A point on the principal axis of a lens such that a ray of light passing throughthis point emerges parallel to its direction of incidence is called as

The point you're referring to is known as the "Principal Focus" or simply "Focus" of a lens. Here's the step-by-step explanation:

1. When a ray of light passes through a lens, it bends due to refraction.

2. The principal axis of a lens is the imaginary line passing through the center of the lens.

3. If the rays of light are incident parallel to the principal axis, after refraction, they either converge at a point (in a converging lens) or appear to diverge from a point (in a diverging lens).

4. This point is called the Principal Focus of the lens.

5. Conversely, if the rays of light are incident towards the lens such that they are directed towards the focus, they will emerge parallel to the principal axis after refraction.

So, the Principal Focus of a lens is the point on the principal axis such that a ray of light passing through this point emerges parallel to its direction of incidence.