Wave behaviour

Waves transfer energy (not matter): i.e. vibration (kinetic energy) of a particle is transferred to the next particle, but the particles remain where they are! Think of a Mexican wave in a stadium! 

Vibration of particles follows a repetitive pattern: each full cycle of this pattern is called an oscillation.

P4.1-1 Describing waves

Wavelength: distance travelled by a wave in one oscillation - - unit: metres.

Amplitude: when particles vibrate in their position, they are displaced from their original position. Maximum displacement is called amplitude – A – unit: metres. The larger the amplitude of a wave, the more energy it transfers. 

Time period: time taken for an oscillation – T – unit: seconds.

Frequency: number of oscillations in 1 second – f – unit: or Hz (called Hertz).

Example 1:

If time period of a wave is 0.5 s, find its frequency.

Wave speed:

Example 2:

Frequency of a wave is 2 kHz, and its wavelength is 50 cm. Calculate its speed.

Transverse and Longitudinal

Waves travel in two different ways: Transverse and Longitudinal

If a slinky is on a table:

1. To produce a transverse wave, move end of the slinky left and right.
2. To produce a longitudinal wave, move end of the slinky back and forth.

P4.1-2 Measuring Wave speeds

Measuring speed of sound:

You need two people for both ways. 

1. First method: the distance between the person and the wall should be at least 50 m, so that the person can feel the difference in time between the clapping and hearing the echo. Sound actually travels a distance of two 50 m (= 100 m) in total till it is heard. 

The person claps as soon as they hear the echo of a previous clapping. They do this for, say, 20 time. Another person records the time for this 20 claps. Let’s say this takes 50 seconds. So the time between each clap and hearing its echo is

1. Two people one with a pot and a ladle, and another with a stopwatch stand at least 100 m apart from each other. First person hits the pots with the ladle and the person with stopwatch sees this first but hears it after that (because of the 100 m distance between them) and can measure the time between seeing and hearing. We know the distance between the two and now have the time, so

Higher only

Us humans, can hear sounds with frequencies from 20 Hz to 20 kHz.

Echo location: how bats find their way by emitting pulses of frequencies from 30 Hz to 100 kHz and hearing the echo.

Echo sounding: how ships find the depth of seabed by producing high frequency sounds and receiving the echo.

Example 3:

A boat uses echo sounding to locate shoal of fish in the sea. The echo is received from the shoal 0.2 s after it was emitted. Find the distance between the boat and the shoal. Speed of sound in water is 1500 m/s.

Measuring speed of water wave

Speed of water wave can also be measured with similar procedure. Create a splash at one end of a swimming pool and measure the time it takes for it to get to other of the pool. Measure the length of the pool, and again

Ripple tank:

Light is shone on the ripples created in the tank by the oscillating paddle. This will create light and dark patches on the screen under the tank. 

Light patch: is the trough of a wave, and dark ones show the crest (as thickness of water is more, and less light passes).

Let’s say 60 dark patches pass through a point on the screen in 30 seconds. This means time period is 0.5 seconds, and as f = 2 Hz.

Wavelength can be measured using a strobe light to “freeze” the motion of wave on the screen. The distance between two dark patches is measured which is the wavelength. 

Now we have both the frequency and wavelength, so wave speed is: