Topic P5- Waves in Matter

Question 1

What do waves do?

Answer 1

-Waves transfer energy in the direction they are travelling without transferring any matter.

Question 2

What happens when a wave travels through a medium (generally)?

Answer 2

• The particles of the medium vibrate and transfer energy between each other.
• The particles stay in the same place and only energy is transferred.

Question 3

What is the amplitude of a wave?

Answer 3

The displacement from the rest position to a crest or trough.

Question 4

What is the wavelength of a wave?

Answer 4

The length of a full cycle of the wave, e.g. crest to crest, compression to compression.

Question 5

What is the frequency of a wave?

Answer 5

The number of complete waves or cycles passing a certain point per second.
It is measured in Hertz.
1Hz is 1 wave per second.

Question 6

What is the period of a wave?

Answer 6

The number of seconds it takes for one full cycle.

Period = 1 ÷ frequency.

Question 7

Describe transverse waves:

Answer 7

• Have sideways vibrations
• the vibrations are perpendicular (90º) to the direction the wave travels.
• E.g. electromagnetic waves, S waves, Ripples and waves in water.
• Transverse waves can travel on the surface of a liquid but cannot travel through liquids.

Question 8

Describe Longitudinal waves:

Answer 8

• Have parallel vibrations
• the vibration is parallel to the direction the wave travels. (same direction)
• E.g. sound waves and P waves.
• Longitudinal waves squash up and stretch out the arrangement of particles in the medium they pass through, making compressions (high pressure, lots of particles)
• and rarefactions (low pressure, fewer particles)

Question 9

What is the equation for wave speed?

Answer 9

wave speed (m/s)= frequency (Hz) x wavelength (m)

1kHz - 1000Hz
1MHz = 1 000 000Hz

Question 10

Describe an experiment to measure the speed of sound:

Answer 10

Using an oscilloscope:

• by attaching a signal generator to a speaker you can generate sounds with a specific frequency.
• Use two microphones and an oscilloscope to find the wavelength of the sound waves generated.
• 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 the distance between the microphones to find one wavelength.
• You can then use the formula v=fxλ to find the speed of the sound waves passing through the air
• the frequency is whatever you set the signal generator to you in the first place.

Question 11

Describe an experiment to measure speed, frequency and wavelength:

Answer 11

• You can generate waves in a ripple tank using a signal generator attached to a dipper.
• The signal generator moves the dipper up and down to create water waves at a fixed frequency.

Frequency:

• float cork in the ripple tank (should bob up/down as wave passes)
• when the cork is at the top of the ‘bob’, start stopwatch
• Count how many times the cork bobs in e.g. 20 secs.
• Divide this number by your time interval (how long you counted for) to get number of ‘bobs’ per second, this is frequency.

Wavelength:

• Use strobe light
• place a card covered with centimetre squared paper behind the ripple tank.
• Turn strobe light on and adjust frequency until waves appear to freeze.
• using squared paper, measure the distance that, e.g. five waves cover.
• Divide this distance by the number of waves to get average wavelength.

Wave Speed:
-use pencil and stopwatch
-place large piece of paper next to tank.
As waves move across the tank, one person should track the path of one of the crests on paper, using pencil.
-make sure line is straight and parallel to the direction wave travels.
-use a ruler, i guess.
-the other person should time how long the first has been drawing for and pick a duration, e.g. 10 seconds,
-and stop drawing when this time has passed.
-calculate the speed of the wave by measuring the length of the line
-use formula distance travelled = speed x time.

REPEAT EXPT 3 TIMES AND TAKE AN AVERAGE.
REMEMBER CONTROL VARIABLES.

Question 12

How do waves behave?

Answer 12

-all waves:
-reflect
-refract
-are absorbed
-are transmitted
…at boundaries.

Question 13

What 3 things happen when a wave meets a boundary between two material interfaces?

Answer 13

• The wave may be absorbed by the second material, transferring energy to the material’s energy stores.
• The wave may be transmitted- it carries on travelling through the new material, often at a different velocity, which can lead to refraction.
• the wave may reflect off the boundary.
• This is where the incoming ray is neither absorbed or transmitted but ‘sent back’ away from the second material.

Question 14

What is the rule for all reflected waves?

Answer 14

Angle of Incidence = Angle of Reflection.

