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# Wave Motion

### Physics (GCSE) Revision (Waves)

StatementResponseComment
When sounds bounce back (reflect) from hard surfaces this is called an . . . ECHO Sound and light are both waves.
When a ray of light is reflected from a shiny surface (e.g. a plane mirror) the angle of incidence is equal to . . . the angle of reflection. Angles are measured from the NORMAL.
A line drawn at 90 degrees to a surface is called a . . . NORMAL Used in measuring angles of reflection and refraction.
When rays of light change direction as they cross the boundary between two different substances this is called . . . REFRACTION All waves do this.
Refraction occurs when light travels between two different materials because . . . the speed of light is different in each material. Light slows down in glass and water compared to air.
Sound waves are . . . LONGITUDINAL waves Compressions and rarefactions travelling through a material ( a push-pull motion on a slinky spring)
Light waves are . . . TRANSVERSE waves Like a side-to-side motion on a slinky spring.
Light waves travel at . . . the speed of LIGHT c = 300 000 km/s
Sound waves travel much slower than light. Their speed in air is approximately . . . 330 m/s Sound travels faster in warm air than in cold.
Sound waves can be reflected and . . . REFRACTED When they cross the boundary between two different substances.
If waves cross a boundary between two different substances at a right angle, there is . . . NO change of direction. Only speed and wavelength change.
What changes when a light ray crosses a boundary between air and glass, travelling into the glass? Speed decreases. Wavelength decreases. It bends towards the normal. These are all part of REFRACTION.
If a ray of light travels from glass into air, its speed will . . . increase. Light travels faster in air than glass.
Waves travelling across the surface of water are . . . transverse waves. The surface is displaced at a right angle to the direction of travel of the waves.
Water waves can be . . . refracted and reflected. Same as any other waves you care to mention.
When water waves travel from deep water to shallow water they are refracted because . . . they travel more slowly in shallow water. There is more drag from the seabed.
When a ray of light travels from glass,Perspex or water into air . . . some light is refracted and some is reflected from the boundary. Usually about 50% of the light is reflected.
For light travelling from glass to air, if the angle of incidence is greater than a certain angle what happens? All of the light is reflected inside the glass. TOTAL INTERNAL REFLECTION. Get the spelling right!
What is the name of the largest angle of incidence at which light can just be refracted at a boundary between glass and air? Critical angle About 42 degrees for glass / air.
Light can travel along an optic fibre because of . . . total internal reflection. No refraction involved.
Optic fibres can be used to send telephone messages in the form of . . . pulses of light. Infra red lasers are used for this.
Sound waves travel through solids, liquids and gases as . . . longitudinal waves. Push a slinky spring away from you and back again.
When a wave moves through a gap, or past an obstacle, it . . . spreads out from the edges. This is called diffraction.
Diffraction of radio waves can improve radio reception in hilly areas because . . . the waves bend over the summit and down into the valleys. This doesn't work with TV or microwaves because they have a shorter wavelength.
Diffraction occurs more strongly when . . . the wavelength is similar to the size of the obstacle. A hill of height 1000m will diffract radio waves of the same wavelength.
When white light passes through a 60 degree prism . . . a spectrum of colours is produced. Isaac Newton explained this by saying that white is a mixture of all the other colours.
Different colours of light have different . . . wavelengths. Red has the longest, violet the shortest.
When a spectrum is produced by a prism the effect is called . . . dispersion. Different colours travel at different speeds in glass - red slows down least.
Which colour is refracted most by a prism? Violet Violet Veers Violently
There are many more kinds of radiation than we can see with our eyes. The full range is called the . . . electromagnetic spectrum It extends to very large and very small wavelengths.
The type of electromagnetic radiation which has the longest wavelength is . . . radio waves. Typically over 1 kilometre in length.
The type of electromagnetic radiation which has the shortest wavelenth is . . . gamma rays. Wavelength is less than the size of an atom.
When radiation is absorbed, the energy it carries makes . . . the substance which absorbs it hotter. Lying in the Sun makes me hot!
