1. Phenomenon
When light crosses obliquely from one uniform medium into another, its direction usually changes — refraction.
Light refraction (intro)
Intro: bending air→water/glass; refraction vs incidence angle.
Light phenomenaReflection, intro refraction, mirrors and lenses.
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Light refraction (intro)
Explore refraction: air → water/glass bends toward the normal with n₁ sin i = n₂ sin r; the reverse bends away and past the critical angle C becomes total internal reflection. On a realistic 3D tank bench, aim the laser, flip the direction, switch media, and connect to everyday illusions.
Memory cue: n₁ sin i = n₂ sin r · denser side, smaller angle.
Key formulas
n₁ sin i = n₂ sin rsin C = n₂ / n₁
How to use
- Review rarer/denser media and the normal.
- Orbit the bench; vary i and the medium; check Snell's law in the readouts.
- Switch to up-going: watch r > i, find C, and see TIR.
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Related knowledge
Read section by section — and check n₁ sin i = n₂ sin r on the 3D tank bench.
2. Rarer vs denser
- Larger n = optically denser (relative).
- Air ≈1, water ≈1.33, glass ≈1.5.
3. Laws
- Coplanar; refracted and incident on opposite sides of the normal.
- Air → water/glass: r < i (toward the normal).
n₁ sin i = n₂ sin rSnell's law.
4. Normal incidence
At i = 0° the ray passes straight through the interface without bending.
- Normal incidence: i = r = 0°, Snell still holds.
5. Reversibility & reverse
Paths are reversible: dense → air gives r > i, bending away from the normal.
- Reverse an air→water path and i and r swap — Snell still holds.
- Switch the bench to up-going to see dense → air bending.
6. Critical angle & TIR
Dense → rare: at the critical angle C, r reaches 90°; past C no ray refracts — everything reflects (TIR).
- TIR only dense → rare; never air → denser.
sin C = n₂ / n₁Air n₂ = 1: water C ≈ 48.6°, glass C ≈ 41.8°.
7. Lab
- Draw the normal; measure i and r.
- Vary i and check Snell's law.
- Flip to up-going, find C, and watch TIR.
8. Life & pitfalls
- Shallow-looking pools and bent chopsticks come from refraction.
- Normal incidence: no bend (i = r = 0).
- Optical fibers guide light by total internal reflection.
Applications
- Pools look shallower and sticks appear bent because of refraction.
- Glasses, camera lenses, and fiber coupling steer light by refraction.
FAQ
Why does a pool look shallower?
Rays bend leaving water; the eye traces them straight and perceives a raised bottom.
Why does a chopstick look bent in water?
Leaving water, the ray bends away from the normal; the eye traces it back, raising the submerged part so it looks kinked upward.
What is total internal reflection and when does it happen?
Going dense → rare, once i exceeds the critical angle C the refracted ray disappears and all light reflects. C ≈ 48.6° water→air, ≈ 41.8° glass→air.
Air into water: is the refraction angle larger or smaller?
Usually smaller — the ray bends toward the normal.
Is refraction reversible?
Yes. Reverse the exit ray and it retraces the path.