P6 Waves

what is the difference between transverse and longitudinal waves?

transverse:

• oscillations are perpendicular to the direction of energy transfer

• EM waves, water waves, waves on a string etc

• have wavelength, amplitude, peaks and troughs

longitudinal:

• oscillations are parallel to the direction of energy transfer

• sound waves, shock waves etc

• have wavelengths, compressions and rarefactions

how do you measure the speed of water waves?

1. use a signal generator attached to the dipper of a water tank so you can create waves at a set frequency

2. use a lamp to see wave crests on a screen below the tank - shadow size must equal wave size

3. the distance between each shadow line is one wavelength - measure the distance between 10 waves then divide by ten - might be easier to take a photo or video with a ruler to do this

4. use the wave speed equation to find speed

what is the difference between transverse and longitudinal waves?

transverse:

• oscillations are perpendicular to the direction of energy transfer

• EM waves, water waves, waves on a string etc

• have wavelength, amplitude, peaks and troughs

longitudinal:

• oscillations are parallel to the direction of energy transfer

• sound waves, shock waves etc

• have wavelengths, compressions and rarefactions

how do you measure the speed of water waves?

1. use a signal generator attached to the dipper of a water tank so you can create waves at a set frequency

2. use a lamp to see wave crests on a screen below the tank - shadow size must equal wave size

3. the distance between each shadow line is one wavelength - measure the distance between 10 waves then divide by ten - might be easier to take a photo or video with a ruler to do this

4. use the wave speed equation to find speed

how do you measure waves on a string?

1. attach a string with a pulley to a signal generator and vibration transducer, and turn on

2. adjust the frequency on the signal generator until there’s a clear wave on the string - frequency you need will depend on the length of string and masses used

3. measure the wavelength by measuring four or five half wavelengths, divide to get mean half wavelength, then double for wavelength

4. use the wave speed equation with the frequency from the generator

how does reflection work?

reflection happens when waves arrive at a boundary between two different materials, and they have the properties to reflect

• angle of incidence = angle of reflection

• reflection can be specular or diffuse - specular reflection happens when a wave is reflected in a single direction by a smooth surface (e.g. mirror)

• diffuse reflection is when a wave is reflected by a rough surface (e.g. paper) and the reflected rays are scattered in different directions - this happens as the normal is different for each incoming ray (which in turn means the angle of incidence is too) - when light is reflected by a rough surface, it appears matte

what is refraction?

refraction is waves changing direction at a boundary between materials

• how much it’s refracted by depends on how much the wave speeds up or slows down, which usually depends on the density of the material (denser material - slows down)

• if a wave crosses the boundary and slows down it will bend towards the normal, if it crosses into a material and speeds up it will bend away

• the wavelength changes when the wave is refracted, but frequency stays the same

• if a wave travels along the normal, it will change speed but will NOT be refracted

• optical density of a material is the measure of how quickly light travels through (higher = slower)

how can you measure refraction?

1. place a transparent block on a piece of paper and trace around it - use a ray box/laser to shine a ray of light at the middle of one side of the block

2. trace the incident ray and mark where the ray emerges - remove block and join up these two with a straight line

3. draw the normal to where the light entered to block, and use a protractor to measure the angle between the incident ray and normal, and the refracted ray and the normal

4. repeat this using blocks made of different materials, keeping the incident angle the same

how are radio waves produced?

• alternating currents are made up of oscillating charges - as they oscillate, the produce oscillating electric and magnetic fields - EM waves

• frequency of the waves produced will be equal to the frequency of the ac

• you can produce radio waves using an ac in an electrical circuit - transmitter

• when transmitted radio waves reach a receiver, the radio waves are absorbed and the energy is transferred to the electrons in the receiver

• if it is part of a complete electrical circuit, it generates the ac with the equal frequency

what are the uses of EM waves?

• radio waves → communication (long wave, 1-10km used for long distances, 10-100m for short)

• microwaves → satellites, microwave ovens

• infrared → increase or monitor temperature

• visible light → fibre optic cables

• UV → tanning, dental cleaning

• X-rays and gamma rays → imaging, medical tracing, treating cancer

how can you measure risk in radiation dose?