Question 15

How does light rays behave when reflected off smooth surfaces?

Answer 15

They reflect off in the same direction. giving a clear reflection.

Question 16

How does light rays behave when reflected off rough surfaces?
Is Angle of Incidence still equal to angle of reflection?

Answer 16

• They reflect off in all 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,
• and so is reflected at a different angle,
• scattering the light.

Question 17

What is white light?

Answer 17

A mixture of all the different colours of light, which all have different wavelengths.

Question 18

What happens when white light is reflected?

Answer 18

All the colours of light in white light are reflected at the same angle,
-white light doesn’t split into the different colours when it reflects, as all the wavelengths follow the rule:

Angle of Incidence = Angle of Refraction.

Question 19

What is refraction?

Answer 19

• Waves travel at different speeds in materials with different densities.
• So when a wave crosses a boundary between two materials (diff. densities), it changes speed.
• The frequency of the wave stays the same, when it crosses a boundary. As v = fλ, this means the wavelength changes- the wavelength decreases if the wave slows down and increases if it speeds up.
• If the wave hits the boundary at an angle to the normal, this change in speed (and wavelength) makes the wave bend- this is called refraction.
• The greater the change in speed, the more it bends.
• If the wave slows down, it will bend towards the normal.
• If the wave speeds up it will bends away from the normal.

Question 20

From air to glass, how does violet light refract in comparison to red light?

Answer 20

• the colours of light all have slightly different wavelengths- shortest to longest it goes:

violet, indigo, blue, green, yellow, orange, red.

The travel the same speed in air, but when they enter a denser substance, like glass, the shorter wavelengths slow down more and so refract more (and bend towards to normal).

Question 21

What is specular reflection?

Answer 21

when waves are reflected in single direction by a smooth surface.
This means you get a clear reflection.

Question 22

What is scattered reflection?

Answer 22

When a wave is reflected by a rough surface and the waves are reflected in all directions.
This happens because the normal is different incident ray so each ray has a different angle of incidence.

no clear reflection.

Question 23

Describe an experiment to investigate reflection using a ray box and a mirror:

Answer 23

• take paper and draw a solid line across it with ruler.
• Then draw dotted line at 90º to solid line (normal)
• Place a plane (flat) mirror so it lines up with solid line.
• using ray box, sine a thin beam of white light at the mirror, so the light hits the mirror where the normal meets the mirror.
• trace the incident and reflected light rays.
• measure the angle between the incident ray and the normal (angle of incidence)
• and the angle between the reflected ray and the normal (angle of reflection) using a protractor.
• repeat steps, varying the angle of incidence.
• no matter the value, angle of incidence always equals angle of reflection.
• plane mirror gives a clear reflection and none of the light is absorbed.
• you could repeat the experiment with different colours of light by using colour filters.

Question 24

What happens when white light is passed through a triangular prism?

Answer 24

The triangular prism disperses white light.
You get a rainbow (colour spectrum of light).
-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- red bends the least, violet bends the most (towards normal).
-Light bends away from normal as it leaves the prism.
-Different colours bend by different amounts.
-Because of the prism’s shape, this spreads the wavelengths out even more.
- you get a spectrum!!

Question 25

How does sound waves travel?

Answer 25

• Sound waves are caused by vibrating objects.
• These vibrations are passed through the surrounding medium as a series of compressions and rarefactions.
• They are a type of longitudinal wave.
• When a sound wave travels through a solid it does so by causing vibrations of the particles in that solid.
• sound waves travel faster in solids than liquids, and faster in liquids than in gases.
• frequency does not change when passing through one medium to another. Because v=fλ the wavelength does change, it gets longer when it speeds up, and shorter when it slows down.
• sound waves refract when they enter different media.

(the sound wave hits a solid object, the air particles hitting the object cause the particles in the solid to move back and forth (vibrate) these particles hit the next and so on, passing the wave through the object as a series of vibrations)

Question 26

What is an echo?

Answer 26

Sound waves will be reflected by hard flat surfaces. echoes are just reflected sound waves.

Question 27

Why can’t sound travel in space?

Answer 27

Space is mostly a vacuum, so there are no particles to move or vibrate to transfer energy in the form of vibrations.

Question 28

How do humans hear sound?