Electromagnetic radiation may also . . . create an alternating current with the same frequency as the radiation. That's how a radio or TV works.
Radio waves are used to . . . transmit radio and TV programmes between different points on Earth's surface. Range is limited by line of sight.
Longer wavelength radio signals can be reflected from an electrically charged layer in Earth's upper atmosphere called . . . the ionosphere. This enables signals to be sent between distant points despite the curvature of Earth's surface
Microwave radiation of short wavelength which can pass through Earth's atmosphere is used to . . . send information to and from satellites and within mobile phone networks. To receive information from a satellite you will need a satellite DISH.
A microwave oven can be used for cooking because . . . water molecules in food strongly absorb microwaves of the correct wavelength. No metal objects and certainly not your hands!
Toasters, grills and radiant heaters make use of . . . infra red radiation. This is emitted from warm bodies such as animals and plants.
A TV remote control uses . . . infra red radiation. You might not see it yourself but try using a video camera !
Light can be sent down optical fibres to allow doctors to see inside a patient's body. The device is called an . . . endoscope. It can be used at both 'ends' - but only by a qualified medical person please !
Sunbeds make use of . . . ultraviolet radiation Careless exposure can result in skin cancer (melanoma) so use Factor 25 at least !
Special coatings which absorb UV and emit visible light are used in . . . fluorescent lamps and security coding. Check out your white shirt at the discotheque ( yeah, I know that dates me!)
X-rays are used to produce shadow pictures of bones because . . . X-rays do not easily pass through bone or metal. It can be used at airports for screening your luggage - so leave those scissors at home.
Gamma radiation is used to . . . kill harmful bacteria in food, sterilise surgical instruments and kill cancer cells. Those party forks you use at barbecues have been inside a nuclear reactor you know.
Microwaves are absorbed by the water in cells which may be . . . damaged or killed by the heat released. So keep your hands out of the microwave and keep your mobile phone at a safe distance too.
Infra red is absorbed by the skin and felt as . . . heat. Nice.
Ultraviolet can pass through the skin to deeper tissues. The darker the skin . . . the more UV it absorbs and the less reaches deeper tissues. See, there's a reason for everything.
X-rays and gamma rays mostly pass through soft tissues but some . . . is absorbed by the cells. No exposure is good exposure.
High doses of UV, X and gamma radiation can kill normal cells. Lower doses can . . . cause normal cells to become cancerous. Very high doses are needed to kill cancer cells, but healthy cells are also killed. It is a high risk strategy.
Information such as speech or music can be converted into . . . electrical signals that can be sent long distances through cables or using electromagnetic waves as carriers. Traditional methods: copper wires and radio signals.
Information can be converted into light or infra red signals and sent . . . along optic fibres. Modern methods: cheaper, less loss of energy so more efficient.
Signals which vary continuously in amplitude and/or frequency are called . . . analogue signals Old fashioned vinyl records and terrestrial TV ( not digital)
Signals can be coded as a series of pulses which have two states, on or off. These are called . . . digital signals Now we're talking !
One advantage of difgital signals over older analogue signals is . . . higher quality - the signals do not change their information during the transmission process. Analogue signals are prone to distortion. On is on and off is still off.
A major advantage of digital signals is . . . their information carrying capacity. More information can be transmitted in a given time than with analogue signals.
As signals travel they become weaker. Random additions to the signal are called . . . NOISE Noise adds to both types of signal.
With analogue signals, different frequencies may weaken to different extents. When amplified . . . the signal becomes less like the original. Its quality deteriorates. Noise is also amplified along with any differences.
Even though digital pulses weaken with distance, the quality is maintained because . . . noise is of low amplitude and below the level recognised as 'on'. It is treated as 'off'. The voltage needed for an 'on' state is much higher than the noise level.
Sounds are produced when object vibrate. The greater the amplitude of vibrations . . . the louder the sound. Turn it up !
