Physics
Physics
Problem
A narrow beam of light is incident on a rectangular glass slab (refractive index 1.5) placed in air. After emerging from the opposite face into air, the emergent ray will a) will bend closer to the normal than the incident ray b) will emerge along the same straight line as the incident ray (without any lateral displacement) c) will travel parallel to the incident ray d) will retrace its path through the slabIntuitive explanation
The key insight is that refraction depends on the change in medium, not on how thick the slab is. When light enters the glass, Snell's law bends it toward the normal; when it exits back into air, Snell's law bends it away from the normal by the same amount. The two refractions are symmetric, so the emergent ray emerges parallel to the incident ray—just laterally displaced. The slab acts like a "parallel shifter," not a permanent deflector.
Intuitive explanation
The key insight is that refraction depends on the change in medium, not on how thick the slab is. When light enters the glass, Snell's law bends it toward the normal; when it exits back into air, Snell's law bends it away from the normal by the same amount. The two refractions are symmetric, so the emergent ray emerges parallel to the incident ray—just laterally displaced. The slab acts like a "parallel shifter," not a permanent deflector.