Answer 28

• Sound waves that reach your eardrum cause it to vibrate.
• these vibrations are passed on to tiny bones in your ear called ossicles, through the semicircular canal and to the cochlea.
• The cochlea turns these vibrations into electrical signals which get sent to your brain.
• the brain interprets the signals as sounds of different pitches and volumes, depending on their frequency and intensity.
• A higher frequency sound wave has a higher pitch.
• Human hearing (audition) is limited by the size and shape of our eardrum, and the structure of all the parts within the ear that vibrate to transmit the sound wave.
• Young people can hear frequencies ranging from about 20Hz (low pitch) up to 20000Hz (high pitch).
• As you get older, the upper limit decreases, and sounds may need to be louder for you to hear them.
• This is mainly due to wear and tear of the cochlea or auditory nerve.

Question 29

How can waves be used to see images?

Answer 29

• If you know the speed of the wave in the medium, you can use the time it tales for the reflections to reach a detector and the formula distance = speed x time to find how far away the boundary is.
• this means we can use ultrasound (frequencies above 20kHz to create images.

Question 30

How is ultrasound used in medicine?

Answer 30

• Ultrasound waves can pass through the body, but are partially reflected at boundaries between different tissues.
  (e. g. between muscles in pregnant woman’s stomach and fluid in her womb, and between fluid in womb and skin of foetus.)
• if you know the speed of ultrasound in the different tissues, you can calculate the distance to the different boundaries. (and divide by two)
• the reflections are processed by a computer to produce an image.
• so ultrasound can be used to form image of a developing foetus.
• It can also be used to examine soft tissues and organs lime the kidneys, liver and bladder.
• completely safe too!!

Question 31

How can ultrasound be used in Industry?

Answer 31

• Can be used to find flaws in objects such as pipes or materials such as wood or 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 inside the object, the wave will be reflected sooner.

Question 32

What can Sonar be used for?

Answer 32

It is used by boats and submarines to fin out the distance to the seabed or to locate objects in deep water.

Question 33

What are EM waves?

Name them all from largest wavelength to smallest wavelength.

Answer 33

• Electromagnetic waves
• Transverse waves
• All travel at the same velocity through air or vacuum.
• they all travel at different speeds in different materials.
• EM waves vary in wavelength
• those with shorter wavelengths have higher frequencies
• our eyes can only detect a small part of this spectrum- visible light.
• Different colours of light have different wavelengths.

Radio Waves, Microwaves, Infrared, Visible Light, Ultraviolet, X rays, Gamma Rays.

• All EM waves transfer energy from a source to an absorber.
• For example, when you warm yourself by an electric heater, infrared waves transfer energy from the thermal store of the heater (source) to your thermal energy store (absorber).
• The higher the frequency of the EM wave, the more energy it transfers.

Question 34

What EM wave is the best to use for satellite communications and why?

Answer 34

Radio waves are refracted by some layers of the atmosphere but microwaves aren’t, making them better for satellite communications.

Question 35

Can radio waves affect the human health?

Answer 35

No- radio waves are transmitted through the body without being absorbed.

Question 36

Can microwaves damage the human health?

Answer 36

Some wavelengths of microwaves can be absorbed, causing heating of cells, which may be dangerous.

Question 37

Can Infra-red affect the human health?

Answer 37

• mostly reflected or absorbed by the skin, causing some heating too.
• IR can cause burns if the skin gets too hot.

Question 38

Can visible light affect the human health?

Answer 38

-mostly reflected or absorbed by the skin, causing some heating too.

Question 39

Can Ultra-violet cause human health issues?

Answer 39

• UV is absorbed by the skin.
• But it has a higher frequency, so it transfers more energy, causing more damage.
• When it enters living cells. it collides with atoms in molecules, which may knock electrons off and cause ionisation.
• this is ionising radiation .
• this damages cells which may cause genetic mutation and cancer.
• it can lead to tissue damage or radiation sickness.

Question 40

Can X-rays and Gamma rays damage the human health?

Answer 40

• X rays and gamma rays are also ionising, so they can cause tissue damage and cancer too.
• They have even higher frequencies so transfer even more energy, causing even more damage.
• they can also pass through the skin and be absorbed by deeper tissue.

