The Bending Ray
Snell's law — why light kinks at every surface
What this lesson covers
Why it matters
A straw in a glass of water looks broken at the surface. The straw is fine — it's the LIGHT that bends on its way to you.
The idea in plain words
One ratio rules every refraction. Tap the terms.
sin i ÷ sin r = μ
A ray hits water (μ = 1.33) at i = 45°. Find r.
r ≈ 32°
- degrees: i — angle of incidence, measured from the NORMAL (not the surface)
- degrees: r — angle of refraction, also from the normal
- no unit: μ — refractive index — how strongly the medium slows light (μ = c/v)
- sin r = sin 45° ÷ 1.33 = 0.707 ÷ 1.33 = 0.53
- r = sin⁻¹(0.53)
Predict first
A light ray passes from air into glass. Which way does it bend?
Entering a denser medium, light slows and turns TOWARDS the normal. The exact amount is Snell's law: sin i / sin r = μ.
- Towards the normal — light slows down in glass — correct
- Away from the normal
- It doesn't bend, it only dims
What you do
Aim the ray into water, glass and diamond. Watch r lag further and further behind i — the denser the medium, the harder the turn.
Check yourself
When light enters a denser medium, its…
The medium can slow the wave and squeeze its wavelength, but the source fixes the frequency.
A ray hits the surface along the normal (i = 0°). It…
sin 0 = 0, so r = 0 too — no bending along the normal. That's why you aimed off-normal to see the kink.
Diamond bends light hardest because…
Bending follows μ, not hardness or clarity — light crawls at c/2.42 inside diamond.
- speed and wavelength decrease; frequency stays the same — correct
- speed increases
- frequency drops
- passes straight through, unbent — correct
- bends the most
- reflects back
- its refractive index is highest (μ ≈ 2.42) — correct
- it is the hardest material
- it is crystal clear