Snell’s Law, explained.
Snell’s law relates incident and refracted angles through refractive indices: n₁ sin θ₁ = n₂ sin θ₂.
Why it happens
The drawing uses indices one and 1.5 and measures angles from the normal. Reversing media permits an incident angle above the glass-to-air critical angle, where no propagating transmitted ray exists.
Snell’s law relates incident and refracted angles through refractive indices: n₁ sin θ₁ = n₂ sin θ₂.
Read the result
Compare angles from the normal. A total-internal-reflection result means no propagating refracted ray under these indices, not an angle beyond ninety degrees.
A worked example
Crossing the critical-angle boundary
Begin with air-to-glass incidence at thirty-five degrees and inspect the smaller transmitted angle.
Reverse the media to glass-to-air, then raise incidence above forty-two degrees.
Read total internal reflection rather than expecting the formula to return a physical transmitted angle.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
The drawing uses indices one and 1.5 and measures angles from the normal. Reversing media permits an incident angle above the glass-to-air critical angle, where no propagating transmitted ray exists.
Where this idea is useful
A practical use
Interpret why an underwater object appears displaced and why a fibre can retain a ray.
A common misconception
“Light always bends toward the normal on entering another material.”
The direction depends on the relative refractive indices. Entering a lower-index medium bends the transmitted ray away from the normal.
What this explanation leaves out
- Ideal isotropic media are assumed. Fresnel intensities, dispersion, absorption and polarisation are omitted; the transmitted direction alone does not specify transmitted energy.
Does the ray drawing say how much energy was reflected?
No. It shows directions and the critical case; partial reflection intensities and polarisation are omitted.
Are you comparing an angle with the normal or with the boundary surface?
Associated thinkers
Associations marked provisional are awaiting source review.
Further reading
Explore the original research or the teaching reference behind this experiment.