Q1: A lens or combination of lenses in which the following defect is completely eliminated is called aplanatic

ANS:A - spherical aberration

A lens or combination of lenses in which the following defect is completely eliminated is called aplanatic: spherical aberration. Aplanatic refers to a lens system that has been designed or configured to eliminate spherical aberration. Spherical aberration occurs when light rays passing through different parts of a lens or lens system focus at different points, leading to blurred or distorted images, particularly in off-axis portions of the lens. Aplanatic lenses are designed to ensure that light rays from a single point source converge to a single focal point, regardless of their distance from the lens's optical axis. This correction is crucial for improving the image quality and sharpness, especially in optical systems such as telescopes, microscopes, and camera lenses.