Amplitude
displacement from rest position to crest
Wavelength
length of a full cycle of the wave (from crest to crest)
Frequency
number of complete waves passing a certain point per second or number of waves produced by a source each second
Unit of frequency
herts (Hz)
1 wave per second
Do waves transfer matter?
- no but they can make matter oscillate (move up and down/side to side) but there's no net travel
Transverse
vibrations that are perpendicular to the direction of energy transfer of the wave.
Examples of transverse waves
- light waves (EM waves)
- slinky spring wiggled up and down
- waves on strings
- S waves (seismic)
Longitudinal waves
vibrations are parallel to the direction of energy transfer of the wave
Examples of longitudinal waves
- sound waves and ultrasound
- a slinky spring when you push the end
- P waves (seismic)
Period
time taken for one whole cycle of a wave
frequency of wave =
the reciprocal of its period
1/T
speed of a wave =
frequency x wavelength
Speed of wave is (usually) independent of
frequency and amplitude of wave
Speed and wavelength can vary depending on
medium that the wave is travelling through
Why does frequency not vary according to medium?
as it is set at its source
When any wave arrives at an obstacle/meet a new material
direction of travel can be changed by reflection/refraction
Reflection of light
- allows us to see objects
- light travelling in same direction reflects from an even surface at the same angle (clear reflection)
- light travelling in same direction reflects from an uneven surface at different angles
Law of reflection
angle of incidence = angle of reflection
What does reflection change
direction of light but doesn't change speed, wavelength or frequency
Refraction
by different boundaries cause objects to look bent
Refraction in a prism
- different wavelengths of light refract by different amounts so white light disperses into different colours when it enters a prism
- boundaries aren't parallel, different wavelengths emerge not parallel
- rainbow
Refraction in rectangular prism
- rectangular block has parallel boundaries so ray bend by same amount when they leave block as when they enters
- rays emerge as parallel
Water waves showing reflection
Sound waves
- longitudinal wave
- caused by vibrating objects
- mechanical vibrations are passed through surrounding medium as a series of compression
Sound travels faster in
solids than liquids and faster in liquids than in gases
Sounds cannot travel through
space as it is a vacuum so there are no particles
Sounds are reflected by
hard flat surfaces
Echoes =
reflected sound waves
Why is there a delay in the echo?
as echoed sound waves have to travel further so take longer to reach your ears
Sound waves refract when
- they enter a different media
When sound waves enter denser material...
they speed up
The higher the frequency
the higher the pitch
(shorter wavelength)
The lower the frequency
the lower the pitch
(longer wavelength)
Frequency is the
number of complete vibrations each second
The bigger the amplitude
the louder the sound
Ultrasound
sound that has a frequency above the range of human hearing (about 20kHz)
Uses of ultrasound
pre-natal scanning and sonar
Infrasound
sound that is too low for humans to hear (below 20Hz)
Electromagnetic waves all
- travel at the same speed (3x10? m/s) in a vacuum
- are transverse waves
EM waves with higher frequencies have
shorter wavelengths
Order of EM waves in increasing frequency and energy and decreasing wavelength
Radio Waves, Microwaves, Infra Red, Visible Light, Ultra Violet, X rays, Gamma rays
Radio waves
- wavelengths longer than 10cm
Long-wave radio (1-10km)
- can diffract around hills into tunnels
- diffraction effect makes it possible for radio signals to be received even if receiver isn't in line of sight of transmitter
Medium-wave radio
- can reflect from ionosphere depending on atmospheric conditions and time of day
Short-wave radio (10m-100m)
- can be received at long distances from transmitter
- reflected from ionosphere an electrically charged layer in earth's upper atmosphere
Microwaves
- used to communicate to and from satellites that can pass easily through earth's watery atmosphere
- signal from transmitter is transmitted into space where it is picked up by satellite receiver dish orbiting above earth
- satellite transmits signal back
Microwaves and Microwave ovens
1. Microwave ovens, microwaves need to be absorbed by water molecules in food to be able to heat it up so use a different wavelength to those used in satellite communications
2. Microwave penetrate up to a few cm into food before being absorbed by water m
Health risks with microwaves
- no conclusive proof
- phone emits microwave radiation
- if some of this radiation were to be absorbed by your body causing heating of your body tissue cells would be burned or killed
Infrared : Temperature
- heat radiation
- given out by hot objects
- can monitor tempertures
- detected by night-vision equipment
- the hotter the object the brighter it appears
Uses of IR
- cooking
- short distance with mobile phones and computers
- remote controls: emitting different patters of infrared waves to send different comands
IR: Optical Fibres
- carry data over long distances quickly
- use IR and visible
- signal is carried as pulses of light/IR and is reflected off sides of a very narrow core from one end of fibre to other
IR: Greenhouse Effect (Step 1)
EM radiation from sun can pass through earth's atmosphere to heat up surface
IR: Greenhouse Effect (Step 2)
When radiation is emitted back from surface it is at lower frequency - IR radiation
IR: Greenhouse Effect (Step 3)
Lots of this IR radiation is absorbed by atmospheric gases called green house gases.
(CO?, CH? and H?O)
IR: Greenhouse Effect (Step 4)
Gases then re-radiate heat in all direction, including back towards the Earth. So atmosphere acts as an insulating layer, stopping Earth losing all its heat at night.
IR: Greenhouse Effect (Step 5)
Without greenhouse effect in our atmosphere earth would be a lot colder but too much leads to global warming.
IR : Skin burns
- IR warms up skin
- too much causes burns
Visible light seeing with your eye
When light enters your eye, it gets refracted through lens and focused onto the retina at back of eye.
Retina then sends messages to brain via optic nerve and brain interprets them.
Visible light photography
Camera uses lens to focus visible light onto a light-sensitive film or electronic sensor that records the image
Lens aperture controls
how much light enters the camera (like the pupil in an eye)
Shutter speed determines
how long film or sensor is exposed to the light
UV uses:
- fluorescence
- bank notes
- security pens
- fluorescent lamps
- can cause skin cancer
Ionising radiation
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EM radiation
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Ionisation is dangerous
- damages DNA molecules and cause mutations
- cells might start dividing over and over again = cancer
- high doses of radiation can kill your cells altogether
X-rays
- look inside objects
- cannot pass easily through denser material
- can cause cancer
- to scan for luggage
Gamma rays
- to treat/diagnose cancer
- radioactive isotope is injected into patient - a gamma camera is then used to detect where radioactive isotope travels in body
- creates image
EM waves towards end of spectrum
can pass through material
EM waves towards middle
are absorbed
The higher the frequency of EM radiation
the more ionising and so the more harmful it is
Microwaves: HARM
- similar frequency to vibrations of many molecules
- increase these vibrations
- heats human body cells
Infrared: HARM
- make surface molecules of any substance vibrate causing a heating effect
- skin burns
Visible: HARM
- blindness
UV: HARM
- sun burn
- skin cancer
- eye damage
X-rays: HARM
- cell mutation
- cancer