GCSE Physics

P3: Waves Specification

Go back to home page

Content

• Units
• Properties of waves
• The electromagnetic spectrum
• Light and sound
• The Earth’s layered structure

Units

(P3.01) Use the following units: hertz (Hz), kilohertz (kHz), megahertz (MHz), metre/second (m/s)

Properties of waves

(P3.02) Describe longitudinal and transverse waves in ropes, springs and water.

• State that wave motion is produced by an oscillation i.e. a source that's moving up and down or to and fro.
• Describe how to produce pulses and progressive transverse waves in ropes, springs and ripple tanks
• State that transverse waves are waves in which the disturbances in the substance through which the waves travel are at right angles (perpendicular) to the direction in which waves themselves travel.
• State that the waves that travel along ropes and across the surface of water are transverse.
• State that longitudinal waves are waves in which the disturbances in the spring are along the same direction as that in which the waves themselves travel.
• State that the waves that travel through springs are usually longitudinal.
• State that in the 'slinky' used in classroom demonstrations they may be transverse, or a complicated mixture of transverse and longitudinal, depending on how the hand shaking one end of the slinky moves.
• Explain the difference between a progressive (travelling) wave (energy travels in one direction) and a standing wave (no net transfer of energy forwards or backwards - usually due to they way in which an incident wave combines with its reflection at a barrier).
• Explain, understand and use the terms crest, trough, wave speed, displacement, phase, wave front.
• Explain the meaning of a plane polarized wave and that it only applies to transverse waves
• Explain and illustrate how plane waves are reflected at a straight barrier
• Explain and illustrate how (a) plane waves are reflected at a concave barrier and (b) how circular ripples are reflected at a plane barrier
• State that light can be diffracted but needs a very small gap because the wavelength is so small

(P3.03) State the meaning of amplitude, frequency, wavelength and period of a wave.

• State that whereas a pulse is a short burst of energy, regular waves travelling along ropes or springs or across the surface of water set up regular patterns of disturbances.
• State that (a) the maximum displacement caused by a wave is called its amplitude, (b) the distance between a particular point on one disturbance and the same point on the next is called the wavelength; and (c) the number of waves each second produced by a source (or passing a particular point) is called the frequency and is measured in hertz (Hz).
• Represent a transverse wave on a displacement-distance graph and read off from it displacement, amplitude & wavelength

(P3.04) Recall that waves transfer energy and information without transferring matter

(P3.05) Recall and use the quantitative relationship between the speed v, frequency f and wavelength λ of a wave:
         -           wave speed = frequency × wavelength       or         v = f λ

(P3.06) Use the quantitative relationship between frequency f and period (i.e. time period) T:
         -           frequency = 1 / time period            OR            f = 1 / T

(P3.07) Use the above relationships in a wide range of contexts including sound waves and electromagnetic waves

• State that waves travelling along a rope or spring, or across the surface of water, can be reflected.
• Explain the terms reflection, refraction, diffraction and interference in relation to waves
• Draw a diagram showing reflection of straight wave fronts at a plane surface & describe a ripple tank experiment that demonstrates this
• State that   angle of reflection r = angle of incidence i
• State that waves travelling across the surface of water can be refracted.
• Draw a diagram showing refraction of straight wave fronts at a straight boundary & describe an experiment that demonstrates this
• State that the change in the speed of water waves when they cross the boundary between two different depths causes a change in their direction (refraction), unless the direction of travel of the waves is along a normal.
• State that refraction at a boundary/barrier (i.e. there is a change in depth) is due to change of wave speed but NOT of wave frequency

(P3.08) Understand that waves can be diffracted through gaps or when they pass an edge and that the extent of diffraction depends on the wavelength and the physical dimension.

• State that when a wave moves through a gap, or past an obstacle, it spreads out from the edges and that this is called diffraction.
• Draw diagrams to show diffraction of straight wave fronts at single slits of different widths & describe experiments that demonstrate this
• Predict the effect of changing the wavelength and the size of the gap on diffraction of waves at a single slit (diffraction is not observable if the wavelength is much smaller that the gap width).
• Explain interference as the superposition of two crests or troughs at points were waves arrive in phase or out of phase
• Describe an experiment to show the interference of water waves using two point sources

The electromagnetic spectrum

(P3.12) Understand that light is part of a continuous electromagnetic spectrum that includes radio, microwave, infra-red, visible, ultraviolet, X-ray and gamma ray radiations and that all these waves travel at the same speed in free space.

