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#### KS3 3.4.4 Wave properties

In this section of the Waves topic we will focus on the Transverse type of wave and look at how it can be reflected, transmitted and absorbed.

Note - an amount in this section has already been covered especially in 3.4.1 Sound which was at the start of the waves topic.

### Transverse Waves

In section 3.4.1 Sound we said that there are just 2 types of wave, longitudinal (eg sound) and transverse (eg water waves)

All waves begin with something vibrating.

A transverse wave is formed when the direction of the vibration is perpendicular (or transverse) to the direction of travel of the wave.

So, a transverse wave would look like:

As you can see, the vibration (on the left) is perpendicular to the direction of the wave.

Notice the repeating pattern of peaks and troughs (maximums and minimums); this is a characteristic of a Transverse wave.

A simple way to make a transverse wave is to drop a stone into some water; the downward movement of the stone sets up a ripple across the surface of the water.
Another simple way to demonstrate a transverse wave is to get a skipping rope or any long rope, keep one end still, then vibrate the other end up and down. You will see a transverse wave on the rope.

Notice - the "thing" that vibrates or which causes the disturbance does not travel along the wave. The dropped stone merely drops into the water, doesn't it; the hand moving up and down doesn't travel along with the resulting wave, does it?
Also the section of water near to the dropped stone doesn't move along the surface "with" the wave and the section of rope near the vibrating handle doesn't move to the other end of the rope.

So, a wave such as a transverse wave is a great way of transferring energy from one place to another without any transfer of matter.

In fact all waves share this property - all waves transfer energy from one place to another without any transfer of matter.

#### The transmission, reflection and absorption of waves

All waves can be transmitted (meaning pass through from one place to another), reflected or absorbed.

Transmission - a simple example of the transmission of a wave is when a sound wave travels a short distance from a person speaking to a person listening. The sound wave is "transmitted" (meaning passes through) the air from the talker to the listener unaffected.

Absorption - if the distance between the talker and the listener is increased we should expect the sound wave to be affected. We should expect that its Amplitude is reduced by the time it reaches the listener compared to when it left the talker.

We call this absorption of the wave.
What happens to the wave's energy?
It is absorbed by the material that the wave passes through (eg the air). This will cause the material to increase in temperature!

If we place a wall between the talker and the listener then the sound wave would be absorbed even more such that the listener may not hear the talker at all.

Another example of the absorption of a wave is when transverse light waves from the Sun pass through sunglasses.
The glasses absorb an amount of the light waves such that the wearer can see better.

Reflection -

The easiest way to illustrate the reflection of waves is to consider transverse water waves moving in a "Ripple Tank".
When we look down at the tops of the water waves in the tank we see parallel lines moving from the vibrating bar that rests on the surface of the water producing the waves.

.

What happens if we put a solid obstacle such as a plastic or metal bar into the water, as shown below?

The hard solid obstacle prevents any transmission or absorbtion of the waves. So, the only option for the waves is that they are reflected when they meet the boundary. And that is what we are seeing in the diagram.

If we simplify the "wave" diagram by turning it into just a "ray" diagram, as shown below, we can see that the reflection of the waves obeys the same Law of Reflection that we met in 3.4.2 Light.

You can see that "the angle of incidence i equals the angle of reflection r." ( i = r)

All types of wave obey this Law of Reflection making it very easy to control the direction of waves.

#### Wave Properties

Transmission, absorption and reflection are properties of waves common to all types of wave.

Amplitude, frequency, wavelength and wave speed are properties that can be different from one wave to the next.

We have already discussed amplitude, frequency and wavelength of waves in 3.4.1 Sound so we will not discuss them here.
We will, however, discuss wave speed.

But before we do, have a go at these questions which are about the properties amplitude, frequency and wavelength of waves.

Wave Speed

Wave speed is the speed at which a wave moves through a medium or through space (in the case of electromagnetic waves).

Or, it is the speed at which energy is transferred as a wave moves from one point to another.

How can we know the speed of a wave?

Its speed must be connected to its Wavelength, mustn't it? ie a wave with a long wavelength might travel 100m in a certain time whilst a wave with half its wavelength will only travel 50m in the same time, so the first wave has a higher speed.

So, the greater the Wavelength the greater the Wave Speed.

Wave speed must also have something to do with Frequency. A wave with a high frequency puts out, say, 10 waves per second, whilst a wave with half its frequency puts out only 5 waves each second. If both waves have the same wavelength, then the one with the higher frequency will have travelled further in one second, so it is faster.

So, the greater the Frequency the greater the Wave Speed.

Summary of wave speed

The speed of a wave depends on its wavelength (the distance travelled by each full wave) and its frequency (the number of full waves in a second).
The greater the wavelength or the frequency of a wave, the greater its wave speed.

To calculate the wave speed we simply multiply the wavelength of the wave by its frequency.

Wave speed = frequency x wavelength

Time for a few more questions!

#### Keywords

Now that we have reached the end of this section we can focus on the keywords highlighted in the KS3 specification. You have already met each one, but it is important to learn them.

• Waves: Vibrations that transport energy from place to place without transporting matter.
• Transverse wave: Where the direction of vibration is perpendicular to that of the wave.
• Longitudinal wave: Where the direction of vibration is the same as that of the wave.
• Transmission: Where waves travel through a medium rather than be absorbed or reﬂected.