C.2 · IB Physics
Refraction (Snell’s law)
Send a ray of light across the boundary between two media. Drag the ray or use the sliders, watch it bend, and find the critical angle beyond which total internal reflection takes over.
Predict first
Start from the defaults (n₁ = 1.5, n₂ = 1.0, θ₁ = 35°). You raise the angle of incidence to 50°. What happens to the refracted ray?
The critical angle is sin⁻¹(n₂ ÷ n₁) = sin⁻¹(1.0 ÷ 1.5) = 41.8°. At 50° the angle is beyond it, so n₁ sinθ₁ > n₂ and total internal reflection occurs.
Check it in the model: Move the Angle of incidence, θ₁ slider from 35° to 50° and read the Refraction angle θ₂ and Critical angle θc readouts.
incident refracted reflected / total internal reflection. Drag the incident ray to change its angle.
critical angle: sinθc = n₂ ÷ n₁ (n₂ < n₁)
speed in a medium: v = c ÷ n
c = 3.00 × 10⁸ m s⁻¹
Try this
- Choose 'Air → glass' and raise θ₁ from 0° to 80°. Does the refracted ray ever reach 90°, and what is the largest θ₂?
- Set n₁ = 1.5 and n₂ = 1.0. Raise θ₁ until the refracted ray disappears and compare the angle with the critical angle readout.
- Tick 'Show wavefronts' with n₁ = 1.0 and n₂ = 2.0. What happens to the wavefront spacing, and why does the ray bend towards the normal?
Common error: total internal reflection needs light to travel from the higher to the lower refractive index, so a critical angle does not exist when going from air into glass.