IB Physics SLTopic C.3 — How Waves BehavePaper 1 & 2i = r~8 min read
Reflection, Refraction & Transmission
Whenever a wave reaches a boundary between two materials, it has to make a decision. Some of it bounces back, some passes through — often bending as it goes — and some gets soaked up. Those three outcomes are reflection, refraction and transmission.
📘 What you need to know
At a boundary a wave can be reflected, refracted, transmitted or absorbed
Reflection: the wave bounces back into the first medium, obeying angle of incidence = angle of reflection
In reflection the frequency, wavelength and speed do not change
Refraction: the wave changes direction as it crosses into a medium of different density, because its speed changes
Into a denser medium it slows and bends towards the normal; into a less dense medium it speeds up and bends away
In refraction the frequency stays the same but the speed and wavelength change — all angles are measured from the normal
Reflection
Reflection happens when a wave hits a boundary and bounces back into the medium it came from instead of passing through. The rule is beautifully simple — measuring both angles from the normal (the line drawn at 90° to the boundary):
Law of reflection
angle of incidence (i) = angle of reflection (r)
The incident and reflected rays make equal angles with the normal: i = r. Reflection leaves the wave’s speed, frequency and wavelength unchanged.
Refraction
Refraction is the change in direction of a wave as it crosses a boundary into a medium of different density. It happens because the wave changes speed — one edge of the wavefront reaches the new medium and slows (or speeds up) before the other edge, swinging the whole wave round, just like a car turning when one wheel hits mud first.
The direction of the bend follows a clear pattern:
Into a denser medium (e.g. air → glass): the wave slows down, its wavelength shortens, and it bends towards the normal (so r < i)
Into a less dense medium (e.g. glass → air): the wave speeds up, its wavelength lengthens, and it bends away from the normal
Entering the denser glass, the ray slows and bends towards the normal, so the angle of refraction r is smaller than the angle of incidence i.
A special case: if a wave hits the boundary along the normal (angle of incidence 0°), the whole wavefront enters at once, so it slows but carries straight on with no change of direction. And through it all, the frequency never changes — which is why the colour of refracted light stays the same. Since v = fλ, if v drops while f holds, the wavelength must shrink to match.
Refraction of Water Waves
Water waves refract when they move between regions of different depth. Crossing from deep to shallow water, there’s more drag from the seabed and less room to oscillate, so the waves slow down and their wavelength shortens — the crests bunch closer together. If they meet the depth change at an angle, they also bend towards the normal.
Crossing into shallow water the waves slow and the wavefronts crowd together — the wavelength drops while the frequency stays the same.
Transmission
Transmission simply means the wave passes through to the far side of the boundary — the opposite of reflection. Refraction is a type of transmission (the special case where the wave also changes direction crossing into a different density). As a wave is transmitted, some of its energy may be absorbed, which lowers the wave’s amplitude but leaves its frequency and speed unchanged. All of this — reflection, refraction and transmission — happens for every type of wave, transverse and longitudinal alike.
🧭 Telling the behaviours apart
Bounces back into the first medium, no crossing → reflection (i = r)
Crosses and bends into a new density → refraction (a type of transmission)
Crosses straight through a substance → transmission; if energy is lost, amplitude drops (absorption)
Measure all incidence, reflection and refraction angles from the normal
Only refraction changes speed and wavelength — reflection changes neither
Quick recap: reflection bounces back with i = r (no change to v, f, λ); refraction crosses and bends because speed changes; transmission is passing through, with absorption reducing amplitude.
WE 1
A ray of light strikes a plane mirror with an angle of incidence of 35°.
(a) State the angle of reflection. (b) State what happens to the wavelength of the light.
Part (a)
By the law of reflection, i = r
angle of reflection = 35°Part (b)
Reflection does not change the wave’s speed or frequency
the wavelength is unchangedBoth angles are measured from the normal, not from the mirror surface.
WE 2
A ray of light passes from air into a glass block at an angle.
State what happens to the light’s (a) speed, (b) wavelength, (c) frequency, and (d) direction.
Reasoning
Glass is optically denser than air, so the light slows down
(a) speed decreases(b) wavelength decreases(c) frequency stays the same(d) bends towards the normalBecause v = fλ and f is fixed, a smaller v forces a smaller λ.
💡 Top tips
Always measure angles from the normal, never from the boundary surface
Reflection changes nothing about the wave; refraction changes speed and wavelength but never frequency
Denser medium → slower → bends towards the normal; less dense → faster → bends away
A ray hitting along the normal (0° incidence) passes straight through without bending
Refraction is a type of transmission — don’t treat them as opposites
⚠ Common mistakes
Mixing up reflection and refraction — refraction always involves crossing into another medium and bending
Measuring angles from the surface instead of the normal
Saying frequency changes on refraction — it never does; speed and wavelength do
Getting the bend backwards: into a denser medium it bends towards the normal
Thinking a wave slows on reflection — reflection leaves speed unchanged
Up next: what happens when waves squeeze through gaps or sneak around edges — Diffraction of Waves.
Want this to actually click before the exam?
Book a free meeting and let’s work through the tricky bits together.
