Waves - 6.1.5 Waves For Detection and Exploration (Physics Only) (HT Only) (GCSE Physics AQA)

Waves For Detection and Exploration (Physics Only) (HT Only)

Differences in Velocity, Absorption and Reflection

• Velocity of waves can vary. Velocity is a property of waves. If the velocity of the wave is changed, then the wave will speed up, slow down or change direction.
• Velocity can make waves reflect. If a wave slows down enough, it can be reflected by a boundary. Similarly, if a wave speeds up enough, it will be absorbed by a boundary.
• Waves can be used for detection and exploration. We have just seen that waves can be reflected or absorbed by changing their velocities. This means that we can use these waves for detection and exploration. These waves can be used to look at structures that can’t be directly observed.

Ultrasound Waves

• Ultrasound waves have a high frequency. Ultrasound waves have an extremely high frequency. The frequency of ultrasound waves is well above 20 kHz, meaning that ultrasound cannot be heard by humans.
• Ultrasound waves can be reflected. Like any other type of wave, ultrasound waves have the ability to be reflected. This occurs when they meet a boundary that they cannot cross.
• The reflections tell us about boundaries. Since the reflection of ultrasound waves occurs at boundaries, we can use these waves to determine boundary locations. For example, if we view ultrasound waves moving through an object, the waves will be reflected at different points. These points will tell us where there is a boundary, where one substance turns into another.
• Ultrasound waves are used in imaging. We have just seen that ultrasound waves can help us to determine boundaries. This means that they can be used in imaging, for example to view babies inside the womb. The ultrasound waves will be reflected when they reach the baby (which acts as a ‘boundary’), therefore allowing an image to be created on the scan.

Seismic Waves

• Earthquakes create seismic waves. Earthquakes, which start off in the Earth’s crust, create waves known as seismic waves. These waves can be classified into two different types, P waves and S waves.
• P-waves are a type of seismic wave. P waves are longitudinal seismic waves. These can travel through both solids and liquids, but will travel at different speeds in these substances. P waves can also be called primary waves.
• S-waves are another type of seismic wave. S waves are transverse seismic waves. These can travel through solids only, not liquids. S waves can also be called secondary waves.
• These waves can be linked to the Earth’s core. Since we know that P waves travel through liquids, but S waves do not, we can start to think about the structure of the Earth’s core. We will explore this in the next section.

Using Seismic Waves

From observations, we know that the core of the Earth is made up of two layers; firstly, a solid inner core surrounded by a liquid outer core. We know this due to investigation using seismic waves.

Scientists have found that P waves are able to travel through and into the Earth’s core completely. This is because P waves are able to travel through both solids and liquids.

On the other hand, S waves are unable to travel through and into the Earth’s core. This is because S waves are not blue to travel through liquids, meaning that they are stopped by the liquid outer core.

Echo Sounding

• Echoes use high frequency waves. When we refer to an ‘echo’, we simply mean a reflected sound wave. When we are using the technique of echo sounding, we have to use high frequency sounds to produce high frequency reflections.
• Echo sounding is a detection technique. We can use these high frequency echos to help us detect objects, or the depth of the sea. The waves are sent into the water, and the echoes are reflected off the sea bed.
• The depth of water can be measured. Using echo sounding, we can measure the depth of water. First, we need to know the time taken for the echo to return. We can do this by timing how long it takes for the sound wave to return as an echo. Next, we need the speed of sound in water. Now that we have speed and time, we can work out the distance to the sea bed (the depth of the water).