Physics - Topic 6 - Waves (Images and Ray Diagrams (Drawing a ray diagram…
Physics - Topic 6 - Waves
Transverse and Longitudinal Waves
Waves transfer energy from one place to another without transferring matter
is the distance between the same point on two waves.
is the number of complete waves per second, measure in
is the highest point of disturbance, measured from the point of least. (Highest point on peak to the straight line)
of waves is the time it takes for a full cycle of waves.
E.g, dropping a twig in a pool creates ripples, these dont carry water away
Strumming on a guitar creates sound, these waves do not carry air away.
Waves travel through a
, transferring energy to one another, but
only energy is transferred, not particles.
Transverse waves have sideways vibrations
All electromagnetic waves
Ripples and waves in water
oscillations are perpendicular
(at 90 degrees) to the direction of energy transfer. Most waves are this type
Waves on a string
Sound Waves in the air, ultrasound
Shock waves, e.g some seismic waves.
Oscillations are parallel to the energy transfer
WAVE SPEED = FREQUENCY X WAVELENGTH
is the speed of energy transfer. The Equation is applied to all waves
Experimenting With Waves
Measure the Speed of Water Ripples Using a Lamp
2)Use a lamp to show the
on a screen below.
3)The distance between each shadow line is
Measure the distance of
10 lines at once and then divide by ten
- this will find an average, good for smaller waves.
1) Using a
signal generator attached to the dipper
of the ripple tank, you can create waves at a
4) You could do this with a photo of a ruler next to the waves if your having trouble
5) Use the formula to calculate the speed.
This method lets you measure waves without disturbing them.
You Can Use The Wave Equation for Waves on Strings
3) Then measure the wavelength. The best way to do this is to
measure 5 half length, find the mean and the double it
4) The frequency is waht the signal generator is set to
2) Adjust the frequency until a clear wave can be seen. This will depend on the string length between the pulley and traducer and the mass.
5) Use the formula to find the speed
See set up on page 74 of the guide
Turn on the signal generator and vibrator transducer. The string will string to vibrate.
Use an Oscillations to Measure the Speed of Sound
2)Start with the microphones at the same point on the speaker, and slowly move on away until the arc of the waves line up
3)Measure the distance between the two microscopes to find one wavelength.
1) Set up the oscilloscope so that the detected waves are shown at the microphones are shown separately.
4) You can use the formula to measure the speed. The frequency is whatever you set the signal generator to.
By using a
and a speaker you can generate sound with specific frequency .Using
two microscopes and an oscilloscope
you can find their wavelength.
speed of sound 300m/s
, your results should roughly match this.
Simple Ray Diagrams
3) The angle of reflection is the angle between the reflected wave and the normal.
2) The angle of incidence is the angle between the oncoming wave and the normal.
Angle of incidence = Angle of reflection
is an imagery line that is perpendicular to the point of incidence
5) Usually indicated with a dotted line
All Waves be Absorbed, Transmitted or Reflected
The wave is
by the material the wave is trying to cross into, and thus becomes part of the materials energy store
When waves reach the boundary between two materials, three things can happen:
The waves are
: they carry on as normal, this often leads to
The waves are
Reflection can Secular or Diffuse
Waves are reflected to different boundaries in different ways
is when the wave is reflected in
i.e light and a mirror.
is when a wave is reflected by a
and the reflected rays are
scattered in varying directions
This happens because the
angle of incident is different
for each ray as the
normal is different for each
When light is reflected by a rough surface tends to be
without good relection
Radio Waves Are Made By Oscillating Charge
Radio waves can be produced using an AC current in a circuit.
The object in which electrons (charges) oscillate is a
When these transmitted waves reach a receiver they are absorbed.
The frequency of the weaves produces is equal to the current
The energy from the waves is carried to the electrons in the receivers material
AC is made up of oscillating charges, as these charges oscillate they produce the oscillating electric and magnetic fields
The electrons oscillate and, if the receiver is part of a complete circuit, AC is produced
Em waves are made of oscillating electric and magnetic fields
The current s frequency is equal to the waves.
Radio waves Are Used Mainly For Communication
Short wave radio signals (10-100m) can also be received at long distances because they can be
reflected from the ionosphere
- electrically charged part of the atmosphere.
Bluetooth uses short wave radio to send signals / data over short distances between devices without wires.
This means the radio signals can be received without the receiver being in line of sight of the transmitter.
Medium - wave signals, can also reflect from the ionosphere, depending on weather condiotions
Long-wave radio (wave length = 1-10km) can go from London to halfway around the world.
