TwiddleLab / Physics / Light and Sight

Refraction: Light Bends

Lesson 3 of 10 · 7 min · Free, no account needed

What you will learn

• Refraction: light changes direction when it crosses into a material where its speed is different.
• Into a slower material (air to water or glass) it bends toward the normal; out into a faster one, away from it. Straight in, no bend.
• The refractive index n = c / v tells how much slower light is: water 1.33, glass about 1.5, diamond 2.42.

Start the lesson

How the lesson goes

1. A straw in water looks broken. Is the light lying, or your brain?

   Start with a fish. To spear it, where do you aim?

2. Shine the laser into the water at different angles.

   Turn the dial to land the beam on the coin. Watch the two angles.

3. Why does it bend? Drive a toy car onto sand.

   On sand the wheels go at half speed. Try straight in, then at an angle.

4. Refraction and the refractive index.

   Refraction: light changes direction where its speed changes, at the boundary between two materials.

   The refractive index n = c / v: speed of light in a vacuum ÷ speed in the material. Air 1.00, water 1.33, glass about 1.5, diamond 2.42.

   Into a material with bigger n (slower), the ray bends toward the normal. Into smaller n (faster), away from it. Along the normal, no bend.

   Simplified: at every surface a little light also reflects. We hide that weak reflection on most screens. The exact rule is Snell's law, n1 sin i = n2 sin r (in Practice).

5. Fill the block with the right material for each target.

   Drag a material into the block, or tap one and then the block.

6. Game: Spear fishing. One throw per fish.

   The spear flies straight and doesn't bend. Light from the fish does.

7. Code: video games draw light by tracing rays.

   This program sends three rays into glass. Step through it.

8. A lamp under water. Draw where its ray goes in the air.

   The ray meets the surface 38° from the normal. Leaving water (n = 1.33) for air (n = 1.00), light speeds up, so the angle grows: 1.33 × sin 38° = sin r gives r ≈ 54°.

9. Your fishing trip. Which throw was your best?

Previous lesson: Mirrors: Angle In, Angle Out · Next lesson: Total Internal Reflection: Light in a Pipe · Part of the course: Light and Sight

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