Question

A point on the principal axis of a lens, to which light rays parallel to the principal axis, converges after passing through the lens is_________.
(A) Centre of curvature
(B) Origin
(C) Focus
(D) Optical center

Answer 1

The focal length is defined as the distance of the principal focus from the center of the lens. The principal focus is the point on which object is placed such that the image is formed at the infinity. Focus can be also seen as the point where light rays coming from an object converges.

Complete step-by-step answer:
A point on the principal axis of a lens, to which light rays parallel to the principal axes, converges after passing through the lens is the principal focus.

Principal focus:
Concave lens: the principal focus of a concave lens is a point where rays parallel to the principal axis converge, after refraction through the lens.
Convex lens: the principal focus of a concave lens is a point, from where rays parallel to the principal axis appear to diverge, after refraction through the lens.
Focal length (f): The distance between the optical center and principal focus of the lens is called the focal length.
Hence, option (C) is the correct option.
Additional information:
The power of a lens is defined as the reciprocal of its focal length where the focal length is measured in meter. The unit of power of lens is Dioptre. Mathematically given below.
$P = \dfrac{1}{f}$
Where,
P=power of the lens,
F=focal length
The principal axis of a spherical lens is an imaginary line passing through the two centers of curvature of the lens.
Note:
For a convex lens, if the object is at first focus then the image is formed at infinity. Similarly, if an object is at
Infinity that means rays of light are coming from infinity the image is formed at the second focus.
There are three rules of refraction which are as follows;
The incident ray traveling parallel to the principal axis of the lens, refracts through the lens, and pass through the focal point or focus on the opposite side of the lens.
The incident ray traveling through the focal or focus point will refract through the lens and the refracted ray is parallel to the principal axis.
An incident ray which passes through the center of the lens travels in the same direction when entered through the lens.