Question

A focal plane is

• A. parallel to the principal axis
• B. perpendicular to the principal axis
• C. at 45° to the principal axis
• D. at 60° to the principal axis

Answer 1

The Correct Option is B

The focal plane is defined as the plane that passes through the focus of a lens or mirror and is perpendicular to the principal axis.  

The principal axis is the straight line that passes through the center of curvature and the optical center (or pole) of the lens or mirror. 

Since the focal plane contains all points where light rays parallel to the principal axis converge (in a converging system) or appear to diverge from (in a diverging system), it must be perpendicular to the axis along which the lens or mirror is aligned. 

Answer: Perpendicular to the principal axis