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C.3.10—Reflection, refraction and transmission at boundaries

Syllabus
First assessment 2025
Objective
—
Level
HL

Explain Reflection, Refraction and Transmission at Boundaries

A boundary can split wave energy

When a travelling wave reaches a boundary, part of its energy may be reflected back into the first medium and part may be transmitted into the second. The transmitted wave is refracted when its speed changes and it crosses the boundary at a non-zero angle.

Behaviour What happens Direction rule
Reflection wave remains in medium 1 angle of reflection equals angle of incidence
Transmission wave enters medium 2 continues across the boundary
Refraction transmitted wave changes direction because speed changes toward the normal if speed decreases; away if speed increases

Read rays and wavefronts together

Angles are measured from the normal. Rays show energy-transfer direction and remain perpendicular to wavefronts. Frequency stays fixed at a stationary boundary; a change in speed therefore changes wavelength and wavefront spacing.

Boundary cases

At normal incidence the transmitted ray does not bend, although its speed and wavelength may change. Unless absorption is stated, reflected and transmitted energy together account for the incident energy; their amplitudes do not generally add directly.

C.3.10 Exam Analysis

Assessment in practice

1–3 marks
How it is assessed

Questions ask for a missing reflected wavelength or the minimum film thickness for constructive reflection. The evidence rewards including refractive index and the correct phase-shift condition.

Command terms

Determine / What is

What earns marks

Draw or identify the top and bottom reflected rays, calculate the optical path contribution 2nt at normal incidence, and count phase reversals before selecting the constructive condition. For the air–film–air minimum-thickness case, use t = λ/(4n).

Watch for

Using 2t instead of 2nt, or applying the air–film–air quarter-wave result without checking the phase changes at the two surfaces.

Representative question

Question 1

[Maximum number: 3]

The refractive index of the coating is 1.63 and the refractive index of the glass is 1.52 .

The thickness of the coating is 143 nm .
Determine the wavelength, in nm , that is missing in the light reflected to the girl assuming that the light is incident normally on the window.