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All 14 Flashcards — Wavefronts and rays
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Question
What is a wavefront?
Answer
A line (or surface) joining all the points of a wave that are **in phase** — for example, all the crests.
Question
What is a ray?
Answer
A line showing the **direction in which the wave travels**, drawn at **right angles** to the wavefronts.
Question
What does 'in phase' mean?
Answer
Two points are **in phase** if they are at the **same point in their cycle** at the same time (e.g. both at a crest).
Question
How are a ray and the wavefronts related?
Answer
The ray is always **perpendicular (90°)** to the wavefronts — the wave advances along the ray.
Question
How far apart are neighbouring wavefronts?
Answer
Exactly **one wavelength, λ** (crest to next crest).
Question
How do you read the wavelength off a wavefront diagram?
Answer
Measure the gap between **two neighbouring wavefronts** — that distance is λ.
Question
What shape are wavefronts near a point source?
Answer
**Circles** (or spheres in 3D) spreading out from the source.
Question
What shape are wavefronts far from a point source?
Answer
Straight, parallel lines — called **plane wavefronts**.
Question
Which given equation links the wavefront spacing to speed?
Answer
$v = f\lambda$ — wavelength λ (the spacing) × frequency f = wave speed v. **Given** in the data booklet.
Question
Wavefronts 0.50 m apart, 3.0 pass per second — wave speed?
Answer
λ = 0.50 m, f = 3.0 Hz, so v = fλ = 3.0 × 0.50 = 1.5 m s⁻¹.
Question
Common mistake when drawing a ray?
Answer
Drawing it **along** a wavefront instead of **across** it — the ray must cross the wavefronts at 90°.
Question
How do you draw the rays from a point source?
Answer
As **straight arrows pointing outwards** from the source, each crossing the circular wavefronts at 90°.
Question
What do the rays look like for plane wavefronts?
Answer
**Parallel** arrows, all at 90° to the straight wavefronts.
Question
Wavefronts 3.0 cm apart, 20 pass each second. Wave speed?
Answer
v = fλ = 20 × 0.030 = **0.60 m s⁻¹**.
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Topic 3.2 hub
Wave model
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