“During refraction of light through the glass slab, incident ray and emergent ray are parallel to each other.” Explain.

During the refraction of light through a glass slab, the incident ray and the emergent ray are parallel to each other due to the way light behaves when it passes through the different media (air and glass) and the shape of the slab. Here’s a detailed explanation:
1. Refraction at the First Surface:
When a light ray strikes the surface of a glass slab (which is denser than air), it undergoes refraction (bending of light) at the air-glass interface. According to Snell’s Law, the angle of incidence ii and the angle of refraction r1r_1 are related by:
sin⁡isin⁡r1=nairnglass\frac{\sin i}{\sin r_1} = \frac{n_{\text{air}}}{n_{\text{glass}}}
where nairn_{\text{air}} and nglassn_{\text{glass}} are the refractive indices of air and glass, respectively. Since the glass is denser, the light bends towards the normal.
2. Traveling Through the Slab:
Once the light ray enters the glass slab, it travels through the material without changing direction (since the slab is uniform and the light moves in a straight line through the glass). The light ray continues its path at the same angle as it entered the glass.
3. Refraction at the Second Surface:
When the light ray reaches the opposite surface of the glass slab (the glass-air interface), it again undergoes refraction. This time, the light moves from the denser medium (glass) to the less dense medium (air). According to Snell’s Law, the angle of refraction at the second surface r2r_2 is greater than the angle of incidence r1r_1, and the light bends away from the normal.
Since the slab is of uniform thickness and the angles at both interfaces are related (i.e., the refraction at the first interface is compensated by the refraction at the second interface), the ray emerges from the glass slab parallel to the incident ray.
4. Why Are the Incident and Emergent Rays Parallel?
• The amount by which the light bends while entering the glass slab (at the first surface) is exactly compensated by the bending at the second surface. Thus, the overall change in direction due to refraction is nullified, and the emergent ray is parallel to the incident ray.
• The displacement between the incident and emergent rays occurs, but the direction remains the same, making them parallel.
Conclusion:
When light passes through a glass slab, the angle of refraction at the first surface is compensated by the angle of refraction at the second surface, resulting in the incident and emergent rays being parallel to each other. The glass slab essentially shifts the light ray without altering its overall direction.