Snell's Law

Overview

This lesson should keep the equation tied tightly to the diagram and the physical meaning of the answer. Students need more than button-pressing practice: they should see refractive index as a measure of how much the wave slows down and therefore how much the ray bends.

Key knowledge and explanations

• Define refractive index, n, as a ratio of wave speeds in different regions and connect that idea to the qualitative refraction work from the previous lesson.
• Introduce Snell’s law using one clear diagram with all angles measured from the normal. ni sin i = nr sin r
• Model the simplified equation in the syllabus form n = sin i / sin r and practise identifying which angle is i and which is r before substituting numbers.
• Work through at least one example that finds n and one that rearranges the equation to find an unknown angle.
• Keep emphasising that a larger refractive index means the wave travels more slowly in the second medium and bends more towards the normal on entry.

Lesson flow

1. Begin with two retrieval questions from the refraction lesson so students recall how to identify the incident and refracted angles from a diagram.
2. Introduce refractive index as a speed ratio, then derive the need for a numerical way to compare different materials.
3. Model Snell’s law with careful calculator work, including a reminder to stay in degree mode and to write the diagram before using the equation.
4. Finish with mixed practice where some questions ask for refractive index and others ask for a missing angle, followed by a short interpretation question about what the answer means physically.

Checks for understanding

• Use a hinge question where students choose which pair of angles should be substituted into the equation from a labelled diagram.
• Give one quick calculation for n and one for a missing angle so students show both substitution and rearrangement.
• Ask students to compare two materials and decide which has the greater refractive index from the given bending information.

Common mistakes or misconceptions

• Students often swap i and r because they read from the surface instead of the normal. Keep the diagram visible during every example.
• Some leave the calculator in radian mode, which produces impossible answers. Build in a quick calculator check before the first question.
• Students may treat refractive index as just another number with no meaning. Always ask what the value tells them about speed and bending.

Follow-up

• Set the refraction question sheet for consolidation, with students annotating each question before calculating.
• Carry forward refractive index as preparation for the next lesson on critical angle and total internal reflection.