Question 41

Describe how Radio waves are used for communication:

Answer 41

• we use radio waves to transmit information like television and radio shows from one place to another.
• radio waves and all EM waves are just oscillating electric and magnetic fields.
• Alternating currents (a.c.) in electrical circuits cause charges to oscillate.
• This creates an oscillating electric and magnetic field, 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 also cause charged particles to oscillate.
• If the charged particles are part of a circuit, this induces an alternating current of the same frequency as the EM wave that induced it.
• So if you’ve got a transmitter and a receiver, you can encode information (e.g. TV show) in an a.c. and then transmit it as a radio wave.
• The wave induces an a.c. in the receiver (the aerial) and boom, you have your information!!

Question 42

How can microwaves be used for communication?

Answer 42

-Communication to and from satellites (including satellite TV signals and mobile phones) uses microwaves with a wavelength that can easily pass through the Earth’s watery atmosphere.

Question 43

How can microwaves be used for cooking?

Answer 43

We can use microwaves of slightly different wavelength to cook food. These microwaves penetrate up to a few centimetres into the food before being absorbed and transferring energy to water molecules in the food, causing the 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 food.

Question 44

How can IR radiation be used to increase or monitor temperature?

Answer 44

• IR radiation is given off by all objects.
• The hotter the object, the more it gives off.
• IR cameras can detect IR radiation and monitor temperature.
• The detect the IR and turn it into an electrical signal, which is displayed on a screen as a picture.
• The hotter an object is, the brighter it appears.
• E.g. IR us used in night vision cameras.
• IR is also used in medical imaging.
• IR cameras can detect increases in temperature caused by infections in a small area, or in the whole body.
• Absorbing IR radiation also causes objects to get hotter.
• Food can be cooked using IR radiation, the temperature of the food increases when it absorbs IR radiation.

Question 45

How do optical fibres work?

What are some of its uses?

Answer 45

• Light signals can travel through optical fibres.
• Light is also used for communication using optical fibres, which carry data over long distances as pulses of light.
• 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.
• Optical fibres are used for telephone and internet cables.
• They’re also used for medical purposes to see inside the body, only a small hole is need for the optical fibre and any instruments to enter the body, which is better than having major surgery.

Question 46

Describe uses of UV?

Answer 46

• Florescence is property of certain chemicals where UV radiation is absorbed and then visible light is emitted.
• That is why florescent colours look so bright, they actually emit light.
• Florescent lights use UV to emit visible light.
• They are energy-efficient so they’re good to use when light is needed for ling periods of time.
• Security pens can be used to mark property.
• Under UV light, the ink will glow, but it is invisible otherwise, helping to identify stolen property.

Question 47

How can UV be dangerous to human health?

Answer 47

-UV rays can cause damage to DNA in skin cells, so it is important to wear sunscreen when exposed to UV light for a long time.

Question 48

Describe uses of X-rays:

Answer 48

-X-rays can be used to view the internal structure of objects and materials, including our bodies.
-They affect photographic film in the same way as light, meaning you can take X-ray photographs.
-But X-ray images are usually formed electronically nowadays.
-Radiographers in hospitals take X-ray images to help doctors diagnose broken bones,
-X-rays are transmitted by flesh but absorbed by denser material like bones or metal.
-To produce X-ray images, X ray radiation is directed through the object or body onto a detecter plate.
-The brighter parts are where fewer X rays are transmitted, producing negative image.
-(plate starts off all white)
-

Question 49

How can X rays affect human health and how do people exposed to X ray radiation frequently protect themselves?

Answer 49

Exposure to X rays can cause cell damage, so radiographers and patients are protected as much as possible, by using lead aprons and shields,
and exposure to the radiation is kept to a minimum.

Question 50

Describe uses of gamma radiation:

Answer 50

• Gamma rays are used to sterilise medical instruments- they kill microbes.
• This is better than trying to boil plastic instruments, which might be damaged by temperatures.
• Food can be sterilised in the same way and again killing microbes.
• This keeps the food fresh for longer, without having it freeze it, cook it or preserve it some other way, and it is perfectly safe to eat.
• Gamma radiation is also used in cancer treatments
• Radiation is targeted at cancer cells to kill them.
• Doctors have to be careful to minimise the damage to healthy cells, when treating cancer like this,.
• We also use gamma radiation in medical imaging.