Understanding the Principles
* Refraction: When light passes from one medium to another (like air to glass), its speed changes. This change in speed causes the light ray to bend, a phenomenon called refraction. The amount of bending depends on the angle of incidence and the refractive indices of the two media.
* Snell's Law: This law describes the relationship between the angles of incidence and refraction, and the refractive indices of the two media:
* n₁sinθ₁ = n₂sinθ₂
where:
* n₁ and n₂ are the refractive indices of the first and second medium, respectively.
* θ₁ is the angle of incidence.
* θ₂ is the angle of refraction.
The Scenario
1. Entering the Slab: When a light ray enters the glass slab from a different medium, it bends towards the normal (an imaginary line perpendicular to the surface) due to the higher refractive index of glass.
2. Traveling Through the Slab: The light ray continues traveling through the glass, maintaining a straight path (until it encounters the next interface).
3. Exiting the Slab: When the light ray exits the glass slab and returns to the original medium (air, for example), it bends away from the normal. The amount of bending is equal and opposite to the bending that occurred when it entered the slab.
Why Parallel?
The key is that the angles of incidence and emergence are equal, but with opposite signs. Here's why:
* Snell's Law Applied Twice: Snell's Law applies at both the entry and exit points. Because the glass slab is rectangular, the two surfaces are parallel. This means the normals at both surfaces are parallel.
* Reversal of Bending: The change in direction when entering the slab is reversed when exiting, resulting in the light ray emerging parallel to its original direction.
Important Note: The emergent ray will be parallel to the incident ray *only if the two surfaces of the glass slab are parallel*. If the surfaces are not parallel, the emergent ray will not be parallel to the incident ray.
Let me know if you'd like to explore specific examples or a diagram to illustrate this further!