The number of complete vibrations each second is called the . . . frequency ( measured in hertz, Hz) The higher the frequency of a sound the higher its PITCH
Electronic systems can produce waves which have a frequency higher than humans can hear called . . . ultrasound or ultrasonic waves Above 20kHz, I give up.
Ultrasonic waves are used in medicine for . . . pre-natal scanning Doesn't harm the foetus like X-rays would.
In industry, ultrasound is used for . . . cleaning delicate mechanisms and for quality control Can detect cracks in welds, moulded castings etc.
The information needed for an ultrasound scan to be useful is . . . the time taken for an echo to return to the detector. The distance to the reflecting boundary can be calculated if the speed of sound in the material is known.
Our knowledge of the structure of the Earth comes from studying how the shock waves from earthquakes travel through it. These are called . . . seismic waves. Earthquake intensity is measured on the Richter scale.
Earthquakes produce waves which can be detected using . . . seismographs or seismometers. They produce a pen and ink trace on a rotating drum of paper.
The outermost layer of the Earth is very thin and is called the . . . crust. If the Earth was the size of an apple, the crust would only be the thickness of the skin.
The Earth's core extends about halfway to the surface. It consists of an inner and an outer core. The outer core is . . . liquid nickel and iron. The inner core is a solid ball of metal.
The layer of rock surrounding the core extends almost to the surface and is called the . . . mantle It has the properties of a solid but it can flow very slowly.
The overall density of the Earth is much greater than the mean densities of the rocks that form the crust. This suggests that . . . the interior of the Earth is much denser. Density increases with depth due to the increasing pressure of the overlying rocks.
What are the two types of seismic waves that travel through the interior of the Earth? P and S waves. There are surface waves as well which only travel through the crust.
Which type of seismic waves are faster? Primary (P) waves They are recorded first by seismographs at the start of an earthquake.
What is the difference between the two types of seismic waves? P waves are longitudinal, S waves are transverse. P waves are compression waves. S waves are shear waves.
Which type of seismic wave cannot travel in a liquid? S waves They need a rigid (solid) medium.
Seismic waves travel in curved paths through the interior of the Earth because . . . they are being refracted. They travel faster as they go deeper.
The speed of seismic waves increases with depth because . . . the density of rock increases with depth. Seismic waves travel faster in denser material.
The speed and direction of P waves change abruptly as they reach the outer core because . . . its density suddenly decreases. Liquids are less dense than solids.
We know that the outer core is a liquid because . . . S waves can't travel through it. Only P waves can pass right through the Earth's core.
Some seismometers cannot detect the S waves from an earthquake. Why is this ? S waves can't pass through the core. If the seismometer is in a 'shadow zone', only P waves can reach it.
The fact that the edges of land masses which are separated by thousands of kilometres of ocean have shapes which fit quite closely suggests that . . . they may have been part of larger land mass which spli apart. e.g. south east America and west Africa
Wegener's theory of continental drift is supported by . . . similar patterns of rocks and fossils on different continents Or perhaps a land bridge which broke apart could explain this
The upper part of Earth's mantle and crust is called the . . . lithosphere It is moving on top of denser material below
The pieces of Earth's lithosphere which are moving are called . . . tectonic plates Like rafts of polystyrene floating on denser water
The Earth's tectonic plates are moving slowly at a speed of about . . . a few centimetres a year We can measure this by timing radio signals
The process which drives plate tectonics is . . . convection currents in the mantle The energy is provided by radioactive decay which releases heat
At the boundaries between tectonic plates we find . . . more earthquakes and volcanic eruptions You can see a map of this in your text book
Where two plates meet they may slide past each other. This is known as a . . . transform fault They rub and produce friction e.g. California
When two tectonic plates approach each other, one of them may be driven down. This is called . . . subduction Old crust is melted to form volcanoes
Sea floor spreading is caused when . . . two tectonic plates move apart New magama rises to fill the gap
Evidence for sea-floor spreading is provided by . . . magnetic reversal patterns in oceanic crust This confirms the theory of plate tectonic and also shows the Earth's magnetic field can flip over.
Created by: vahajuddin

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