Question

Define Principal axis of a lens.

Answer 1

Hint: An axis is simply an imaginary line that differentiates different coordinates or quadrants, or it can be defined as an imaginary line that passes through a center of symmetry. In an optical system containing a lens, whether it be a convex or concave lens, is a line through which light can pass through a mirror un-deviated.

Complete step by step answer:
When drawing an optical system containing a lens, we usually draw the lens first, and after that, we will draw a line or a ray that passes through the center of the lens un-deviated. This line is usually used as a measurement system in an optical system. The distances measured upward from the optical axis are taken as positive, while the distances below this axis are taken as negative. This axis is known as the principal axis.
A principal axis is defined as a line which is passing through both the curvatures of the lens. It can also be defined as the line which connects the two centers of the lens. A principal axis connecting the centers of two centres in a convex lens is given below.

Note: Focal Point: It is a point on the principal axis where all the light rays after passing through a lens converge (in the case of a convex lens) or fail to converge (in the case of a concave lens).
The distance from the center of the lens to the focal point is called the focal length of a lens. The focal length is negative for a concave lens while it is a positive quantity for a convex lens.
Vertical Axis: Vertical axis is an imaginary axis that splits the lens into two equal halves.