AMU 2026 — science PYQ

1. Understanding the Terms

• Aperture: The effective diameter of the light-transmitting area of a lens. Using a "stop" (an iris or diaphragm blocks outer rays) reduces this diameter.

• Spherical Aberration: An optical defect where light rays striking the outer edges (marginal rays) of a lens focus closer to the lens than light rays passing near the center (paraxial rays).

• Chromatic Aberration: An optical defect caused by the dispersion of light, where a lens fails to focus all colors to the same convergence point because the refractive index varies with wavelength.

2. Effect of Reducing Aperture

A. Effect on Spherical Aberration

Spherical aberration primarily happens because marginal rays (passing through the outer periphery of the lens) bend more sharply than paraxial rays. When you reduce the aperture using a physical stop, you block these peripheral marginal rays. Since only paraxial rays near the principal axis are allowed to pass through, the difference in focus points vanishes, causing a significant decrease in spherical aberration.

B. Effect on Chromatic Aberration

Chromatic aberration depends directly on the dispersive power of the material of the lens ($\omega$) and its focal length ($f$). The standard longitudinal chromatic aberration ($x_{\text{CA}}$) for a distant object is given by:

$$x_{\text{CA}}=\omega f$$

Since reducing the aperture size does not alter the underlying material properties (refractive index $n$) or the geometry responsible for focal length ($f$), the overall spread between the red and violet focal points stays identical along the principal axis. Thus, chromatic aberration remains unchanged relative to the focal point.

Conclusion

By limiting the clear opening of the lens, the extreme marginal rays are eliminated, reducing the longitudinal spherical aberration, while the material-dependent chromatic spreading remains constant.

Correct Answer

The correct option is (b) Spherical aberration decreases, but chromatic aberration remains unchanged.