Chapter 4: Waves in matter
The chapter problem: the harbour survey
A survey boat is checking whether a harbour channel is deep enough for a ferry. The boat sends a pulse of sound straight down to the sea floor and listens for the reflection.
These are the facts to use all the way through the chapter.
- The speed of sound in sea water is 1500 m/s.
- A pulse takes 0.020 s to travel to the sea floor and back.
- The echo sounder sends sound at a frequency of 50 kHz.
- The ferry needs a channel at least 12 m deep.
- On the boat, a ripple tank makes water waves with a frequency of 5.0 Hz and a wavelength of 0.12 m.
- The speed of sound in air is 330 m/s.
- Later in the chapter, a medical scanner uses ultrasound at 2.0 MHz, and a seismic wave from an earthquake travels 60 km in 12 s.
By the end of the chapter you will work out the depth of the water under the boat, the wavelength of the echo sounder, and the speed of the waves in the ripple tank. You will decide whether the channel is deep enough for the ferry. You will also describe what happens when a wave meets a boundary, and how ultrasound and earthquake waves are used to explore things we cannot see.
First contact: no working yet. Estimate. The pulse takes 0.020 s to travel to the sea floor and back. Is the depth closer to 1.5 m, 15 m or 150 m? Write your estimate down.
If you are not sure where to start
Sound travels 1500 m through sea water in one second. The pulse takes 0.020 s, which is two hundredths of a second, so it travels about 30 m. That 30 m is the trip there and back, so the depth is about 15 m.
By the end of this chapter
You will be able to:
- describe a wave in terms of its amplitude, wavelength, frequency and period;
- use the equation to link wave speed, frequency and wavelength;
- use the equation to find a period from a frequency;
- describe the difference between a transverse wave and a longitudinal wave;
- recall the speed of sound in air;
- describe a method to measure the speed of sound in air and the speed of ripples on a water surface;
- describe what happens to a wave at the boundary between two different materials: reflection, transmission and absorption;
- state the law of reflection and draw a ray diagram to show it.
A few skills are for Higher tier only: how sound waves make solids vibrate and so set the limits of hearing, and how ultrasound and seismic waves are used for detection and exploration.
Where it is used
Waves carry energy and information across the world and through the human body. Doctors use ultrasound to see inside a patient without surgery. Engineers use sound to find cracks inside metal. Survey boats use echo sounding to map the sea floor. Seismologists use earthquake waves to work out the structure of the Earth's core. Every one of these methods rests on the ideas in this chapter.
Your turn to connect it
Where in your own week do you meet a wave? Write two examples. One should be a wave you can hear, and one should be a wave you cannot hear.