Topic 6: Waves Flashcards
What are waves?
Repeated vibrations that transfer energy
What are the two types of wave?
- Transverse
- Longitudinal
What are transverse waves?
Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer
What are the characteristic of a transverse wave?
- The energy transfer is perpendicular to wave motion
- They transfer energy, but not the particles of the medium
- They can move in solids and on the surfaces of liquids but not inside liquids or gases
- Some transverse waves (electromagnetic waves) can move in solids, liquids and gases and in a vacuum
What is the highest point above the rest position known as?
Peak or Crest
What is the lowest point below the rest position known as?
Trough
What are examples of transverse waves?
- Ripples on the surface of water
- Vibrations in a guitar string
- Seismic Wave (S-Wave)
- EM Waves
How are transverse waves represented?
- Drawn as a single continuous line, usually with a central line showing undisturbed position
- Curves are perpendicular to direction of energy transfer
- Up and Down is direction of vibration whilst Left and Right is direction of energy transfer
What are longitudinal waves?
Waves where the points along its length vibrate parallel to the direction of energy transfer
What are the characteristics of a longitudinal wave?
- The energy transfer is in the same direction as the wave motion
- They transfer energy, but not the particles of the medium
- They can move in solids, liquids and gases
- They can not move in a vacuum (since there are no particles)
What are the key features of a longitudinal wave?
- Points close together: Compressions
- Points spaced apart: Rarefractions
What are some examples of longitudinal waves?
- Sound Waves
- P-Waves (Seismic Wave)
- Pressure Waves
How are longitudinal waves represented?
- Usually drawn as several lines to show that the wave is moving parallel to direction of energy transfer
What are the difference in structure, material and speed of a wave in a transverse and longitudinal wave?
Structure: Peaks and Troughs, Compressions and Rarefactions
Material: Both move in solids, liquids and gases
Speed: Both dependent on material travelling in it
What are the difference in vibration, density and pressure of a wave in a transverse and longitudinal wave?
Vibration: Perpendicular to energy transfer, Parallel to energy transfer
Density: Constant Density, Changes in Density
Pressure: Constant Pressure, Changes in Pressure
What is amplitude?
- The distance from the undisturbed position to the peak or trough of a wave
- Maximum or minimum displacement from the undisturbed position
What is the symbol and unit for amplitude?
Symbol - A
Unit - Metres (m)
What is wavelength?
The distance from one point on the wave to the same point on the next wave.
How can wavelength be measured in a traverse wave?
One peak to the next peak
How can wavelength be measured in a longitudinal wave?
Centre of one compression to the centre of the next
What is the symbol and unit for wavelength?
Symbol - λ (lambda)
Unit - Metres (m)
What is frequency?
The number of waves passing a point in one second
What is the symbol and unit for frequency?
Symbol - f
Unit - Hertz (Hz)
What is the time period of a wave?
- Time taken for a single wave to pass a point
- Time taken for one full cycle of a wave
What is the symbol and unit for time period?
Symbol - T
Unit - Seconds (s)
What is the equation for frequency?
f = 1/T
What is wave speed?
The distance travelled by a wave each second
What is the symbol and unit for wave speed?
Symbol - v
Unit - Metres per Second (m/s)
What is the wave equation?
Wave Speed = Frequency x Wavelength
Where:
v = wave speed in metres per second (m/s)
f = frequency in Hertz (Hz)
λ = wavelength in metres (m)
What are the three ways of measuring sound?
- Between two points (Distance travelled/Time taken)
- With echoes (2 * Distance to Wall/Time taken)
- Oscilloscope (Distance between microphones/Time between peaks)
Are sound waves transverse or longitudinal?
Longitudinal, they transfer energy by the molecules vibrating and knocking into neighbouring molecules
What changes when a sound wave moves from one medium to another?
- Wave speed
- Frequency
- Wavelength
True or False: The more molecules present, the faster the wave can transfer energy.
True, therefore:
Sound Waves travel fastest in solids and slowest in gases
What is reflection?
When a wave hits a boundary between two media and does not pass through but instead stays in the original medium
What is the law of reflection?
Angle of Incidence = Angle of Reflection
What surfaces reflect waves?
- Flat surfaces are most reflective and strongest
- Rough surfaces are least reflective as light scatters in all directions
- Opaque surfaces reflect light which is not absorbed by material
What is transmission?
A wave passes through a substance and emerges from other side successfully
What’s the relationship between transparency and transmission?
For light waves, the degree of transmission depends on transparency. More transparent materials allow more light to pass through and be transmitted.
How does transmission affect wave amplitude?
When waves are transmitted through materials, they are usually partially absorbed. This results in the transmitted wave having a lower amplitude compared to the incident wave. For example, sound waves become quieter after passing through a wall due to partial absorption.
What is absorption?
Where energy is transferred from the wave into the particles of a substance, and can be partial or completely absorbed
How can light be absorbed?
If frequency of light matches energy level of electrons, light is absorbed and reemitted over time as heat
Why do we see objects as colours?
