P12 Wave properties Flashcards
What do waves transfer?
Waves transfer energy from one place to another. Nothing physical actually travels - just the energy.
What are the two types of wave?
Transverse and longitudinal
Wavelength
The distance between two identical points on the wave
Frequency and the unit
The number of waves passing a fixed point every second. The unit for frequency is Hertz (Hz).
What causes a wave?
Vibrations/oscillations
What are transverse waves?
Waves where the displacement/oscillations) is at right angles (perpendicular) to the direction of the wave/ direction of the energy transfer.
What are longitudinal waves?
Waves in which the displacement/oscillations is parallel to the direction of the wave/direction of energy transfer
Frequency equation
frequency (Hz) = number of waves / time (s)
What is ‘period’?
The time for one complete oscillation.
Period equation
period (s) = 1 / frequency (Hz)
The wave equation
All waves obey the wave equation:
wave speed (m/s) = frequency (Hz) x wavelength (m)
v = fλ
Give examples of types of error
Systematic error, Random error, Reading error, Zero error
State three things that happen when a wave meets an interface of a material.
Reflection, Transmission, Absorption
State both the name of the ray going into a mirror, and the name of its angle to the normal, before it is reflected.
Incident ray and angle of incidence
State the name of the type of reflection we get from smooth, shiny surface, like mirrors.
Specular reflection
State the name of the type of reflection we get from tough, matt surfaces, like a piece of paper.
Diffuse reflection
State the law of Reflection.
The angle of incidence equals the angle of reflection.
Specular reflection
- The surface of the mirror is very very smooth
- When 3 parallel rays hit the surface of the mirror each ray reflects at the same angle, equal to the angle of incidence
Diffuse reflection
- On a very rough surface, such as paper
- When 3 parallel rays hit the surface of the paper, they reflect off at different angles depending on the angle of the interface where it hits
- The reflected rays are scattered in different directions
Everyday examples of refraction
Telescopes, Magnifying glass
What is refraction?
- When plane waves cross a boundary at a non-zero angle to the boundary, each wavefront experiences a change in speed and direction. If the wave crosses at a zero angle (straight on) the wave may change speed but will not change direction.
- Refraction happens when waves are transmitted from one medium to another and there is a difference in optical density of the two media.
- A more optically dense substance will refract a wave more
Measuring water waves using a ripple tank
If we measure:
a) Frequency
by recording the waves with a stop clock in the background, then playing back in slow motion to count how many waves pass a fixed point in 10s
b) Wavelength
by measuring the length of multiple waves and dividing it by the number of waves
We can use v = fλ
State the equation that links velocity, time and distance
velocity = distance / time
Why is a sound wave classified as a longitudinal wave?
The oscillations are parallel to the direction of energy transfer
Calculate the frequency of a sound wave that has a period of 0.004 seconds
1 / 0.004 = 250
250 Hz
What is the normal hearing range for a human?
between 20Hz and 20kHz (20,000Hz)
What are sounds above the normal human hearing range classified as?
Ultrasound
State the name of the process that occurs when a wave travels through a different media type at an angle
Refraction
State the name of the process that occurs when a wave rebounds of a surface
Reflection
What are ultrasound waves?
Sound waves above the highest frequency that humans can detect
Seismic waves
- Shock waves that are produced whenever there is an earthquake
- They can be detected using a seismometer
- There are 2 different types of seismic waves: P-Waves, S-Waves (primary and secondary)
What happens when ultrasound waves meet at a boundary?
- Some of the sound waves are partially reflected
- This means that when a pulse of ultrasound is directed at an object, some of the wave is reflected back whenever it hits a boundary of the object
Uses of ultrasound
- Medical imaging
- Industrial imaging
- Echo sounding
Industrial imaging to find flaws in materials
- Ultrasound waves enter the material
- Ultrasound waves are usually partially reflected back by the far side of the material
- If there is a crack inside the material, some of the wave is reflected back sooner, indicating a flaw
- The depth of each feature of a flaw can be calculated using s=vt which allows us to form an image of the flaw
Pre-natal scanning of a foetus
- The transducer produces and detects ultrasound waves
- Ultrasound waves enter and pass through the body
- Each ultrasound pulse from the transducer, is partially reflected from the different tissue boundaries in its path
- Reflections from different boundaries at different depths return to the transducer at different times
- The depth (distance) of each structure/boundary can be calculated using s=vt and can be used to construct an image
Advantages of using ultrasound waves for medical scanning
- They are non-ionising, so it is harmless when used for scanning
- Reflected at boundaries between different types of tissues, so they can be used to scan organs and different types of tissues in the body
Why is a gel placed between the patients skin and the ultrasound probe?
