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Topic 6 Waves By Bethan Poole (6.1 Waves in air, fluids and solids :wave:…
Topic 6 Waves
By Bethan Poole
6.3 Black body radiation
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6.3.1 Emission and absorption of infrared radiation
All bodies emit and absorb infrared radiation
Perfect black body absorbs all of the radiation incident on it- black body doesn't reflect or transmit any radiation
Good absorber is a good emitter a perfect black body would be the best emitter
The hotter the body the faster it emits infrared radiation
Rate radiation is emitted depends on
Nature of the surface
The surfaces temperature
6.3.2 Perfect black bodies and radiation
Temperature of an object related to the balance between radiation absorbed and radiation emitted
Temp. of object/body determines
Rate at which it emits radiation
Wavelength of the radiation it emits
As the temp. increases amount of radiation emitted at all wavelengths increases but the intensity of shorter wavelengths increases faster
As an object is heated the wavelengths shorten causing it to turn red and then when heated further white
i.e on a sunny day the ground will increase in temperature- as it absorbs radiation from the sun faster that it emits it- eventually the amount emitted will equal the amount absorbed so the ground will be at constant temp.
Temp of earth depends on many things
How much energy received from sun
How much energy reflected back to space
How much energy it emits to space
Earth's atmosphere affects how much of the radiation emitted from the surface escapes into space
6.1 Waves in air, fluids and solids
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6.1.1 Transverse and longitudinal waves
Longitudinal
Oscillations are parallel to the direction of the energy transfer
All waves transfer energy from one place to another
Particles making up a wave oscillate about a fixed point- transfers energy on to the next particle
Transverse
Oscillations perpendicular to direction of energy transfers
e.g. if a stone is dropped in water , waves ripple outward carrying the energy
6.1.2 Properties of a wave
All waves have a
frequency
- number of waves passing a fixed point per second (Hz)
amplitude
- maximum displacement that any particle achieves from its undisturbed position (m)- seen as wave height- indicates to amount of energy a wave is carrying (more energy= higher amplitude)
wavelength
- Distance from one point on a wave to the equivalent of the next wave
Period
- Time take to complete one oscillation (s)
Waves speed
Rate at which energy is transferred
Ripples on the surface of water are slow enough so that speed can be observed using a stopwatch
When waves change medium speed and wavelength change but frequency doesn't change
Speed and wavelength are directly proportionate
6.1.3 Reflection of waves
When waves reach a boundary between two mediums they can be reflected, refracted, absorbed or transmitted
Ray diagrams show what happens
Rays must be drawn with a ruler
Each straight section should have a single arrow - indicates direction of movement
Should draw a normal at right-angle to boundary
All relevant angles labelled
When waves reflected at a surface angle of incidence= angle of reflection
Refraction
When a wave passes through a different media it can be refracted- change direction
Direction of refraction depends on
Angle it hits the boundary
Materials involved
Light waves- way in which a material affects refraction called- refractive index
When light travels
From material with high refractive index to lower- bends away from normal
From material with low refractive index to higher- bends towards from normal
Due to difference in wave speeds in different media
When light enters a medium it travels slower in...
First part of light wave to enter medium slows down
Rest of wave continues at a higher speed
Causing wave to change direction
Reflection
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6.1.4 Sound waves
Amplitude is the loudness
Frequency and wavelength relate to the pitch- higher frequency means higher pitch
Normal hearing range for humans- 20Hz-20KHz
Can travel through liquids solids and air- always heard due to vibrations- solids whole object vibrates due to oscillations at same frequency
Ear
- causes ear drum and other structures to vibrate, which is heard as sound
Limited range of conversion causes limited human hearing
Sound waves converted to vibrations examples- Ear drum, by a microphone or glass being shattered by an opera singer
6.1.5 Waves for detection and exploration
Ultrasound
Ultrasonic waves- frequency grater than 20 KHz so can't be heard by humans
When they meet a boundary it is partially refected
Determine how far away a boundary is by measuring time taken for the reflection to reach the detector
Used in industry to detect defects without cutting into them
Used in medicine including pre-natal scans and kidney stone detection
Echo sounding
AKA Sonar- used to detect objects in deep water and measuring depth of water
Send an ultrasound pulse into water- reflected back when hits the surface
Time between pulse being sent and reflection detected is used to calculate distance travelled- must be halved to find depth
Seismic waves
P-waves
(Primary)
Longitudinal
Travels at speed of sounds (2x speed of S-Waves)
Travels at different speeds through solids and liquids
S-Waves
(Secondary)
Transverse
Not able to travel through liquids
Time difference between arrival of S-waves and P-waves at different detectors provides info about location of earthquake and materials travelled through
Structure of Earth
During an earthquake...
