Nature of Light Beam Traveling Across Media Interface

Based on the principles of wave optics and refraction across a plane interface:

1. Relation Between Rays and Wavefronts:
   In an isotropic and homogeneous medium, light rays (the directions of energy propagation) are always perpendicular (normal) to the wavefronts.

2. Nature of the Beam in the First Medium:
   A plane wavefront has a constant normal vector at all points across its surface. Since the rays are everywhere orthogonal to the planar wavefronts, they are mutually parallel to one another. Therefore, the incident light beam travels as a parallel beam in the first medium.

3. Behavior at the Plane Interface:
   When a parallel beam of light (represented by plane wavefronts) strikes a planar interface separating medium-1 and medium-2, every ray in the beam is incident at the exact same angle of incidence, $\theta_1$.

   By Snell's Law of refraction:
   $n_1 \sin \theta_1 = n_2 \sin \theta_2$

   Since $n_1$, $n_2$, and $\theta_1$ are constant for all rays of the incident beam, the angle of refraction $\theta_2$ is identical for all transmitted rays.

4. Nature of the Beam in the Second Medium:
   Because all transmitted rays emerge at the same angle $\theta_2$ relative to the surface normal, they remain mutually parallel to each other in the second medium. The corresponding refracted wavefronts are also planar. Thus, the light propagates as a parallel beam in the second medium as well.

Therefore, light travels as a parallel beam in each medium.

Correct Option: (A)