This is due to the fact they diffract (bend) around the curvature of the earth, and hills or mountains.
The radio waves used for TV and Fm radio have very short wavelengths,m they must be in direct sight of the transmitter.- it cannot bend or travel far through buildings.
These waves are EM radiation and have a wavelength longer than about 10cm
EM Waves and Their Uses
Microwave ovens use a different wavelength
The waves get a few cm into the food before absorbtion and the transferring of energy to the water causing it to heat up.
The water molecules than transfer the heat to the rest of the food.
The microwaves need to be absorbed by water molecules, different wavelengths are used to the satellite ones.
Infrared radiation Can Be Used to Increase or Monitor Temperature
IR is given out by hot objects, the hotter, the more given out.
Infrared cameras detect IR and monitor temp. The camera detects IR and turns it into a electric signal, which is then displayed as a picture. The hotter the brighter it appears. Used to detect thermal stores
Absorbing IR causes the object to heat up. Food can be cooked using it e.g a toasters heating element.
Electric heaters work the same.They contain a long wire that heats when a current floes through it. The wire emits IR and some visible light.
The emitted IR is then absorbed by objects as the energy is transferred to thermal stores.
Microwaves are used by Satellites
It is then picked up by a satellite receiver dish which sends it back to earth.
It is then received by a dish on the ground. There is a slight time delay between being sent and received due to the long distances.
For satellite TV, the signal from the transmitter goes to space.
Communication to and from satellites require microwaves, However they must be able to pass through the earths water atmosphere.
Fibre Optic Cables Use Visible Light To Transmit Data
They work due to reflection. The light rays bounce back and forth until the reach the end of the fibre.
Visible light is used in optical fibres.
Optic fibre is thin glass or plastic fibres, that carry data. They do this over long distances as pulses of visible light.
It is not easily absorbed or scattered whilst travelling through the fibre.
UV Radiation Gives a Suntan.
UV is produced by the sun. Expose will either tan you or burn you.
Some people use tanning beds that have UV lamps, over exposure however is dangerous and can lead to cancer.
Security pens use a special ink visible in only UV light, good for identifying property.
Fluorescent light generate UV radiation, which absorbed and re-emitted as visible light, bu a layer of phosphorus on the inside of a light bulb. Efficient- provide light for sustained periods of time
Florescence is a property of certain chemicals, UV is absorbed and then visible light is emitted instead,hence they look bright.
X-rays and Gamma rays are Used in Medicine
Both can be used to treat cancer. High dosage will kill living cells, therefore they target the cancerous ones using this method.
Gamma radiation is also a medical tracer. It is injected and followed through the body with its progress recorded. It is used as it it passes out of the body with ease and can also be detected
X-ray passes through flesh with ease. But not through denser material like bone. So its he amount of radiation absorbed (or not) that gives you the image.
Both X-rays and gamma rays can be very harmful to people. So radiographers wear lead aprons and stand behind a lead screen or leave the room to minimise exposure.
Radiographers use X-rays to take 'photos' to show broken bones
Dangers Of Electromagnetic Waves
Some EM radiation can be harmful
High frequency waves such as X-rays, UV and gamma, transfer high amounts and therefore cause alot of damage.
UV radiation can age skin permanently, blind you and lead to cancer as it damages surface cells.
Low frequency waves, like radio waves will not do anything and pass through without being absorbed.
X-ray and gamma waves are ionising. They can knock electrons of atoms essentially. This can lead to gene mutation or cell destruction and also cancer.
Em radiation entering tissue is often times harmless. However depending on how much energy is transferred depends on whether it causes havoc ornot.
You Can Measure Risk Using The Radiation Dose in Sieverts
Whilst the high frequency waves are dangerous, they can also be useful, hence before their use thee risks evaluated, as are the benefits to see if it should be used.
For examples, X-rays can cause cancer. However the risk is much smaller than potential, risk of not finding and treating the injury
Radiation dose is a measure of the risk of harm of the body being exposed.
This is not a measure of the total radiation absorbed
The risk depends on the total amount of radiation absorbed and how harmful they type of radiation is
A sievert is a large unit. Often millisieverts are used.
Risk Can Be Different For Different Parts Of The Body
Radiation dose (mSv)
A CT scan uses X-rays and a computer to build a picture of the interior body.
The table below shows the different parts of the body when having CT scans .
IF its on their chest its 4 times more likely to cause damage than a head scan.