• State that the speed of electromagnetic (EM) waves through free space (i.e. a vacuum) is often referred to as the 'speed of light' c.
• State that EM waves are transverse.
• State that electromagnetic radiation can be transmitted, reflected, or absorbed by an object.
• State that this transmission, reflection and absorption depends on what the object is made of and the properties of its surface.
• State that when radiation is absorbed by a substance, the energy it carries (a) heats the substance and (b) will create an alternating current (AC) with the same frequency as the radiation itself in conductors - but only with longer wavelengths i.e. radio waves and microwaves.
• Electromagnetic radiation can be reflected.
• Because of diffraction, radio signals can sometimes be received in the shadow of hills. Waves having a longer wavelength are more strongly diffracted.
• The fact that electromagnetic radiation can be refracted and diffracted supports the idea that it travels as waves
• Absorption of EM radiation causes heating, ionisation and cell damage - the amount depending on the wavelength of the radiation being absorbed.

(P3.13) Recall the order of the electromagnetic spectrum in decreasing wavelength and increasing frequency including the colours of the visible spectrum

• The various types of electromagnetic radiation form a continuous spectrum extending far beyond each end of the visible spectrum:

• Recall that the colour of light depends on its frequency, that red light has a lower frequency (but longer wavelength) than blue light
• Recall that all colours, despite differing frequencies, travel at the same speed - the speed of light

(P3.14) Recall some uses of electromagnetic radiations including:

     - radio waves: broadcasting and communications.

• State that radio waves are used to transmit radio and TV programmes between different points on the Earth’s surface.
• State that radio waves are suitable for broadcasting because they are so readily diffracted (due to the fact that their wavelengths are comparable to the size of buildings, hills etc).
• Draw a diagram to show how shorter wavelength radio waves are reflected from an electrically charged layer in the Earth’s upper atmosphere called the ionosphere and explain that this enables them to be sent between distant points despite the curvature of the Earth’s surface.

     - microwaves: cooking and satellite transmissions.

• Microwave radiation, with wavelengths strongly absorbed by water molecules, is used for cooking.
• Explain how information in narrow beams can be transmitted using microwaves (small transmitting dish)
• Microwave radiation of wavelengths that can pass easily through the Earth’s atmosphere is used to send information (a) to and from satellites and (b) within mobile phone networks

     - infra-red: heaters, grills, night vision and remote controls.

• Toasters and radiant heaters such as central heating radiators
• Grills
• Describe the use of infra-red radiation in night photography
• Remote controls for TVs and VCRs (video cassette recorders)
• Optical fibre (fibre-optic) communication

     - visible light: optical fibres and photography.

• One example of light sent along optical fibres is in endoscopes used by doctors to see inside patients’ bodies. Another example: Fibre-optic Xmas trees
• Taking pictures with a camera.
• Lasers. Lasers produce an intense, virtually monochromatic, beam or light. Laser light differs from other light in that the waves are in phase right across the beam. The light can travel as a very narrow beam over great distances. Various uses: e.g. in supermarket barcode scanners; in CD players to read the CD; to get a straight line in construction; surgery.
• Light is also, of course, used for seeing

     - ultraviolet: sun beds, crime prevention and fluorescent lamps.

• Sun beds - though some UV wavelengths are harmful to the skin
• Special coatings that absorb ultraviolet radiation and re-emit the energy as light are used for security coding (invisible ink) and in fluorescent lamps.

     - X-rays: observing the internal structure of objects and materials, medical applications.

• X-radiation is used to produce shadow pictures of materials that X-rays do not easily pass through, including bones and metals

     - gamma rays: sterilising food and medical equipment.

• Gamma rays kill harmful (and non-harmful) bacteria in food.
• Describe the use of gamma rays to detect the malfunction of organs and as treatment for killing cancerous body tissue.

(P3.15) Recall the detrimental effects of excessive exposure of the human body to electromagnetic waves of increasing frequencies including:

     - microwaves: internal heating of body tissue.

• Microwaves are absorbed by the water in cells, which may be damaged or killed by the heat generated;

     - infra-red: skin burns.

• Infra red radiation is absorbed by skin and is felt as heat.
• Excessive sun exposure results in sunburn.
• Use of clothing and shade to protect from sun.