• radiation dose, measured in Sieverts, is a measure of risk of harm from the body being exposed to radiation

• it is not a measure of the total amount of radiation that has been absorbed

• the risk depends on how harmful the type of radiation is and the total amount

• risk can be different for different parts of the body

• 1000 mSv = 1 Sv

what are the differences between a convex and concave lens?

convex:

• lens bulges outwards, and causes rays of light parallel to the axis to converge at the principal focus

• the principal focus of a convex lens is where rays hitting the lens parallel to the axis all meet

concave:

• caves inwards, causes light to spread out (diverge)

• the principal focus is the point where rays hitting the lens parallel to the axis appear to all come from - you can trace them back until they appear to meet up at a point behind the lens

the axis of a lens is a line passing through the middle

there is a principal focus on each side of the lens - the distance from the centre of lens to principal focus = focal length

what are the three rules for refraction in a convex lens?

• an incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side

• an incident ray passing through the principal focus refracts through the lens and travels parallel tot he axis

• an incident ray passing through the centre of the lens carries on in the same direction

what are the three rules for refraction in a concave lens?

• an incident ray parallel to the axis refracts through the lens, and travels in line with the principal focus (so it appears to have come from the principal focus)

• an incident ray passing through the lens towards the principal 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

what is the difference between a real image and a virtual image?

• real → where the light from an object comes together to form an ‘image on a screen’ (like the one formed on an eye’s retina)

• virtual → when the rays are diverging, so the light from the object appears to be coming from a different place - like in a mirror, as you appear to be ‘behind’ the mirror, and in a magnifying glass

how do you draw a ray diagram for an image through a convex lens?

1. pick a point on the top of the object - draw a ray going from the object to the lens parallel to the axis of the lens

2. draw another ray from the top of the object going right through the middle of the lens

3. the incident ray that’s parallel to the axis is refracted through the principal focus on the other side of the lens - draw this passing through the principal focus

4. the ray passing through the middle doesn’t bend

5. mark where the rays meet - that’s the top of the image

6. repeat this process for a point on the bottom of the object (when the bottom is on the axis, the bottom of the image is too)

how will distance from the (convex) lens affect the image?

• an object at 2F will produce a real, inverted image the same size as the object and at 2F (other side)

• between F and 2F, it’ll make a real, inverted image bigger than the object, and beyond 2F

• an object nearer then F will make a virtual image the right way up, bigger than the object, on the same side of the lens

how do you draw a ray diagram for an image through a concave lens?

1. pick a point on the top of the object - draw a ray going from the object to the lens parallel to the axis of the lens

2. draw another ray from the top of the object going right through the middle of the lens

3. the incident ray that’s parallel to the axis is refracted so it appears to have come from the principal focus - draw a ray from the principal focus (dotted before it reaches the lens)

4. the ray passing through the middle of the lens doesn’t bend

5. mark where the refracted rays meet (that’s the top of the image)

6. repeat for bottom if not on axis

!concave lenses always create virtual images!

how do magnifying glasses use convex lenses and how do you calculate magnification?

magnifying glasses work by creating a virtual image:

• the object being magnified must be closer to the lens than the focal length

• since the image produced is a virtual image, the light rays don’t come from the place where the image appears to be

magnification = image height / object height

what is visible light?

visible light is a tiny part of the EM spectrum - a range of wavelengths that we perceive as colours

• each colour has its own narrow range of wavelengths - violets at 400nm and reds at 700nm

• colours can mix together to make other colours - the only colours you can’t make are the primary light colours - red, green and blue

• when all of these colours are put together, it creates white light

how does colour depend on absorbed and reflected wavelengths?

• different objects absorb, transmit, and reflect at different wavelengths of light in different ways

• opaque objects are those that do not transmit light - when visible light hits them, they absorb some wavelengths and reflect others

• the colour of an object depends on which wavelengths of light are most strongly reflected (e.g. a red apple reflects red and absorbs others)

• for opaque objects that aren’t a primary colour, they may be reflecting the wavelengths corresponding to that colour or the wavelength of the primary colours that mix to make that colour

• white objects reflect all of the wavelengths of visible light equally

• black objects absorb all wavelengths of visible light - we see lack of colour

how does transparency depend on absorption, transmission and reflection?

• transparent and translucent objects transmit light - not all light that hits the surface of the object is absorbed or reflected - some can pass through

• some wavelengths of light may be absorbed or reflected by transparent and translucent objects - the object’s colour is related to the wavelengths of light transmitted and reflected by it

how do colour filters work?

• colour filters are used to filter out different wavelengths of light, so only certain colours are transmitted - the rest are absorbed

• a primary colour filter only transmits that colour e.g. if white light is shone at a blue filter, only blue light will be let through

• if you look at a blue object through a blue filter, it will still look blue - however a red object would appear black when viewed through a colour filter - all of the light reflected by the object will be absorbed by the filter

• filters that aren’t for primary colours let through both the wavelengths of light for that colour and the ones that mix for it

how do objects absorb and emit infrared radiation?

