P5 - waves in matter Flashcards
what do waves do?
transfer energy in the direction they’re travelling
what happens when waves travel through a medium?
the particles of the medium vibrate and transfer energy between each other; overall the particles stay in the same lattice but energy is transferred
what is the amplitude of a wave?
the displacement from the rest position to a crest or trough
what is the wavelength of a wave?
the length of a full-cycle of the wave e.g. from crest to crest or compression to compression
what is the frequency?
the number of complete waves or cycles passing a certain point per second
what is the period of a wave?
the number of seconds it takes for one full cycle; period = 1/frequency
what type of vibrations do transverse waves have?
sideways
describe transverse waves
in transverse waves, the vibrations are perpendicular to the direction the wave travels
what are some examples of transverse waves?
- all electromagnetic waves
- S waves
- ripples/waves in water
can transverse waves travel through liquids?
they can travel on the surface of a liquid, but they can’t travel through liquids
what type of vibrations do longitudinal waves have?
parallel
describe longitudinal waves
in longitudinal waves, the vibrations are parallel to the direction the wave travels
what are some examples of longitudinal waves?
- sound waves
- P waves
what do longitudinal waves do to the arrangement of the particles in the medium they pass through?
longitudinal waves squash up and stretch out the arrangement of particles in the medium they pass through, making compressions and rarefactions
what is the equation for wave speed?
wave speed = frequency x wavelength
what are the units for frequency?
Hz
what can you use to measure the speed of sound?
an oscilloscope
when experimenting with an oscilloscope, what can be used to generate sounds with a specific frequency?
by attaching a signal generator to a speaker, you can generate sounds with a specific frequency
when experimenting with an oscilloscope, what can be used to find the wavelength of the sound waves generated?
you can use two microphones and an oscilloscope to find the wavelength of the sound waves generated
how is an oscilloscope used to measure the speed of sound?
- set up the oscilloscope so the detected waves at each microphone are shown as separate waves
- start with both microphones next to the speaker, then slowly move one away until the two waves are aligned on the display but have moved exactly one wavelength apart
- measure distance between microphones to find wavelength
- use formula to find speed of sound waves passing through the air (frequency is whatever you set signal generator to in the first place)
what can you measure using a ripple tank?
- speed
- frequency
- wavelength
what is used to generate waves in a ripple tank?
a signal generator attached to a dipper
how does the signal generator attached to a dipper generate waves in a ripple tank?
the signal generator moves the dipper up and down to create water waves at a fixed frequency
to measure frequency with a ripple tank, what is needed?
a cork and a stopwatch
how is frequency measured using a ripple tank?
- float cork in ripple tank; should bob up and down as waves pass
- when cork is at top of a ‘bob’, start stopwatch
- count how many times the cork bobs in e.g. 20 secs
- divide this number by time interval to get number of ‘bobs’ per second; this is frequency of wave
to measure wavelength with a ripple tank, what is used?
a strobe light
how is wavelength measured using a ripple tank?
- place card covered with cm squared paper behind ripple tank
- turn on strobe light and adjust its frequency until waves appear to ‘freeze’
- using squared paper, measure distance that e.g. five waves cover; divide this distance by number of waves to get an average wavelength
to measure wave speed with a ripple tank, what is used?
a pencil and a stopwatch (and a partner)
how is wave speed measured using a ripple tank?
- place a large piece of paper next to the tank
- as waves move across tank, one person track path of one crest on paper using pencil; make line straight and parallel to direction wave travels, could use ruler to help
- other person time how long first has been drawing for; pick duration e.g. 10 secs and stop drawing when time passed
- calculate speed of wave by measuring length of line and using distance = speed x time
what happens to waves at boundaries?
they are absorbed, transmitted and reflected
describe how waves may be absorbed at boundaries
the wave may be absorbed by the second material, transferring energy to the material’s energy stores e.g. how microwaves work
describe how waves may be transmitted at boundaries
the wave may be transmitted; it carries on travelling through new material, often at different speed which can lead to refraction
describe how waves can be reflected at boundaries
the wave may reflect off the boundary; this is where incoming ray is neither absorbed or transmitted but ‘sent back’ away from second material
what does reflection of light allow us to do?
see things
what is the one rule for all reflected waves?
angle of incidence = angle of reflection
what are both the angle of incidence and the angle of reflection measured from?