     - ultraviolet: damage to surface cells and blindness.

• Ultraviolet (UV) radiation can pass through skin to deeper tissues.
• The darker the skin, the more ultraviolet it absorbs and the less reaches into deeper tissues.
• High doses of ultraviolet radiation, X-radiation and gamma radiation can kill normal cells.
• UV can damage the cells in the retina at the back of the eye.
• Use of good quality sunglasses and sun barrier cream to reduce danger

     - X-rays and gamma rays: cancer, mutation.

• X-radiation and gamma radiation mostly pass through soft tissues, but some is absorbed by the cells.
• Lower doses of ultraviolet radiation, X-radiation and gamma radiation can cause normal cells to become cancerous. High doses of can kill normal cells.
• List safety precautions that should be taken when using X-rays and gamma rays

Light and sound

(P3.16) Recall that light waves are transverse waves that can be reflected, refracted and diffracted.

• Whereas sound waves travel through solids, liquids and gases as longitudinal waves, light waves, being electromagnetic, are transverse waves
• Light, being electromagnetic, can travel through a vacuum, unlike sound.
• Recall that light travels much faster than sound. In air, the speed of light is 3 x 10⁸ m/s, whereas the speed of sound is just over 300 m/s
• The speed of light depends on nature and properties of the medium (its refractive index)
• Give examples of effects that show that light travels in straight lines
• Draw diagrams to show how shadows are formed with both point and extended sources, and use the terms umbra and penumbra

• State the laws of reflection: (a) angle of incidence = angle of reflection and (b) the incident ray, reflected ray and normal are all in the same plane
• Describe an experiment to show that, for reflection of light in a plane mirror, the angle of incidence equals the angle of reflection.
• Draw ray diagrams to show the action of convex and concave mirrors on a parallel beam of light
• Define the terms principal focus, focal length, radius of curvature and centre of curvature
• Explain why convex mirrors are used as driving mirrors
• Explain why concave mirrors are used (a) as reflectors in car headlamps and (b) as make-up, shaving and dental mirrors

• Define the term refraction: Rays of light change direction (are refracted) when they cross the boundary between one transparent substance and another, unless they meet the boundary at right angles (along a normal).
• Give everyday examples to show that light can be refracted and describe an experiment to study refraction
• Recall that light is refracted because it changes speed in another medium (think of 'car entering sticky tar patch' analogy)
• Recall that refractive index is a measure of how much the wavefront gets 'knocked off course' i.e. refracted.
• Draw diagrams of the passage of light rays through rectangular blocks and recall that lateral displacement occurs for a parallel-sided block
• Recall that light is refracted because it changes speed when it enters another medium
• The reflective and refractive behaviour of waves in springs and water suggests that light and sound (a) also travel as waves and (b) are refracted because they travel at different speeds in different substances (media)
• The spectrum is produced because white light is made up of many different colours. Different colours of light are refracted by different amounts: red light is refracted least and violet light most.

• When rays of light pass through prisms, their direction will be changed.
• The various colours of light are refracted by different amounts; red light is refracted least and violet light most.
• Draw a diagram for the passage of a monochromatic (i.e. single colour) light ray through a prism
• When white light is used, a spectrum is produced, because white light is made up of many different colours.
• Explain the meaning of the term dispersion (dispersion is the differential refraction of different colours)
• Draw a diagram of how a prism is used to produce a spectrum from white light.

• Explain, using a diagram, the action of a lens in terms of refraction by thinking of it as being made up of a number of small prisms
• Draw diagrams showing how parallel rays of light pass through converging (i.e. convex) and diverging (i.e. concave) lenses
• Identify the position of the focus (or to be precise the principal focus) of both a converging and a diverging lens and know the meaning of focal length
• Describe the difference between a real and a virtual image
• Construct ray diagrams to show the formation of both real and virtual images by converging lenses
• Explain the use of a converging lens as a magnifying glass.
• Describe with the aid of a ray diagram how a single lens is used in a camera to produce a real image on a film (or image sensor in a digital camera).
• State that the image is smaller than the object and nearer to the lens.
• Describe how the image position and size vary according to the object distance and the length of the camera.

• Explain why diffraction of light is not normally observed
• Describe a simple Young's double-slit experiment to show light has a wave-like nature

• Describe some evidence that light is a transverse wave

(P3.17) Describe the role of total internal reflection in transmitting information along optical fibres and in prisms.

