Why is the emergent ray?

An emergent ray is the ray of light that exits a transparent medium—such as a glass slab or prism—after being refracted within it. It represents the final path of the light ray after it has passed through and emerged from the medium.

When light travels through a medium like a glass slab, it undergoes the following transitions:

• The ray that first enters the slab is called the incident ray.
• Inside the slab, the ray bends and becomes the refracted ray.
• The ray that finally exits the slab is known as the emergent ray.

Key Features of the Emergent Ray

• Parallel to the Incident Ray: In a glass slab with parallel sides, the emergent ray is parallel to the incident ray but displaced sideways.
• Bends Away from the Normal: When light moves from a denser medium (like glass) to a rarer medium (like air), it bends away from the normal.
• Undergoes Two Refractions: The light ray bends twice:
  - First, when it enters the medium (air to glass)
  - Second, when it exits (glass to air)
• Lateral Displacement: Although the emergent ray runs parallel to the incident ray, it is laterally displaced (shifted slightly sideways).