Recall and use the equation n = sin i / sin r

3.2.2 Refraction of Light

What is Refraction?

When light travels from one medium to another (for example, from air into water), it changes speed and bends. This bending is called refraction 🌈. Think of a pencil in a glass of water – it looks bent at the surface because the light rays are being redirected.

Snell’s Law (The Core Equation)

Snell’s Law tells us how much the light bends. The equation is:

\$n = \frac{\sin i}{\sin r}\$

Where:

  • \$i\$ = angle of incidence (angle between the incoming ray and the normal)

  • \$r\$ = angle of refraction (angle between the refracted ray and the normal)

  • \$n\$ = refractive index of the second medium relative to the first

Why Does Light Bend?

Light travels slower in denser media. When it enters a medium where it slows down, the part of the wave that enters first bends toward the normal, causing the whole ray to change direction.

Calculating Refractive Index

  1. Measure the angle of incidence \$i\$ using a protractor.

  2. Measure the angle of refraction \$r\$.

  3. Plug the values into Snell’s Law to solve for \$n\$.

Example: Light hits water at \$30^\circ\$ from the normal. The refracted angle is \$20^\circ\$. Then:

\$n = \frac{\sin 30^\circ}{\sin 20^\circ} \approx 1.33\$

That’s the refractive index of water! 🌊

Common Refractive Indices

MediumRefractive Index \$n\$
Air1.00
Water1.33
Glass (typical)1.50
Diamond2.42

Practical Tip: Using a Prism

Place a triangular prism in a beam of light. The light will split into a spectrum because each colour has a slightly different refractive index. This demonstrates how refraction can separate colours, just like a rainbow 🌈.

Quick Quiz

  • What happens to the refractive index if the light goes from a less dense to a more dense medium?

  • Calculate \$n\$ if \$i = 45^\circ\$ and \$r = 30^\circ\$.