Options Are Wrong:

Option B exaggerates the index difference and claims solo use for convergence, which is not standard for achromats.

Option C reverses the usual roles (it states convex flint and concave crown), which is contrary to the conventional achromatic doublet design.

Option D cannot be true because B and C are incorrect.

Common Pitfalls:

Assuming “higher refractive index” means vastly larger; confusing which element is positive or negative in the doublet; overlooking that dispersion, not just index, drives achromat choice.

Final Answer:

Flint glass has a slightly higher refractive index than crown glass and higher dispersion, making it suitable for use with crown glass to correct chromatic aberration.