• When light travels from glass, Perspex or water into air some of the light is reflected from the boundary.
• If the angle between the ray and a normal is greater than a certain angle (called the critical angle), all of the light is reflected inside the glass, Perspex or water.
• This is called total internal reflection. Explain with the aid of diagrams what is meant by critical angle and total internal reflection
• Describe an experiment to find the critical angle of glass or Perspex
• Draw diagrams to show the action of totally reflecting prisms in periscopes and binoculars
• When light travels down an optical fibre, all the light may stay inside the fibre until it emerges from the other end. This is because light travels down optical fibres by repeated total internal reflection.
• More information can be carried optically than by sending electrical signals through cables of the same diameter.
• There is also less weakening of the signal in optical fibres.
• One example of light sent along optical fibres is in endoscopes used by doctors to see inside patients’ bodies. Another example: fibre-optic Xmas trees. Also telecommunications.

(P3.18) Understand the difference between analogue and digital signals

• Information such as speech or music can be converted into electrical signals
• These signals can be sent long distances (a) through cables or (b) using electromagnetic waves as carriers.
• Information can also be converted into light or infra red signals and sent along optical fibres.
• Signals that vary continuously in amplitude and/or frequency - in the same way that the sound waves of speech or music do - are called analogue signals.
• Signals can also be coded as a series of pulses. The signal then has only two states, on or off. Signals of this type are called digital signals.

(P3.19) Describe how digital signals can carry more information

• Information signals are carried by (a) cables, or (b) optical fibres, or (c) EM carrier waves
• One reason why signals degrade (i.e. quality deteriorates) over distance is that they gradually weaken (i.e. lose energy)
• A second reason why signals degrade is that random additions to the signal may also be picked up (these additions are called noise)
• With analogue signals, different frequencies within the signal may weaken to different extents.
• When an analogue signal gets amplified, these differential weakenings together with the noise component also get amplified; so the amplification process does not improve the quality of the signal
• One advantage of digital signals is their information carrying capacity – more information can be sent in a given time via a than with analogue signals.
• A second advantage of digital signals is that, even though they get weaker with distance, they do not lose their information during transmission. Thus the quality of a digital signal is maintained. Noise signals rarely confuse the issue because the noise levels are nearly always too small compared with the size of genuine information pulses to be regarded as being genuine information pulses by the receiving equipment.

(P3.20) Recall that sound waves are longitudinal waves that can be reflected, refracted and diffracted

• Sounds are produced when objects vibrate.
• Sound waves travel through solids, liquids and gases as longitudinal waves
• Describe an experiment to show that sound is not transmitted through a vacuum
• Recall that the speed of sound in air is a little over 300 m/s. It is substantially faster in a liquid and still faster in a solid (but always vastly slower than EM radiations such as light)
• The number of complete vibrations each second is called the frequency (hertz, Hz). The higher the frequency of a sound the higher its pitch.
• The greater the amplitude of the vibrations, the greater is the loudness.
• Compare the amplitudes and frequencies of sounds from diagrams of oscilloscope traces.

• Explain that echoes are sounds that have bounced back (reflected) from hard surfaces
• Explain how distances can be measured using echo sounding
• Sounds are also refracted, i.e. their direction is changed when they cross the boundary between two different substances at an angle other than a right angle.
• The observation that sound is diffracted supports the idea that it travel as waves.
• Because of diffraction, sounds can sometimes be heard in the shadow of buildings. Waves having a longer wavelength are more strongly diffracted.
• The interference of sound waves can sometimes be noticed when walking across the path of two stereo loudspeakers as a variation in sound intensity.
• Describe experiments to show show diffraction of and interference of sound waves

(P3.21) Recall that the frequency range for human hearing is 20 Hz – 20 000 Hz

• 20 000 Hz can be written as 20 kHz.
• Electronic systems can be used to produce electrical oscillations with any frequency.
• These electrical oscillations can be used to produce ultrasonic waves, which have a frequency higher than the upper limit of the hearing range for humans.
• Some animals such as bats and dolphins emit ultrasonic sounds. Even more animals, such as dogs, can hear them.
• Ultrasonic waves are partly reflected when they meet a boundary between two different media.
• The time taken for the reflections of ultrasonic pulses to reach a detector (usually placed near to the source) is a measure of how far away such a boundary is.
• This idea is used in medicine for pre-natal scans and for viewing internal organs
• This idea is also used in industry to detect flaws in metal castings.
• In industry, ultrasonic waves in liquids can also be used for cleaning delicate mechanisms without having to take them apart.

