Refraction (Snell's Law)
Send a light ray into the interface between two media — adjust the incidence angle and each medium's refractive index to see it bend according to Snell's law, or totally internally reflect once you pass the critical angle.
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What you can adjust
- Angle of incidence (θ₁)
- 0 – 89 °
- Refractive index n₁ (top medium)
- 1 – 2.5
- Refractive index n₂ (bottom medium)
- 1 – 2.5
Measured from the normal (the dashed line perpendicular to the interface), not from the interface itself.
The medium the ray starts in. 1.0 = vacuum/air, ~1.33 = water, ~1.5 = glass.
The medium the ray crosses into. If n₁ > n₂ and the incidence angle is steep enough, the ray totally internally reflects instead of crossing.
About this simulation
What it shows
A light ray hits the boundary between two materials and bends, with the angles measured from the normal line. Past a certain angle it reflects completely instead of crossing.
The key idea
Light changes direction when its speed changes, following Snell's law, n₁ sin θ₁ = n₂ sin θ₂. Going into a material with a higher index bends the ray towards the normal, and going out of one can cause total internal reflection beyond the critical angle.
Try this
Set Refractive index n₁ (top medium) to 1.5 and Refractive index n₂ (bottom medium) to 1, then raise Angle of incidence (θ₁) past about 42°: the ray reflects totally.