Bent Pencil (Refraction)

Why does a pencil look bent in a glass of water? Shine a laser between different materials and watch the light change direction.

Laser starts in
Laser goes into
Angle in
40°
Angle after bending
28.9°
Critical angle
None

The maths, live

Snell's law n₁ sin θ₁ = n₂ sin θ₂1 × sin 40° = 1.33 × sin θ₂
So sin θ₂ = n₁ sin θ₁ ÷ n₂sin θ₂ = 0.483, θ₂ = 28.9°
Critical angle sin θc = n₂ ÷ n₁Only when n₁ > n₂

Try starting in glass and going into air, then make the angle bigger. What happens past the critical angle?

How it works

Light travels at different speeds in different materials. When it crosses into a new material at an angle, it changes direction. This is called refraction.

The refractive index (n) says how much a material slows light down. Snell's law tells you the new angle: n₁ sin θ₁ = n₂ sin θ₂. Going from a slow material into a faster one at a steep angle, light can't escape at all and reflects back. That is total internal reflection, and it's how optical fibres work.

Try this

  1. Shine the laser straight down (0°). Does it bend?
  2. Which material bends light the most?
  3. Start in glass, go into air and increase the angle until the light gets trapped.