Only that colour has been reflected and all other frequencies of visible light have been absorbed
What must you remember when drawing a ray diagram?
- Incident ray has an arrow pointing towards boundary and angle is labelled i
- Reflected ray ahs an arrow pointing away from boundary and angle is labelled r
What are sound waves?
Vibrations of air molecules
What are the two components of longitudinal sound waves?
Compressions (regions of higher density) and rarefactions (regions of lower density)
How do sound waves cause solids to vibrate?
The fluctuating pressure from sound waves causes the solid to vibrate in sync with the wave
What are the two main solid components that transfer sound waves in the human ear?
The eardrum (made of tissue and skin) and three small bones
What is the process of hearing sound in the human ear?
Sound waves travel down the auditory canal, create pressure variations on the eardrum causing it to vibrate, vibrations transfer to three small bones, then to the inner ear where nerve cells detect the sound and send a message to the brain
What is the range of frequencies a human can hear?
20 Hz to 20,000 Hz
How can sound waves be used to analyze hidden structures?
Through reflection, absorption, transmission, and the speed of sound in different substances
Name four applications of sound waves for exploring structures
Echo sounding (ocean floor), ultrasound (inside human body), ultrasound crack detection (rail tracks), and reflection seismology (detecting oil and gas underground)
In which state of matter does sound travel fastest?
Solids (sound travels faster in solids than liquids, and faster in liquids than gases)
What is ultrasound?
Sound waves with a frequency above the human hearing range of 20,000 Hz
What can happen to ultrasound when it meets a boundary between two media?
It may be partially reflected, partially transmitted, and partially absorbed
When is the percentage of reflected sound waves greatest?
When the difference in speed of sound between the two media is large
What is echo sounding and what is it used for?
A technique using ultrasound that is reflected off the ocean bottom to calculate water depth
How do you calculate the depth of water using echo sounding?
Multiply the speed of sound in water by the time taken for the sound to return, then divide by 2
What are ultrasound transducers able to do?
Emit ultrasound and receive ultrasound
Name three medical uses of ultrasound
Constructing images of a foetus in the womb, generating 2D images of organs and internal structures, medical treatments such as removing kidney stones
Why is ultrasound preferred over other medical imaging techniques?
It is non-invasive and believed to be harmless
How is ultrasound used in industry?
To check for cracks inside metal objects and generate images beneath surfaces
How can ultrasound detect cracks in metal?
A crack will cause some waves to reflect earlier than the rest, showing up as additional pulses on an oscilloscope trace
What are the two types of waves produced by earthquakes?
P-waves (primary waves) and S-waves (secondary waves)
What type of wave are P-waves and what can they pass through?
P-waves are longitudinal waves that can pass through both solids and liquids
What type of wave are S-waves and what can they pass through?
S-waves are transverse waves that can only pass through solids (not liquids)
Which seismic waves are faster: P-waves or S-waves?
P-waves are faster than S-waves
What is infrasound?
Sound below the frequency of human hearing (<20 Hz)
How do seismic waves help scientists understand Earth’s structure?
By timing wave arrivals at different points and observing which waves can pass through different parts of Earth
What evidence suggests that Earth’s outer core is liquid?
S-waves cannot travel through Earth’s outer core (only P-waves are detected on the opposite side of an earthquake)
What evidence suggests that Earth’s mantle is solid?
Both P-waves and S-waves can pass through it
What evidence suggests Earth’s inner core is solid?
The size and positions of P-wave shadow zones which indicate large refraction is taking place
What is the definition of electromagnetic waves?
Transverse waves that transfer energy from the source of the waves to an absorber
Name a property that all electromagnetic waves share.
They are all transverse waves
Name a property that all electromagnetic waves share.
They can all travel through a vacuum
Name a property that all electromagnetic waves share.
They all travel at the same speed in a vacuum
How many types of electromagnetic waves are there in the EM spectrum?
7 types
How is the electromagnetic spectrum arranged?
Based on wavelengths or frequencies, from longest wavelength (lowest frequency) to shortest wavelength (highest frequency)
What happens to the energy of electromagnetic radiation as frequency increases?
The energy increases
Why is high energy radiation harmful?
It is highly ionising and can damage cells and tissues, potentially causing cancer
Which types of electromagnetic waves are considered high energy and harmful?
UV, X-rays, and Gamma rays
What makes low energy radiation useful?
It is useful for communications and less harmful to humans
What is visible light?
The range of wavelengths which are visible to humans, about 0.0035% of the whole electromagnetic spectrum
Which colour of visible light has the longest wavelength?
Red
Which colour of visible light has the shortest wavelength?
Violet
Which colour of visible light has the highest frequency?
Violet
Which colour of visible light has the lowest frequency?
Red
What mnemonic can help remember the order of colours in the visible spectrum?
Roy G. Biv or Richard Of York Gave Battle In Vain (Red, Orange, Yellow, Green, Blue, Indigo, Violet)
What happens to EM waves with shorter wavelengths regarding energy?