Different Lenses Produce Different Kinds of Images
The principle focus of a convex lens is where the rays hitting the lens parallel to the axis all meet
On a concave lens it is the point where the rays hitting the lens parallel to the axis come from.
The axis of a lens is a line passing through the middle of the lens
There is a principle focus on each side, the distance from the centre of the lens to one of the said points is called the focal length.
A concave lens caves inwards. It causes the parallel rays of light to spread out. (diverge)
A convex lens bulges outwards. It causes light parallel to the axis to be brought together at the principle focus. (converge)
There Are Three Rules for Refraction in a Convex Lens
An incident ray passing through the principle focus refracts through the lens and travels parallel to the axis.
An incident ray passing through the centre of the lens carries on in the same direction.
An incident ray parallel to the axis refracts through the l;ens and passes through the principle focus on the other side.
Three Rules For Refraction of Concave Lens
An incident ray passing through the centre of the lens carries on in that direction.
An incident ray passing through the lens towards the principle focus refracts through the lens and travels parallel through the axis.
An incident ray parallel to the axis refracts through the lens and travels in line with the principle focus (it will appear to have come from the principle focus)
Images and Ray Diagrams
Lenses Can Produce Real and Virtual Images
A virtual image is when the rays diverge so the appear to be coming from somewhere entirely different then they are.
When looking in a mirror, it is a virtual image because we appear behind the mirror
A real image is where the light from one object comes together to form an image on a screen. Like the image on the retina
Another example is through a magnifying glass as it is larger than it actually is.
Drawing a ray diagram for an image through a convex lens.
Pick a point on top of the object. Draw a line going from the object to the lens parallel to the axis.
Draw another from the top of the object to the middle of the lens.
The incident ray on the top is refracted through the principle focus on the other side so draw that line.
The ray passing through the middle of the lens doesn't bend
Mark where the rays meet . That's the top of the image
When the bottom of the image is on the axis, it is also on the axis on the other side.
Distance from the Lens Affects the Image
Kinda hard to explain without pictures so : Page 83, bottom of the page
Drawing a Ray Diagram for an Image Through a Concave Lens
The one on top is refracted so it looks as if it has come from the principle focus. Draw this line and include a dotted on from the principle focus to the start of the line at the lens.
Draw another line from the top to the middle of the lens.
The ray at the middle doesn't bend
Pick a point on the top of an object. Draw a line going from the object, to the lens, parallel to the axis.
There they cross, behind the lens, is the top of the image.
A Concave lens will always create a virtual image. It will be the right way up, smaller and on the same side of the lens as the object.
Magnifying Glasses Use Convex Lense
Since the image is virtual, the light rays do not come from where they appear.
You can't project a virtual image on a screen.
- useful in exams if asked
The object must be closer to the lens than the focal legnth
Magnification = image height / object height
Colour and Transparency Depend on Absorbed Wavelengths
With opaque objects that aren't a primary colour. They either reflect the wavelength the most, or it can mix together to make that colour.
White objects reflect all wavelengths equally
The colour of opaque objects depends on which wavelength of light are most strongly reflected. E.g apples appear red because the wavelengths corresponding to red are reflected the most
Black absorbs all wavelengths of visible light equally
Opaque objects do not transmit light. When visible light hits them they absorb some wave length of light and reflect others.
Transparent and translucent objects allow some wavelengths through
Different objects transmit, absorb and reflect different wavelengths of light in different ways.
Some wavelengths may be absorbed or reflected and this will determine the objects colour (translucent and transparent)
Colour Filters Only Let Through Particular Wavelength
A primary colour filter only transmits that colour. If white light is shone through a blue colour filter, only blue light is shown.
If you look at a blue coloured object, through a blue filter, it will be blue. Blue light is reflected onto the objects surface and transmitted by the filter.
If it was red through a blue filter, it would be black as any light reflected by the object would be absorbed.
Filters that are not primary colours let through the wavelength of light for that colour and the wavelength of the primary colour that can be added together to make the colour.
Colour filters are used to filter out different wavelengths so only certain colours are transmitted, the rest are absorbed
Visible Light Is Made Up of a Range of Colours
Each colour has its own narrow range of wavelengths and frequencies. Violet 400nm - res at 700nm
Colours can also mix together
Em waves cover a large spectrum. We can only see a small part of the the visible light spectrum, which we perceive as individual colours.
When all are put together it creates white light
Infrared Radiation and Temp
Every Object Absorbs and Emits IR
An object that hotter than its surrounding emits more IR than it absorbs as it cools down.
