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Waves transfer energy from one plane to another.

Waves can be:

• mechanical: they need a medium to trace through
• electromagnetic: the can travel in a vacuum without a medium

Waves may be either transverse or longitudinal. One whole wave is called an oscillation.

## Transverse Waves

The ripples on a water surface are an example of a transverse wave

The individual particles oscillate perpendicular to the direction of wave energy travel

## Longitudinal Waves

The ripples on a water surface are an example of a transverse wave.

Longitudinal waves show areas of compression and rarefaction.

Sound waves travelling through air are longitudinal.

The individual particles oscillate parallel to the direction of wave energy travel

For any mechanical wave, the individual particles that make up the wave do not travel far from their rest position. They just oscillate back and forth by plus or minus the amplitude of the wave.

Question: Describe the difference between transverse waves and longitudinal waves. (2 marks)

The amplitude of a wave is the maximum displacement of a point on  a wave away from its undisturbed position.

The wavelength of a wave is the distance from a point on one wave  to the equivalent point on the adjacent wave.

The frequency of a wave is the number of waves passing a point  each second. Measured in Hertz, Hz.

The time period of a wave is how many seconds on wave takes to happen. It is 1/frequency.

The wave speed is the speed at which the energy is transferred (or  the wave moves) through the medium.

All waves obey the wave equation:

wave speed  = frequency × wavelength

•        a speed of 300 000 000 m/s
•        a wavelength of 500 m
Calculate the frequency of the radio wave.
Give the unit.

Question: An electromagnetic wave has a frequency of 2 400 000 000 Hz
The speed of all electromagnetic waves is 300 000 000 m/s
Calculate the wavelength of the electromagnetic waves.

## Measuring the Speed of Sound Waves Air

Stand approximately 50m from a wall

Make a loud noise, start the timer when you make the noise

Stop the timer when you hear the echo

The speed of the sound is calculated using speed = distance/time

The distance the wave travels is 2 x the distance you are standing from the wall.

## Measuring the Speed of Waves on Water

Use a ripple tank screen to observe your waves.

The frequency of the waves being generated is read from the frequency generator.

The wavelength of the waves is found by measuring the length of ‘n’ waves and dividing the by ‘n’ (to improve accuracy)

The wave speed can be found using wave speed  = frequency × wavelength

In this example: Frequency = 40Hz

Length of 5 waves = 9cm = 0.09m

Length of 1 wave = 0.09/5 = 0.018 m

wave speed  = frequency × wavelength

wave speed = 40 x 0.018 = 0.072 m/s

Question: One student bangs two bricks together.
The sound wave produced is reflected from the wall and travels back to the students.
Describe how they can determine the speed of sound (4 marks)

Question: Some students want to determine the wave speed of water waves in a ripple tank.
Describe a method the students could use. (4 marks)

## Refraction

When a wave enters another medium the frequency stays the same but the speed and wavelength change. The wave will change direction because of the change in speed. This is observed as refraction.

If the wave slows down, the wavelength will decrease. If the wave speeds up, the wavelength will increase.