v = fλ with prefixes and standard form, the required practical with a ripple tank and a string, and Edexcel's speed of sound in a metal rod.
What you will learn
- Use v = fλ in any arrangement, with kHz, MHz, cm and nm
- Measure wavelength and frequency in a ripple tank
- Find the speed of waves on a string from the loops
- Find the speed of sound in a metal rod, and say why a stopwatch will not do (Edexcel)
The wave equation
In one second, f waves leave the source, each one λ long.
So the front of the wave has moved f × λ metres in that second.
wave speed = frequency × wavelength
v = f λ
v in m/s, f in Hz, λ in metres. Recall this one for every board. Rearranged: f = v ÷ λ and λ = v ÷ f.
- A dolphin call: 6 kHz in sea water at 1500 m/s
- 6 kHz = 6000 Hz
- λ = v ÷ f = 1500 ÷ 6000 = 0.25 m
- 0.25 m = 25 cm
Speed of sound from an echo. Stand a measured distance from a large wall, make a sharp sound and time the gap until the echo. The sound goes to the wall and back, so it travels twice the distance: speed = 2 × distance ÷ time. Timing many echoes in a row (clapping in time with them) and dividing makes the error from reaction time a smaller fraction of the time measured.
The rest of this lesson and its 13 practice questions are locked.
In this lesson
- The wave equation
- Prefixes and standard form
- Required practical: ripple tank and string
- Edexcel core practical: sound in a solid
- Try it: Waves on a string
- Try it: Your first wave speed
13 practice questions, marked as you go; about 25 minutes in all.
Key terms
Ripple tank, Waves on a string, Solid
In this unit: Waves and the electromagnetic spectrum
Transverse and longitudinal waves, v = fλ and the wave practicals, reflection and refraction, and the electromagnetic spectrum with its uses, dangers and the infrared practical.