An object cooler than surroundings absorbs more than it emits as it warms up.
The hotter, the more infrared radiation is given of f in a given time
All objects are conditionally emitting and absorbing infrared radiation. Infrared radiation is emitted from the surface of an object
You Can Investigate Emission With a Leslie Cube
Hold a infrared detector at a set distance. Record the amount of IR it detects
Repeat this measurement for each of the vertical sides.
Wait for it to warm. Hold a thermometer against each side, should be the same temp
More IR should be detected from the black surface of the cube. More on the matte one aswell
Boil water and fill the cube
Repeat more than once
Place an empty Leslie cube on a heat-proof mat.
Be careful with hot water
Black Body Radiation
Black Bodies are Ultimate Emitters
The intensity and distribution of the wavelengths emitted by an object depend on the objects temperature. Intensity is power per unit area.
As the temp of the object increases and the intensity of every emitted wavelength increases
All objects emit EM due to the energy in their thermal store. This radiation isn't just in the IR part of the spectrum. It covers a range of wavelengths and frequencies.
The intensity increases more rapidly for shorter wavelengths
A perfect black body absorbs all radiation that hits it.
Radiation Affects Earth Temp
During the day, lots of radiations is transferred from the sun causing an increase in local temps.
At night it decreases
The overall temp depends on the amount of radiation it emitts, absorbs and reflects.
Overall the temp of the Earth stays fairly constant.
Changes in the atmosphere change the overall temp. It it absorbs more radiation without emitting the same amount it will rise until they are equal again
You Hear Sound When Ear Drums Vibrate
The cochlea turns these vibrations to electric signals which go to the brain allowing us to sense sound
Different materials can pick up different frrquencies. We hear from 20Hz to 20kHz
These vibrations are passed on to the tiny bones in the ear (ossicles), through the semicircular canals and to the cochlea
Human hearing is limited to the size and shape of the ear drum, as well as the structure of all parts involved
Sound waves that reach the ear drum cause it to vibrate
Sound Waves Can Reflect and Refract
Sound waves will be reflected by hard flat surfaces. This is what echoes are.
They refract when reaching different media. The denser, the faster they travel. This is because in entering a different media, its wavelength changes but not the frequency so it must speed up.
Sound Travels As A Wave
When travelling through a solid, the sound waves cause particles to vibrate
Sound cannot travel in space as it is a vaccum
Sounder travels in this order (fastest to slowest): solid, liquid, gas
Eventually sound will travel through the ear and hit the ear drum
Sound waves are caused by objects vibrating. They are passed to surrounding mediums as vibrations and rerafractions
Ultrasound Waves Get Particularly Reflected At Boundaries
This means you can point a pulse of ultrasound at a object and wherever there are boundaries between one substance and another, some ultrasound is reflected back
The time it takes for the reflection to reach the detector can be used to measure how far the boundary is.
When a wave passes between a medium, some is reflected off the boundary and some is transmitted. This is partial reflection
Ultrasound Is Useful In Lots Of Different Ways
These echoes that reflect back can be processed by a computer to give a video image of a foetus
No one knows for sure whether it is safe or not however
They can pass through the body, som earerelected back others pass through
It can be used to find flaws in objects such as pipes or materials like wood and metal
Ultrasounds will usually pass through and be reflected back by the far side
However if a crack or flaw is detected they will be reflected back sooner.
Ultrasound is sound above what we can hear (20000 Hz) Electrical devices can be made which produce oscillations over a range of frequencies
Exploring Structure Using Waves
Earthquakes and Explosions Cause Seismic Waves
Seismic waves travel all through the earth and are detected with seisometers
Seismologists work out the time it takes each shock wave to reach these monitors, noting which don't receive them
Some are absorbed, others refracted when reaching new material
Most of the time they are refracted and change speed gradually resulting in a curved graph. A sharp change leads to a sharp change on the graph
P Waves Travel through the Crust, S-Waves Can't
P Waves: Longitudinal. Travel through solids and liquids. Travel faster than S waves
S-Waves: Transverse can't travel through liquids (or gases). They're slower than P-waves
By observing how seismic waves are absorbed and refracted scientists have been able to work out where properties of the earth change dramatically. These have helped us work out the size of the core and internal structure
Detection and Exploration
Reaching a boundary can cause a number of things to happen
It can completely or partially reflect, refracted or absorbed, or carry on at a different speed
Different properties depending on the material they travel through
Mapping the progression and path can give you information on the property of that structure