To avoid most of the ultrasound waves being reflected back at the skin surface
Primary Waves (P-Waves)
- Longitudinal waves, their vibrations are parallel to the direction of energy transfer
- Can travel through solids and liquids and can therefore pass through all of the Earth’s layers
- The density of Earth increases with depth, so as the primary waves pass through it they change speed and refract gradually
- At some points the p-wave may experience dramatic changes in the Earth’s structure, causing a more dramatic refraction of the wave
Secondary waves (S-Waves)
- Transverse waves, their vibrations are perpendicular to the direction of the energy transfer
- They can only travel through solids, so can’t pass through all of the Earth’s layers
- The density of Earth increases with depth, so as the S-waves pass through the mantle, they change speed and refract gradually
- S-waves are slower than p-waves
Uses of seismic waves
- To understand the structure of the Earth
- Studying how seismic waves are absorbed and refracted, allows scientists to determine where the properties of the Earth changes dramatically
- Scientists have used these observations to understand the internal structure of the Earth
Examples of transverse waves
- Water waves
- Electromagnetic waves
Amplitude
The maximum displacement of a wave from its undisturbed position
Angle of incidence
Angle between the incident ray and and normal
Angle of reflection
Angle between the reflected ray and normal
What are infrasound waves?
Sound waves below the minimum frequency that humans can detect
Compare sound and light waves
- Light and sound waves both transfer energy without any transfer of matter.
- They both have wavelength, amplitude and frequency and can all be reflected, transmitted or absorbed by different materials.
- They are both caused by vibrations.
- However, light is cause by vibrations that are perpendicular to the direction of the wave, and are transverse, whereas sound waves are caused by vibrations that are parallel to direction of travel and are longitudinal.
- Light waves travel much faster than sound waves in air.
Properties of light waves
- caused by vibrations that are perpendicular to the direction of the wave
- can travel up to 3 x 10^8 m/s
- can travel through a vacuum
- the denser the media, the slower the travel
- energy is transferred
- no matter is transferred
- can be reflected, transmitted or absorbed
Properties of sound waves
- caused by vibrations that are parallel to the direction of the wave, longitudinal waves
- speed of travel varies but in the air speed is approximately 340 m/s
- can not travel through a vacuum, particles are needed for longitudinal waves to travel
- the denser the media, the faster the travel
- energy is transferred
- no matter is transferred
- can be reflected, transmitted or absorbed
Compare p-waves and s-waves
- Secondary waves are slower than primary waves
- Primary waves are longitudinal, secondary waves are transverse
- Primary waves can pass through solids and liquids, secondary waves can only pass through solids
- Primary waves can pass through all the Earth’s layers, whereas secondary waves cannot, they can only pass through the mantle.
- The density of the Earth increases with depth, so as primary waves pass through the Earth’s layers, and as secondary waves pass through the mantle, the waves change speed and refract gradually
- Both waves are seismic waves and are produced by earthquakes
- Both waves refract as they travel deeper through the Earth. P-waves refract more dramatically when they pass through into a different layer
Echo sounding
- Uses pulses of high-frequency sound waves to detect objects in deep water and to measure water depth below a ship
- An echo is the reflection of sound waves from a smooth surface
- The pulses from the transmitter are reflected at the sea bed directly below the ship and detected by a receiver at the same depth as the transmitter
- The time taken by each wave to travel to the sea bed and back is measured. The total distance travelled by the wave = vt, where v is the speed of sound in water
- This is twice the depth of the sea bed below the surface. So, the depth of water below the ship = 1/2vt
Describe the effect of amplitude on a sound wave
As the amplitude of a sound wave increases, the loudness of the sound will increase
Reflection vs Refraction - when does it happen?
Reflection - mirrors
Refraction - lenses
Reflection vs Refraction - medium
Reflection - Wave stays in the same medium. It is reflected at a boundary between two different materials
Refraction - Wave transmitted from one medium to another
Reflection vs Refraction - wave speed and direction
Reflection - Wave direction changes
Refraction - Speed and direction changes
