Refraction & total internal reflection, explained.
Light changes direction when it crosses between media with different refractive indices at oblique incidence. Entering a higher-index medium bends the ray toward the normal. Total internal reflection requires travel from higher to lower index above the critical angle.
The relationship to remember
A worked example
From air (n = 1) into glass (n = 1.5) at 30°, the refracted angle is about 19.47°. From glass to air, the critical angle is about 41.81°.
Inside the default experiment
- sin θ₂ = n₁ sin θ₁ / n₂ = 0.3824.
- θ₂ = arcsin(0.3824) = 22.48° from the normal.
- Total internal reflection is not possible in this direction.
When this model applies
Flat interface between transparent media. Angles are measured from the normal; ray intensity and partial reflection are not modeled.
Make a prediction. Then test it.
Travel from glass to air. Find the angle where the refracted ray disappears.
Try the interactive experiment ↗Frequently asked questions
Wondering about the why? Start here.
When does total internal reflection occur?
When n₁ > n₂ and the incident angle exceeds arcsin(n₂/n₁). At the critical angle, the refracted ray travels along the boundary.
Does light always bend at a boundary?
At normal incidence, its direction stays the same. There is also no directional change if both media have the same refractive index.