the normal
what is the normal that the angle of incidence and angle of reflection are measured from?
an imaginary line that’s at right angles to the surface at the point the light hits it (drawn with dotty line)
how does the reflection of visible light let us see things?
light bounces off objects and into our eyes
how do light rays reflect off smooth surfaces?
all in the same direction, giving a clear reflection
e.g. a mirror
how do light rays reflect off rough surfaces?
in all different directions; the angle of incidence still equals the angle of reflection for each ray, but the rough surface means each ray hits the surface at a different angle, so is reflected at a different angle- scattering light
e.g. paper
what is white light?
a mixture of all the different colours of light, which all have different wavelengths
what happens to all the colours of light in white light?
they are reflected at the same angle; white light doesn’t split into the different colours when it reflects, as all wavelengths follow rule
what is refraction?
when waves bend
for light, when we say density, what do we mean?
optical density; how the material affects the speed of light
why might wave travel at different speeds in different materials?
waves travel at different speeds in materials with different densities
what happens to a wave’s speed when it crosses a boundary between materials e.g. glass to air?
it changes
what happens to a wave’s frequency when it crosses a boundary?
the frequency of a wave stays the same when it crosses a boundary
what happens to a wave’s wavelength when it crosses a boundary?
the wavelength decreases if the wave slows down; it increases if it speeds up
how does refraction work?
if a wave hits a boundary at an angle to the normal, this change in speed and wavelength makes the wave bend, this is refraction; the greater the change in speed, the more it bends
if a wave slows down, what does it bend towards?
the normal; whereas if it speeds up it will bend away from the normal
what does sound travel faster in?
denser material; so going from water to air, wavelength increases
what do electromagnetic waves travel more slowly in?
denser material; going from air to glass, their wavelength would decrease and they would bend towards the normal (if they refracted)
what can affect how much an electromagnetic wave refracts?
wavelength; shorter wavelengths bend more
what do the colours of light all have different of?
wavelengths
in order of longest to shorter wavelengths, what is the order of the colours of light?
red
orange
yellow
green
blue
indigo
violet
how do the speeds of the colours of light compare?
they travel at the same speed in air, but when they enter a denser substance e.g. glass, the shorter wavelengths slow down more so refract more
what are the two different types of reflection?
specular or scattered
what is specular reflection?
when waves are reflected in a single direction by a smooth surface; this means you get a clear reflection e.g. when light is reflected by a mirror
when does scattering reflection occur?
when waves are reflected by a rough surface (like paper) and waves reflected in all directions
why does scattering reflection occur?
beach the normal is different for each incident ray, so each ray has a different angle of incidence; rule, angle of incidence = angle of reflection still applies
how does a surface appear when light is reflected by something rough?
matt as you don’t get a clear reflection
what can you use to investigate reflection?
a ray box and a mirror
how can you use a ray box and a mirror to investigate reflection?
- take a piece of paper, draw solid line (normal) across using ruler then draw dotted line at 90 degreed to solid line
- place a plane (flat) mirror so it lines up with solid line
- using ray box, shine thin beam of white light at mirror so light hits mirror where normal meets mirror
- trace incident and reflected light rays
- measure angle between incident ray and normal (angle of incidence) and angle between reflected ray and normal (angle of reflection) using protractor
- repeat these steps, varying angle of incidence
- should see that reflected ray is as thin and bright as incident ray; plane mirror give clear reflection and none of light is absorbed
- can repeat experiment for different colours of light using colour filters
what do you need to draw for reflection/refraction?
ray diagrams
what is the angle of incidence between?
the incident ray and the normal
what is the angle of refraction between?
the refracted ray and the normal
what does the angle of refraction vary with?
the angle of incidence
what does the angle of refraction depend on
the light’s wavelength; the shorter the wavelength, the more it refracts
what do triangular prisms do to white light?
disperse it
why do different wavelengths of light refract by different amounts in glass?
they travel at different speeds
what is the result when white light passes through a triangular prism?
you get a rainbow
how do you get a rainbow from white light passing through a triangular prism?
- the light bends towards the normal as it enters the prism, as glass is denser than air
- different wavelengths (colours) of light bend by different amounts
- light bends away from the normal as it leaves the prism
- besides the fact that different wavelengths bend by different amounts, the prism’s shape spreads the wavelengths out even more
- on the far side of the prism, you see a spectrum (rainbow)
what can you investigate refraction of light with?
a prism
what equipment is needed to investigate refraction flight using a prism?