Seismic waves travel outward from earthquake
Seismic waves travel in a curved path (earth's density increases with depth)
Detectors measure when and where waves arrive
Get evidence from P-Wave and S-Wave shadows
P-Wave Shadow Zone
Can travel through liquid outer core
Refracted at boundary between semi-solid mantle and liquid outer core
Study of it helps determine size and composition of inner and outer core
Refracted at boundary of liquid outer core and solid inner core
S-Wave Shadow zone
Can't travel through liquid outer core- results in large shadow zones- opposite side of earth to where earthquake originated
Shadow provides evidence for the size of the Earth's core
6.2 Electromagnetic radiation
6.2.1 Types of electromagnetic waves
Transverse- transfer energy from the source to an absorber
Form a continuous spectrum and they all travel at the same velocity in air or a vacuum
Spectrum
From low to high frequency
From low energy waves to high energy waves
Order: radio, microwave, infrared, visible light, ultraviolet, x-rays and gamma rays
Our eyes only detect visible light
Wavelength affects how it is absorbed, transmitted, reflected and refracted by different substances- affects its uses
6.2.2 Properties of EM waves
Radio Signals
Can be caused by oscillations in circuits
Frequency of wave matches frequency of the electrical oscillation
When radio waves are absorbed by a conductor they may created an alternating current with the same freq. as the radio wave- how signal is received
When this oscillation is induced in an electrical circuit it creates an electrical signal that matches the wave
Hazards
EM waves can be generated or absorbed over a wide freq. range due to
Electrons between energy levels due to heat or electrical excitation can generate waves
Changes in atoms nucleus can generate waves
UV waves, X-rays and Gamma rays
Carry enough energy to have a hazardous effect of humans
UV
Can cause skin to age prematurely
Increases risk of skin cancer
X-rays and Gamma Rays
Ionising radiation
Can damage cells by ionising atoms
Can damage cells by ionising atoms
Effect depends on does (sieverts) and type of radiation
6.2.5 Visible light
Visible light
When a light is incident on an object it can be absorbed, reflected or transmitted
Reflection on smooth surface-
Specular reflection
Reflection on a rough surface when light is scattered-
Diffuse reflection
All objects are either
Transparent
Transmit light coherently so objects on the other side can be clearly seen
Translucent
Transmit light but rays are scattered so can't be clearly seen
Opaque
Either reflect or absorb all light incident on them so no light passes through
Colour
Each colour has a narrow band of wavelength and frequency
When object appears coloured...
It is reflected the light of that particular wavelength
It's absorbing all other wavelengths
If all wavelengths reflected equally- white
If all wavelengths absorbed- appears black
Filters
Work by absorbing some wavelengths and not others- transmitted wavelengths control what colour the filter allows to pass through
When looking at objects through filters...
If object same colour as filter it will appear its true colour
If object a different colour
Red and blue object in a red filter =red and black- filter allows red light through but not blue
Red and blue in green filter= black. filter doesn't let blue or red pass through
6.2.4 Lenses
Lenses
Convex
Wider in the middle
Parallel waves entering are brought to a focus at the principal point (focal point)
Sometimes called converging lenses
Distance from lens to focal point is focal length
Concave
Wider at edges
Parallel waves entering spread out
Makes rays appear to have come from the principal focus on the side they started
Sometimes called diverging lenses
Images and magnification
Convex lenses produce real or virtual images- concave only produce virtual
Real image on the opposite side of lens to objects and can be projected on a screen
Virtual- same side as object- only seen through the lens
Ray diagrams
When drawing...
Draw principal axis- horizontal line running straight through the lens
Use correct lens symbol and mark principal foci on either side of lens (draw a dot labelled F)
Mark position of object as an arrow
Magnification= image height/object height
- a ratio
Concave represented by
Convex represented by
6.2.3 Uses and applications of EM waves
Radio
Uses
TV
Radio
Bluetooth
Suitable
Low energy waves- not harmful, makes them ideal
Microwaves
Uses
Satellite communications
Cooking food
Suitable
Travel in straight lines through the atmosphere
Makes them ideal for transmitting signals to satellites in orbit and transmitting them back down
Infrared
Uses
Electrical heaters
cooking food
Infrared cameras
Suitable
Glow hot a electricity flow through them- transmit infrared energy that is absorbed by the food and converted back into heat
Ultraviolet
Uses
Energy efficient light bulbs, security marking, sunbeds
Suitable
In light bulbs UV waves produced by the gas in the bulb when it is excited by the current
UV waves absorbed by bulb coating- fluoresces giving of visible light
X-rays
Uses
Medical imaging/ treatment
Suitable
Penetrate soft tissue but not bone
Photographic plat behind will show shadows
Visible light
Uses
Fibre optic communications
Suitable
Visible light travels down optical fibres from one end to another without being lost down the sides
Gamma rays
Uses
Sterilising food, treating tumours
Suitable
Most energetic and can destroy bacteria and tumour