They carry higher energy
Why are EM waves with higher energy more dangerous?
They are more ionising and can cause more damage to cells
How do microwave ovens transfer energy?
They transfer energy by radiation to the thermal store of food, as water molecules absorb microwave radiation
What type of radiation do hot objects emit?
Infrared radiation
Name three types of EM radiation emitted by the Sun.
Visible light, infrared waves, and ultraviolet waves
What happens to visible light when emitted by the Sun?
It allows living creatures to see
What happens to infrared waves when emitted by the Sun?
They heat up the Earth
What happens to ultraviolet waves when emitted by the Sun?
They cause suntans and sunburn
What are the four possible interactions when EM waves move from one medium to another?
Transmission, absorption, reflection, and refraction
What factors determine how a material interacts with EM waves?
The wavelength of the waves and the material the wave is travelling through
How does glass interact with visible light?
It transmits and/or refracts visible light
How does glass interact with UV radiation?
It absorbs UV radiation
How does glass interact with IR radiation?
It reflects IR radiation
What causes refraction of EM waves?
The difference in velocity of the waves in different substances
How do EM waves behave when they encounter materials like water or glass?
They slow down
What happens to the speed of EM waves when they enter a denser medium?
They slow down
What happens to light rays when they pass from a less dense to a more dense medium?
They bend towards the normal
What happens to light rays when they pass from a more dense to a less dense medium?
They bend away from the normal
What happens to light rays when they pass along the normal?
They do not bend at all
What properties change during refraction?
Speed and wavelength change, but frequency remains constant
Why doesn’t light change color when it refracts?
Because the frequency does not change during refraction
When investigating infrared radiation
what is the independent variable?
When investigating infrared radiation
what is the dependent variable?
What should be controlled when investigating infrared radiation?
Identical flasks (except colour), same amounts of hot water, same starting temperature, and same time interval
Which coloured surface loses heat fastest through radiation?
Black
Which coloured surface loses heat slowest through radiation?
Silver
What happens when an electromagnetic wave is absorbed by an electron?
The electron moves up to a higher energy level
What happens when an electron moves down to a lower energy level?
It emits an electromagnetic wave
What energy range is typically associated with electron transitions in atoms?
Visible and ultraviolet range (sometimes X-rays)
How are radio waves produced?
By connecting an antenna to a high frequency alternating current power source
What happens in a transmitting antenna?
The charge from the alternating current oscillates up and down, producing radio waves
What happens in a receiving aerial?
It absorbs radio waves, creating an alternating current with the same frequency as the transmitted wave
What is the relationship between frequency and ionisation potential?
As frequency increases, so does the ionisation potential
Which three types of EM radiation can ionise atoms?
Ultraviolet, X-rays, and gamma rays
What is radiation dose?
A measure of the risk of harm resulting from exposure to ionising radiation
What unit is radiation dose measured in?
Sieverts (Sv) or millisieverts (mSv)
What is the typical background radiation per year?
About 1.5 to 3.5 mSv per year
What amount of radiation is enough to cause death?
8 Sv
Why are microwaves potentially dangerous?
Certain frequencies are absorbed by water molecules, which could cause internal heating
Are microwaves used in mobile phones harmful?
They emit very small amounts of energy which are not known to cause harm
How do microwave ovens prevent radiation from escaping?
By using metal walls and a metal grid in the glass door
What damage can UV radiation cause to eyes?
Severe eye damage
How do sunglasses protect eyes from UV radiation?
They absorb ultraviolet light
How does UV radiation damage skin?
It is ionising, can kill cells or cause them to malfunction, resulting in premature aging and skin cancer
How does sunscreen protect skin?
It absorbs ultraviolet light
Why are X-rays and gamma rays the most dangerous EM waves?
They are the most ionising, can penetrate the body, cause internal damage, and mutation of genes
What precaution do doctors take when using X-rays?
They leave the room when taking X-rays to avoid unnecessary exposure
What are radiation badges used for?
To measure the amount of radiation exposure in people working with radiation
Which EM waves are used for satellite communications?
Radio waves and microwaves
Which EM waves are used for television and radio broadcasting?
Radio waves
Which EM waves are used to kill bacteria?
Ultraviolet, X-rays, and gamma rays
Which EM waves are used in security scanners at airports?
X-rays
Which EM waves are used in night vision equipment?
Infrared
Which EM waves are used for cooking food?
Microwaves
Which EM waves help diagnose broken bones?
X-rays
Which EM waves are used in cancer treatments?
Gamma rays
Why are radio waves suitable for communications?
They can travel long distances without much energy loss and can pass through buildings
Why are microwaves suitable for satellite communications?
They can pass through the atmosphere with little absorption
Why is infrared suitable for night vision?
It is emitted by all warm objects, and special cameras can detect it
Why are X-rays suitable for medical imaging?
They can pass through soft tissue but are absorbed by denser materials like bone