- a light source e.g. a ray box
- coloured filters
- a triangular glass prism
- a piece of paper
how can you use a prism to investigate refraction of light?
- place a red filter in front of the ray box
- shine a thin light beam into prism at angle to normal (some light will be reflected)
- trace incident and emerging rays onto paper and remove prism
- draw reflected ray by joining ends of other two rays with straight line
- repeat using blue filter, keep angle of incidence the same
- should see blue light refracting more at each boundary
- repeat with more colours or without filters so white light disperses
- could also try changing shape/material of prism
how does sound travel?
as a wave
what type of waves are sound waves?
longitudinal
what are sound waves caused by?
vibrating objects
how does sound travel as a wave?
- vibrations from vibrating objects are passed through the surrounding medium as a series of compressions and rarefactions
how does a sound wave travel through a solid?
by causing vibrations of the particles in that solid
does sound travel faster in solids or liquids?
solids
does sound travel faster in liquids or gases?
liquids
what happens to the frequency of sound when it passes from one medium to another?
it doesn’t change
what happens to the wavelength of sound the it passes from one medium to another?
it gets longer when it speeds up and shorter when it slows down
what can sound waves do as they enter different media?
refract
what do hard flat surfaces do to sound waves?
reflect them
what are echoes an example of?
reflected sound waves
why can’t sound travel in space?
because it’s mostly a vacuum; there are no particles to move or vibrate
what do sound waves cause your eardrum to do?
vibrate
how do people hear sound?
- eardrum vibrates when sound waves reach it
- vibrations passed to ossicles, through semicircular canal and to cochlea
- cochlea turns vibrations into electrical signals to be sent to brain
- brain interprets signals as sounds of different pitches and volumes depending on frequency and intensity
what are ossicles?
tiny bones within the ear
what does a higher frequency sound wave have?
a higher pitch
what is human hearing limited by?
the size and shape of our eardrum and the structure of all parts within ear that vibrate to transmit sound wave
what frequency range can young people hear?
from around 20-20,000Hz
what happens to the upper limit of human hearing as you get older?
it decreases
why does the upper limit of human hearing decrease as you get older?
mainly due to wear and tear of the cochlea or auditory nerve
what happens to waves at boundaries?
they get partially reflected
- when a wave reaches a boundary between two media it can be absorbed, transmitted (and possibly refracted) or reflected
what is it called if part of a wave is transmitted and part is reflected at a boundary?
partial reflection
what is ultrasound used in?
- medicine
- industry
- sonar
how can ultrasound be used it medicine?
- ultrasound waves can pass through the body but are partially reflected at boundaries between different tissues e.g between muscles in a pregnant woman’s stomach and the fluid in her womb, and between fluid in the womb and skin of foetus
- if you know speed of ultrasound in different tissues, you can calculate distance to different boundaries
- reflections are processed by a computer to produce an image
besides to view a developing foetus, what else can ultrasounds be used to view within the human body?
soft tissues and organs like the kidneys, liver and bladder
how can ultrasound be used in industry?
- to find flaws in objects such as pipes or materials like wood/metal
- ultrasound waves entering a material will usually be reflected by the far side of the material
- if there is a flaw such as a crack in an object, the wave will be reflected sooner
how is ultrasound used in sonar?
sonar is used by boats and submarines to find out the distance to the seabed or to locate objects in deep water
what type of waves are electromagnetic waves?
transverse
what speeds do electromagnetic waves travel at through air or space/vacuums?
they all travel at the same speed/velocity through air or space/vacuums; they travel at different speeds in different materials
what can the wavelengths of different EM waves vary around?
from around 10^-15m to more than 10^4m
what do EM waves with shorter wavelengths have higher of?
shorter wavelengths = higher frequencies
how do we group EM waves?
based on wavelength and frequency
how many basic types of EM wave are there?
7
what do the different groups of electromagnetic waves merge to form?
a continuous spectrum
which part of the spectrum of electromagnetic waves can our eyes detect?
visible light
from longest to shortest wavelength, what is the order of different colours of light?
- red
- orange
- yellow
- green
- blue
- indigo
- violet
what is the order of electromagnetic waves from longest wavelength/lowest frequency to shortest wavelength/highest frequency?
