C.3.1—Wavefronts and rays

Define a wavefront

A wavefront is a line or surface joining points that are in phase. Adjacent wavefronts are separated by one wavelength. In a uniform medium, the wavefronts are perpendicular to the direction of propagation.

Use rays to show propagation

A ray is a line showing the direction in which the wave transfers energy. Draw rays perpendicular to the local wavefronts; straight, parallel wavefronts give parallel rays, while circular wavefronts from a point source give radial rays.

Read the geometry

When a wavefront diagram changes direction at a boundary, compare the ray direction and the spacing of wavefronts on each side. The wavefront construction helps distinguish a change in speed from a change in frequency.

Common trap

Do not draw rays parallel to wavefronts. A ray follows energy propagation and is normal to the wavefront at each point.

How it is assessed

The packet contains one directly relevant ray-diagram item and one unrelated polarization item. Use the ray evidence for diagram construction; no reliable frequency claim is made for the unrelated item.

Command terms

Sketch / Label

What earns marks

Define wavefronts as in-phase lines or surfaces and draw rays perpendicular to them in the propagation direction. For a diagram question, label the image or ray construction only after identifying the correct normal direction.

Watch for

Drawing rays along wavefronts instead of perpendicular to them, or treating the mixed polarization evidence as a wavefront question.

Representative question

[Maximum number: 3]

Sketch two appropriate rays on the diagram to show the formation of the image. Label the image with the letter I.

C.3 Wave phenomena is secure when you can connect the physical picture to the equation and its limits.

Wavefronts and rays
Reflection, refraction and Snell’s law
Refractive index and total internal reflection
Superposition, coherent sources and interference
Young’s double-slit pattern

C.3.1—Wavefronts and rays

Model Wavefronts and Rays

C.3.1 Exam Analysis

Assessment in practice

Retrieve the Core C.3 Wave Phenomena Model

Question 1