Refraction

Overview

Students need to connect three things in this lesson: the standard ray diagram, the observed change in direction, and the physical reason that the wave changes speed at the boundary. Keep the qualitative explanation secure here so that Snell’s law feels like an extension rather than a new idea in the next lesson.

Key knowledge and explanations

• Reuse the vocabulary of normal and incidence angle, then add angle of refraction.
• Demonstrate a ray box passing through a rectangular glass or acrylic block and trace the path clearly at both boundaries.
• Explain that light bends because its speed changes when it enters a different medium.
• Emphasise the direction rule qualitatively: into a slower medium the ray bends towards the normal; into a faster medium it bends away from the normal.
• Show that in a rectangular block the emergent ray is parallel to the incident ray, even though it is laterally displaced.

Lesson flow

1. Start with a retrieval prompt on reflection vocabulary so students immediately draw a normal and label the incident ray correctly.
2. Demonstrate refraction through a transparent block and ask students to describe what changes and what stays the same as the ray crosses the boundary.
3. Guide students through annotated ray diagrams, including one example through a rectangular block and one example of a ray leaving the block back into air.
4. Finish with short written explanations that link bending direction to the idea of a speed change rather than to the thickness or colour of the material.

Checks for understanding

• Use a hinge question where students decide whether a ray should bend towards or away from the normal when entering glass from air.
• Ask students to label an unannotated refracted-ray diagram with all three required terms.
• Give one quick sketch of a rectangular block and ask students to complete the emergent ray.

Common mistakes or misconceptions

• Students often say the ray bends because the boundary pushes it. Keep the explanation rooted in a change in wave speed.
• Some confuse the angle of refraction with the angle to the surface. Keep measuring from the normal every time.
• Students may think the ray keeps curving through the whole block. Contrast the straight-line path within each medium with the direction change only at the boundaries.

Follow-up

• Set a short sequence of boundary diagrams so students practise predicting the bending direction in both directions.
• Carry forward the idea that the amount of bending tells us something about the medium, which leads directly into refractive index and Snell’s law.