- radio waves
- micro waves
- infra-red
- visible light
- ultraviolet
- x-rays
- gamma rays
what is a mnemonic to help to remember the order of electromagnetic waves from longest to shortest wavelength?
Rich
Men
In
Vegas
Use
X-ray
Glasses
where do electromagnetic waves transfer energy from/to?
from a source to an absorber
what is an example of electromagnetic waves transferring energy from a source to an absorber?
when you warm yourself by an electric heater, infra-red waves transfer energy from the thermal energy store of the heater (source) to your thermal energy store (absorber)
do higher frequency EM waves transfer more or less energy?
the higher the frequency of the EM wave, the more energy it transfers
what causes things to be a certain colour?
some materials absorb some wavelengths of light but reflect others
why are radio waves better for satellite communication than microwaves?
radio waves are refracted more by some layers of the atmosphere but microwaves aren’t
what do differences in how EM waves are transmitted, reflected and absorbed have implications for?
human health
how do differences in how EM waves are transmitted, reflected and absorbed have implications for human health?
- radio waves are transmitted through the body without being absorbed
- some wavelengths of microwaves can be absorbed, causing heating of cells which can be dangerous
- infra-red and visible light are mostly reflected or absorbed by skin, causing some heating (IR can cause burns if skin gets too hot)
- ultraviolet is absorbed by skin; has higher frequency so transfers more energy and causes more damage
- x-rays and gamma rays are also ionising so can cause tissue damage and cancer too; they have even higher frequencies so transfer more energy and cause more damage; they can also pass through skin and be absorbed by deeper tissues
what happens when UV enters living cells?
it collides with atoms in molecules, which may knock electrons off and cause ionisation (it’s ionising radiation); this damages cells which may cause genetic mutation and cancer and can lead to tissue damage or radiation sickness
what can we use radio waves for?
communications
how are radio waves used to transmit information like television and radio shows from one place to another?
- radio waves/all EM waves are just oscillating electric and magnetic fields
- alternating currents in electrical circuits cause charges to oscillate
- this creates an oscillating electric and magnetic field (an EM wave)
- this EM wave will have the same frequency as the current that created it, so it can create a radio wave
- EM waves can also cause charged particles to oscillate
- if the charged particles are part of a circuit it induces an alternating current of the same frequency as the EM wave that induced it
- so if you have a transmitter and a receiver, you can encode information in an a.c. and transmit it as a radio wave
- the wave induces an a.c. in the receiver and then you receive the information
what are microwaves used for?
communications and cooking
how are microwaves used for communications?
communication to and from satellites uses microwaves with a wavelength that can pass easily through the earth’s watery atmosphere
how are microwaves used to cook food?
microwaves can penetrate up to a few cm into food before being absorbed and transferring energy to water molecules in the food, causing water to heat up; the water molecules then transfer this energy to the rest of the molecules in the food by heating, which quickly cooks the food
what can infra-red be used for?
to increase or monitor temperature
how can infra-red be used to increase or monitor temperature?
- infra-red radiation is given off by all objects; the hotter the object, the more it gives off
- infra-red cameras can detect IR radiation and monitor temperature
- they detect the IR and turn it into an electrical signal, which is displayed on a screen as a picture; the hotter the object is, the brighter it appears e.g. IR is used in night vision cameras
- also used medical imaging as IR cameras can detect increases in temperature caused by infections in a small area e.g. an infected wound or in the whole body
- absorbing IR radiation also causes objects to get hotter; food can be cooked using IR as the temp of the food increases when it absorbs IR radiation e.g. from a toaster’s heating element
what can light signals travel through?
optical fibres
what is light used for?
to look at things and for communication through optical fibres
what do optical fibres do?
carry data over long distances as pulses of light
how do optical fibres work?
by bouncing light off the sides of a very narrow core; the pulse of light enters the core at a certain angle at one end and is reflected again and again until it emerges at the other end
what are optical fibres used for?
