Wave speed
Waves - OCR A-Level Physics
Key Definition
Wave speed
The distance travelled by a wavefront per unit time. Measured in m s⁻¹. Depends on the medium, not the source.
The distance travelled by a wavefront per unit time. Measured in m s⁻¹. Depends on the medium, not the source.
$$v = f\lambda$$
- This equation applies to all waves - mechanical, electromagnetic, and sound.
- Combining with $T = 1/f gives v = \lambda/T$.
- The speed of electromagnetic waves in a vacuum is c = 3.00 × 10⁸ m s⁻¹.
- In a given medium, wave speedThe distance travelled by a wavefront per unit time. is fixed. If frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). increases, wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). must decrease.
Worked Example [2 marks]
A sound wave has a frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). of 440 Hz and travels at 340 m s⁻¹ in air. Calculate its wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)..
Show Solution
1
Use v = fλ, rearranged to $\lambda = v/f = 340/440$
[1]2
$\lambda = 0.773\;\text{m} (3\;\text{s.f.})$
[1]Answer
$\lambda = 0.77\;\text{m}$
Common Mistake
MEDIUM
Students often: Confusing wave speedThe distance travelled by a wavefront per unit time. with frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz).. Assuming that changing the frequency of a source changes the wave speedThe distance travelled by a wavefront per unit time. in a given medium.
Instead: In a given medium, speed is fixed. If frequency increases, wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). must decrease to keep $v = f\lambda constant$.
Instead: In a given medium, speed is fixed. If frequency increases, wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). must decrease to keep $v = f\lambda constant$.