Define and use the terms normal, angle of incidence and angle of refraction

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – Refraction of Light

3.2.2 Refraction of Light

Learning Objective

Define and use the terms normal, angle of incidence and angle of refraction when analysing the refraction of light at a plane surface.

Key Definitions

  • Normal: An imaginary line drawn perpendicular to the surface at the point where the incident ray meets the surface.

  • Angle of incidence (i): The angle between the incident ray and the normal.

  • Angle of refraction (r): The angle between the refracted ray and the normal.

Understanding the Geometry

When a ray of light passes from one transparent medium to another, its speed changes, causing the ray to bend. The bending is described by the relationship between the angles measured from the normal.

Suggested diagram: Ray of light incident on a plane surface showing the normal, angle of incidence and angle of refraction.

Snell’s Law

Snell’s law relates the angles of incidence and refraction to the refractive indices of the two media:

\$n1 \sin i = n2 \sin r\$

where:

  • \$n_1\$ = refractive index of the medium in which the incident ray travels.

  • \$n_2\$ = refractive index of the medium into which the ray refracts.

  • \$i\$ = angle of incidence.

  • \$r\$ = angle of refraction.

Example Calculation

Light travels from air (\$n1 \approx 1.00\$) into water (\$n2 \approx 1.33\$). If the angle of incidence is \$30^\circ\$, find the angle of refraction.

  1. Write Snell’s law: \$1.00 \sin 30^\circ = 1.33 \sin r\$.

  2. Calculate \$\sin 30^\circ = 0.5\$, so \$0.5 = 1.33 \sin r\$.

  3. Solve for \$\sin r\$: \$\sin r = \dfrac{0.5}{1.33} \approx 0.376\$.

  4. Find \$r\$: \$r = \sin^{-1}(0.376) \approx 22^\circ\$.

Summary Table

Medium 1 (incident)Medium 2 (refracted)Angle of Incidence \$i\$Angle of Refraction \$r\$Relation
Air (\$n_1=1.00\$)Water (\$n_2=1.33\$)\$30^\circ\$\$\approx 22^\circ\$\$n1\sin i = n2\sin r\$
Water (\$n_1=1.33\$)Glass (\$n_2=1.50\$)\$45^\circ\$Calculate using Snell’s law\$n1\sin i = n2\sin r\$

Using the Terms in Practice

When answering exam questions, always:

  1. Identify the point where the ray meets the surface.

  2. Draw the normal at that point.

  3. Label the incident ray, refracted ray, angle of incidence \$i\$ and angle of refraction \$r\$.

  4. Apply Snell’s law if required to calculate unknown angles or refractive indices.

Common Mistakes

  • Measuring angles from the surface instead of from the normal.

  • Confusing the incident and refracted media when assigning \$n1\$ and \$n2\$.

  • Forgetting that the normal is always perpendicular to the surface, regardless of the shape of the surface.