- telephone and internet cables
- medical purposes (to see inside the body)
why are optical fibres good for medical purposes/seeing inside the body?
only a small hole is needed for the optical fibre/any instruments to enter the body which is better than having major surgery
what is ultraviolet used for?
fluorescent lamps
what is fluorescence?
a property of certain chemicals, where ultraviolet radiations is absorbed and then visible light is emitted; this is why fluorescent colours look so bright (they emit light)
what do fluorescent lights do?
use UV to emit visible light
what is good about fluorescent lights?
they’re energy efficient so are good to be used for long periods of time
how can UV light help to identify stolen property?
security pens can be used to mark property; under UV light the ink will glow, but it’s invisible otherwise so can help to identify stolen property
why is it important to wear sunscreen if you’re outside in strong sunlight?
UV rays can cause damage to the DNA in skin cells
what can x-rays be used for?
to view the internal structure of objects and materials e.g. bodies
how can x-ray photographs be taken?
they affect photographic film in the same way as light
how are x-ray images usually formed these days?
electronically
what do radiographers do?
they work in hospitals and take x-ray images to help doctors diagnose broken bones
why are x-rays good to view bones etc?
x-rays are transmitted by flesh but absorbed by denser material like bones or metal
how are x-ray images produced?
x-ray radiation is directed through the object or body onto a detector plate; the brighter bits of the image are where fewer x-rays get through, producing a negative image (the plate starts off all white)
what can exposure to x-rays cause?
cell damage
how are radiographers and patients protected from potential cell damage by x-rays?
lead aprons and shields and exposure to radiation kept to a minimum
what are gamma rays used for?
- sterilising things
- cancer treatments
- medical imaging
what can gamma rays be used to sterilise?
- medical instruments
- food
why are gamma rays used to sterilise things?
they kill microbes
what is sterilising using gamma rays a good alternative to?
trying to boil plastic instruments (which might be damaged by high temperatures)
what are advantages of sterilising food?
- keeps the food fresh for longer
- perfectly safe to eat
how are gamma rays used in cancer treatment?
radiation is targeted at cancer cells to kill them
what do doctors have to be careful to do when using gamma rays for cancer treatment?
minimise damage to healthy cells
how can waves be used for medical imaging?
- when waves meet a boundary, they can be absorbed/transmitted/refracted/reflected
- what happens depends on the type of wave and the media that are up the boundary
- so waves can be used to study things hidden from view
- different waves are used for imaging different things
what can x-rays be used to image?
bones
how are x-rays used to image bones?
- x-rays mostly transmitted by soft tissue, but absorbed by denser materials like bones/metal
- although x-rays mostly transmitted by soft tissue, a little absorption occurs too, the absorption varies between tissues
- so if you use a lot of x-rays you can produce high resolution images in 2D and 3D of soft/hard tissues
what sort of things can x-rays be used to diagnose?
bone fractures or dental problems
what are high resolution images in 2D and 3D of soft and hard tissues by x-rays called?
computerised tomography (CT scans)
what can gamma rays be used to see?
how things move through the body
what are gamma rays transmitted by?
skin, soft tissue and bone
if gamma rays are produced inside a patient, where can they be detected?
outside the body
how can gamma rays be used to see how things move through the body?
- radio tracers are radioactive isotopes that patients either swallow or are injected with
- as they move around the body, they produce gamma rays
- the gamma rays are detected by a gamma camera outside the patient and used to form an image
- the tracer is often part of a molecule that the body uses e.g. glucose containing radioactive carbon-14
- by looking at where the tracer ends up, doctors can see how the body is working
what is an example of how a radioactive tracer emitting gamma rays can help a doctor identify an issue?
cancerous tumours use more energy than healthy tissue, so they’ll absorb more glucose containing the radioactive carbon and show up as bright spots on the image
what can infra-red show/tell you?
injuries and infections
how can infra-red show injured/infected areas?
infected/injured areas are usually hotter than other areas so give off more IR; IR cameras can detect these differences in temperature and use them to create an image
what makes IR particularly useful?
useful if you need to take lots of temperatures quickly (so can’t use thermometers)
e.g. IR cameras have been used to check people at airports for signs of infection (fevers)
why do doctors have to make compromises with medical imaging?
some waves are dangerous
what compromises are made with medical imaging?
a good enough image to diagnose problems whilst putting the patient at as low risk as possible
is ultrasound safe?
yes, as far as we can tell
why can ultrasound not be used to image lots?
give a fairly fuzzy image and can only be made to make images of soft tissues
are x-rays safe?
they are ionising, so can cause damage to cells
why are x-rays used if they are ionising?
they give clear images of bones and CT scans give useful high resolution images that you can’t get from ultrasound
are gamma rays safe?
