A WAVE FRONT is an imaginary line connecting all points of a wave that are at the same phase (e.g. all the crests).
Wave fronts are PERPENDICULAR to the direction of wave travel.
For PLANE WAVES (parallel wave fronts):
All points move in the same direction.
Shown as parallel lines with arrows indicating direction of travel.
For CIRCULAR WAVES (from a point source):
Wave fronts are concentric circles spreading outward.
Frequency and wavelength shown by spacing between lines.
CLOSER WAVE FRONTS: shorter wavelength or wave fronts bunching up as wave slows.
FURTHER APART: longer wavelength or wave fronts spreading as wave speeds up.
REFRACTION using wave fronts:
When a wave crosses into a new medium, speed changes.
Part of the wave front enters the new medium first → that part slows (or speeds up) first.
The wave front pivots → wave changes direction.
Explaining Refraction with Wave Fronts
WHY REFRACTION OCCURS:
At a boundary, the wave speed changes.
If wave slows: wave fronts bunch together (shorter wavelength), wave turns towards the normal.
If wave speeds up: wave fronts spread apart (longer wavelength), wave turns away from the normal.
FREQUENCY DOESN'T CHANGE during refraction — only speed and wavelength change.
v = fλ → if v decreases and f stays constant → λ decreases.
EXAMPLE — Light entering glass:
Light travels slower in glass than air.
Wave front entering glass slows down → bends towards the normal.
Wavelength shortens inside glass.
Frequency unchanged — the eye perceives the same colour.
SEISMIC WAVE REFRACTION:
P-waves travel faster in denser rock.
As P-waves descend through Earth, density increases → speed increases → waves curve upward.
This is why P-waves travel in curved paths through the Earth.
Radio Waves and Electrical Circuits
RADIO WAVES can be PRODUCED by oscillations in electrical circuits.
An oscillating current in an aerial (antenna) produces oscillating electromagnetic field → radio wave emitted.
Frequency of radio wave = frequency of electrical oscillation.
RADIO WAVE ABSORPTION AND CURRENT INDUCTION:
When a radio wave is ABSORBED by a conductor (receiving aerial):
The oscillating electromagnetic field drives electrons in the conductor.
An ALTERNATING CURRENT (AC) is induced with the SAME FREQUENCY as the radio wave.
This is the basis of all radio and wireless communication receivers.
REFRACTION OF RADIO WAVES:
Radio waves refract in the ionosphere (upper atmosphere).
This allows long-distance communication — waves bent back to Earth's surface.
Different frequencies refract differently — some pass through, some are reflected.
⚠️ Common Mistake
During refraction, FREQUENCY stays constant — only SPEED and WAVELENGTH change. v = fλ, so if v decreases and f is constant, λ must decrease proportionally. Wave fronts are perpendicular to the direction of travel — closer wave fronts = shorter wavelength, not higher frequency.
📐 Key Equations
v = f × λ (speed = frequency × wavelength — frequency unchanged in refraction)
📌 Key Note
Wave fronts ⊥ direction of travel. Refraction: wave crosses boundary → speed changes → wave fronts pivot → direction changes. Frequency constant, speed and wavelength change. Into slower medium: bends towards normal, wavelength decreases. Radio waves: produced by oscillating current; absorbed → induces AC at same frequency.
🎯 Matching Activity — Wave Fronts and Refraction
Match each statement to the correct refraction or wave front concept. — drag the symbols on the right to match the component names on the left.
Wave slows at boundary
Drop here
Frequency during refraction
Drop here
Wave front orientation
Drop here
Radio wave reception
Drop here
Unchanged — only speed and wavelength change
Always perpendicular to the direction of wave travel
Wave fronts bunch together — wavelength decreases, wave bends towards normal
Oscillating radio wave induces AC at same frequency in the receiving aerial
⭐ Higher Tier Only
HT only — draw and interpret wave front diagrams to explain refraction. Explain refraction in terms of change of speed at a boundary. Describe how radio waves are produced by oscillating electrical circuits and how they induce alternating currents when absorbed.
🔬 Triple Science Only
Wave front diagrams and radio wave production (HT only, physics only) — not in Combined Science.
🎯 Test Yourself
Question 1 of 2
1. A light wave enters glass from air and slows down. What happens to its wavelength and frequency?
2. How are radio waves produced for transmission?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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