1. Direction: dense → rare

Here light goes from glass or water into air. Relative to air they are denser: r > i and the ray bends away from the normal.

2. Snell's law

• Coplanar; refracted and incident on opposite sides of the normal.
• Until TIR, readouts satisfy n₁sin i = n₂sin r.

• n₁ sin i = n₂ sin rSnell's law (before TIR).

3. Critical angle

The incidence that makes r = 90° is the critical angle C. Beyond it, light cannot refract into the rarer medium.

• Glass n≈1.5 → C≈41.8°; water n≈1.33 → C≈48.8°.

• sin C = n₂/n₁With air n₂≈1, sin C = 1/n₁.

4. TIR conditions

• Light must travel from denser toward rarer medium.
• Incidence exceeds critical: i > C.
• Almost no light enters air; angle of reflection equals incidence.

5. Pin method for n (required)

1. On a semicircular glass block, pin the incident and refracted paths; measure i and r.
2. Find n from Snell's law (air/glass angles).
3. Or use critical angle: n = 1/sin C for glass→air.

6. Apps & pitfalls

• Optical fiber: core n > cladding; large-angle rays TIR and stay guided.
• Diamond sparkle: high n → small C → more rays TIR before exiting the table.
• Pitfall: air→water never shows TIR; need dense→rare and i>C.