they are ionising
why are gamma rays used if they are ionising?
they can be used to get informations on how the body’s working
is imaging with IR radiation safe?
yes
if it’s safe, why don’t we use IR radiations to image more?
it can’t tell us about very much, only temperature
what do colour and transparency depend on?
absorbed wavelengths
what is colour about?
differences in absorption, transmission and reflection of different wavelengths by different materials
what is white light?
a mixture of all the different colours of light, which all have different wavelengths
what do different objects do to different wavelengths of light?
absorb/transmit/reflect different wavelengths of light in different ways
what are opaque objects?
objects that don’t transmit light
what do opaque objects do when visible light waves hit them?
they absorb some wavelengths of light and reflect others
what does the colour of an opaque object depend on?
which wavelengths of light are reflected
why does a red apple appear red?
because the wavelengths corresponding to the red part of the visible spectrum are reflected
what do colours do when they mix together?
make other colours
which colours can you not make by mixing?
the primary colours
what are the primary colours?
pure red, green and blue
why do black objects appear black?
black objects absorb all wavelengths of visible light, your eyes see black as the lack of any visible light/colour
what do transparent and translucent objects do?
transmit light
what happens when some wavelengths of light are absorbed or reflected by translucent and, to a lesser extent, transparent objects?
these objects will appear to be the colour of light that corresponds to the wavelengths mot strongly transmitted by the object
what do colour filters do?
only let through particular wavelengths
what are colour filters used to do?
filter out different wavelengths of light so that only certain colours/wavelengths are transmitted (the rest are absorbed)
if there was a blue filter in front of a blue object, what colour would the object appear?
blue
if there was a blue filter in front of a red object, what colour would the object appear?
black
what do filters that aren’t for primary colours do?
let through both the wavelengths of light corresponding to that colour and the wavelengths of th primary colours that can be added together to make that colour
what can cyan be made from?
blue and green light mixed together
what would a cyan colour filter let through?
the wavelengths of light that correspond to cyan, blue and green
how do lenses form images?
by refracting light, which changes it’s direction
what are the two main types of lens?
concave and convex
what does a concave/diverging lens do?
- caves inwards
- causes parallel rays of light to diverge/spread out
what is the axis of a lens?
a line passing through the middle of the lens
what is the principal focus/focal point of a concave lens?
the point where rays hitting the lens parallel to the axis appear to come from; trace them back until they appear to meet up at a point behind the lens
what does a convex/converging lens do?
- bulges outwards
- causes parallel rays of light to converge/move together
where is the principal focus of a convex lens?
where rays hitting the lens parallel to the axis all meet
what is the distance from the centre of the lens to the principal focus called for any lens?
the focal length
on which side of a lens is the principal focus?
there’s a principal focus on each side of any lens
what are the two types of image that lenses can form?
- a real image
- a virtual image
what is a real image?
where the light from an object comes together to form an image on a ‘screen’
what is an example of a real image?
the image formed on an eye’s retina
what is a virtual image?
when the rays are diverging, so the light from the object appears to be coming from a completely different place
what is an example of a virtual image?
the image in a mirror is a virtual image
what type of image does a concave lens always produce?
a virtual image
what can lenses be useful for?
correcting problems with vision
what can’t short sighted people do?
focus on distant objects
what type of lens is used to correct short-sightedness?
concave lenses
why are concave lenses used to correct short-sightedness?
- the eye contains a convex lens to focus incoming light on the back of the eye (the retina) where it forms an image
- if the eyeball is too long or the lens is too powerful, the eye lens can’t produce a focused image onto the retina; images of distant objects are brought into focus in front of the retina instead (so image on retina is blurry)
how are concave lenses used to correct short-sightedness?
to correct short sight you put a concave lens in front of the eye; this diverges light before it enters the eye, so it can then be focused on the retina, producing a sharp/clear image
what can’t long-sighted people do?
focus on near objects
what type of lenses are used to correct long-sightedness?
convex lenses
why are convex lenses used to correct long-sightedness?
- long sight happens if the lens is too weak or the eyeball is too short
- this means that images of near objects are brought into focus behind the back of the eye, so image on retina is blurry
how are convex lenses used to correct long-sightedness?
to correct long sight a convex lens is put in front of the eye; this means the light starts to converge before it enters the eye, so it can then be focused on the retina