• all objects are continually emitting and absorbing IR from the surface of that object

• the hotter an object is, the more IR radiation it radiates in a given time

• an object that’s hotter than the surroundings emits more IR than it absorbs as it cools down - an object that’s cooler than its surroundings absorbs more IR than it emits as it warms up

• objects at a constant temp emit IR at the same rate that they are absorbing it

• black surfaces are better at absorbing and emitting than white surfaces

• matte surfaces are better at absorbing and emitting than a shiny one

how do you investigate emission with a Leslie cube?

a Leslie cube is a hollow watertight metal cube whose four vertical faces have different surfaces (e.g. matt black, shiny black, shiny metal, dull metal)

1. place an empty Leslie cube on a heat-proof mat

2. boil water in a kettle and fill the cube with it

3. wait a while for the cube to warm up, then hold a thermometer against each side - should be the same temperature

4. hold an infrared detector 10cm away from one of the cube’s vertical faces, and record amount it detects

5. repeat this for each face - same distance each time

6. you should detect more IR from the black surface than the white one, and more from the matte than the shiny

7. repeat experiment

what is black body radiation?

• a perfect black body is an object that absorbs all radiation that hits it - none is reflected or transmitted

• all objects emit EM radiation due to the energy in their thermal energy store - covers a range of wavelengths and frequencies, not just IR

• the intensity and distribution of the wavelengths emitted depend on the object’s temperature - intensity is the power per unit area

• as the temp increases, the intensity pf every emitted wavelength increases

• however, the intensity increases more rapidly for shorter wavelengths than longer - causes the peak wavelength to decrease

how does radiation affect the earth’s temperature?

• temp depends on the amount of radiation it reflects, absorbs and emits

• during the day, lots of radiation is transferred to the earth from the sun and absorbed - increase in local temperature

• at night, less radiation is being absorbed than emitted - drop in local temp

• changes to the atmosphere can cause a change to the earth’s overall temp - if the atmosphere starts to absorb more radiation without emitting the same amount - the overall temp will rise until absorption = emission

what are sound waves?

• soundwaves are caused by vibrating objects - these vibrations are passed through the surroundings as a series of compressions and rarefactions

• sound generally travels faster in solids than in liquids, and faster in liquids than gases

• when a sound wave travels through a solid it does so by causing the particles in the solid to vibrate

• sound waves can’t travel in a vacuum as there are no particles to vibrate

how can you hear sound?

1. sound waves that can reach your ear drum cause it to vibrate - these vibrations are passed on to tiny bones in your ear and to the cochlea

2. the cochlea turns these vibrations into electric signals which get sent to your brain and allows you to sense the sound

3. different materials can convert different frequencies into vibrations - humans can hear sound in the range of 20Hz - 20kHz

4. human hearing is limited by the size and shape of the ear drum

what is ultrasound?

ultrasound is sound waves above the range of human hearing (above 20,000Hz) - ultrasound gets partially reflected at boundaries:

• when a wave passed from one medium to another, some of the wave is refracted off the boundary and some is reflected

• this means that you can point a pulse at a point and where there are boundaries between one substance and another, some of it is bounced back

• the time it takes for the reflections to reach a detector can be used to measure how far away the boundary is

what are some uses of ultrasound?

• medical imaging e.g. pre natal scanning - ultrasound waves pass through the body but when they reach a boundary some are reflected, the exact timing and distribution of these echoes are processed by a computer to produce a video image

• industrial imaging e.g. finding flaws in materials - ultrasound waves entering a material will usually be reflected by the far side of the material - if there’s a flaw such as a crack the wave will be reflected sooner

what are seismic waves?

seismic waves are caused by earthquakes and explosions:

• when there is an earthquake somewhere, it produces seismic waves which travel out through the earth - detected using seismometers

• seismologists work out the time it takes for the shock to reach every seismometer, and which parts of the earth don’t receive the waves

• when they reach a boundary they are absorbed or refracted - most of the time if they are refracted they change speed gradually (curved path), but if the property change is sudden, the path has a kink

what are the two types of seismic waves?

• P-waves → longitudinal, travel through solids and liquids, faster than S-waves

• S- waves → transverse, can’t travel through liquids (or gases), and are slower than P-waves