When a light ray incident on a rectangular glass slab....it gets refracted by glass slab.....and this result..... the refraction it comes out from glass......Since.... the glass slab is rectangular.....and both sides of the glass slab have the same medium.....so... light refracts in such a way that incident and emergent rays are parallel to itself....okk

Introduction:
When a light ray passes through a rectangular glass slab immersed in any medium, it undergoes a phenomenon known as refraction. Refraction occurs due to the change in the speed of light as it travels from one medium to another. The refraction of light causes the light ray incident on the slab to emerge parallel to itself.

Explanation:
To understand why the light ray emerges parallel to itself, let's break down the process step by step:

1. Incident Ray:
The incident ray is the incoming light ray that strikes the rectangular glass slab. It travels from one medium (e.g., air) and enters another medium (e.g., glass).

2. Refraction at the Entry Point:
When the incident ray enters the slab, it undergoes refraction at the entry point. This refraction is governed by Snell's law, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. As a result, the light ray bends towards the normal (an imaginary line perpendicular to the surface of the slab) as it enters the glass slab.

3. Propagation Inside the Slab:
As the light ray propagates inside the slab, it undergoes multiple internal reflections at the two parallel surfaces of the slab. These internal reflections occur because the light ray hits the surfaces at an angle greater than the critical angle, causing total internal reflection.

4. Refraction at the Exit Point:
When the light ray reaches the exit point of the slab, it again undergoes refraction at the interface between the slab and the medium it is emerging into (e.g., air). The angle of refraction is determined by Snell's law, and it is such that the light ray bends away from the normal.

5. Emergence Parallel to Itself:
The key point to note is that the angle of incidence at the exit point is equal to the angle of incidence at the entry point. This is because the angles of incidence and refraction for a given light ray are always equal on opposite sides of a glass slab. Due to this symmetry, the light ray emerges parallel to its original direction.

Conclusion:
In summary, a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself due to the principles of refraction and the symmetry of angles of incidence and refraction. The refraction at the entry and exit points, as well as the internal reflections inside the slab, result in the light ray maintaining its original direction, albeit slightly displaced.