The Earth’s layered structure

(P3.09) Understand that the different ways in which longitudinal and transverse waves are transmitted through the Earth, and their paths and times of
travel, provide evidence for the Earth’s layered structure: crust, mantle, outer (liquid) core, inner core

• Draw a diagram of the Earth's layered structure, showing that (a) the Earth is nearly spherical (slightly flattened at the poles), (b) it has a thin crust, (c) the mantle extends almost halfway to the Earth’s centre, and (d) the core, with just over half of the Earth’s radius (comprising an inner core and an outer core)
• The mantle has all the properties of a solid except that it can flow very slowly.
• The core seems to be made of nickel and iron; the outer part of the core is liquid and the inner part is solid (due to great pressure).
• The overall density of the Earth is much greater than the mean densities of the rocks that form the crust. This indicates that the interior of the Earth is made of material different from, and denser than, that of the crust.
• The oceanic crust is thinner but denser than the continental crust.

• Earthquakes produce shock waves (called seismic waves) that shake up the surface of the Earth and travel down inside the Earth
• Seismic waves can be detected by instruments (seismometers) on the Earth's surface. The seismometers produce seismographs.

• Know that during earthquakes several types of wave are produces including (a) P-waves (primary waves), which are longitudinal waves that travel quickly through both solids and liquids and (b) S-waves (secondary waves), which are transverse waves that travel through solids but NOT liquids and are a bit slower than P-waves
• Draw, on the same diagram as drawn to show the Earth's layered structure, how P-waves, S-waves and surface waves travel through the earth, showing how P-waves can travel through to the "shadow" side.
• Surface waves do not travel through the interior of the Earth like P- and S-waves do, but they cause the most destruction at the surface.
• Describe how the mantle-core structure of the Earth would explain why P- and S-waves behave differently inside the Earth (S-waves won't pass through core because transverse waves can't pass through liquids.)
• Explain how the seismographic record can be used to find seismic wave speeds.
• P-waves travel slightly faster than S-waves, which travel slightly faster than surface waves.
• Draw a rough sketch of a typical seismogram after an earthquake, showing P-, S- and surface waves.

(P3.10) Recall that the Earth’s outermost layer, the lithosphere, is composed of plates in relative motion and that plate tectonic processes result in the formation, deformation and recycling of rocks.

• The edges of land masses (continents) that are separated by thousands of kilometres of ocean (a) have shapes which fit quite closely and (b) have similar patterns of rocks and fossils. This suggests that they were once part of a single land mass that has split and been moved apart.
• The Earth’s lithosphere (the crust and the upper part of the mantle) is cracked into a number of large slabs (tectonic plates)
• These tectonic plates are constantly moving at relative speeds of a few centimetres per year
• The cause of continental drift is still being debated. (Old theories suggested it is a result of massive convection currents deep within the Earth’s mantle driven by heat released by natural radioactive processes. The Earth also contains heat preserved since the origin of the solar system.)
• Use the theory of plate tectonics to explain how earthquakes and volcanoes arise at plate boundaries
• The forces at the plate boundaries contribute to the rock cycle

(P3.11) Understand that at plate boundaries, plates may:

     - slide past each other, causing earthquakes

• Earthquakes and/or volcanic eruptions occur at the boundaries between tectonic plates.
• Earthquakes happen when two tectonic plates slide past each other, the rocks become stressed, eventually break and energy is released as shock waves (called seismic waves) that shake up the surface of the Earth and travel down inside the Earth
• This sliding causing earthquakes is happening along the Californian coast, for example
• The epicentre of an earthquake is the point on the Earth's surface that is directly above the point where an earthquake originates. The epicentre is directly above the focus, which is the actual location of the energy release inside the earth.
• A tidal wave (tsunami) is usually produced as a result of an earthquake or volcanic eruption under water.

     - move towards each other, taking rock into the mantle.

• The upper part of the lithosphere is the Earth's crust, which is thinner under oceans and thicker under continents
• As the tectonic plates collide, a thinner, denser oceanic plate can be driven down (subducted) beneath a thicker granitic continental plate. This is called a subduction zone.
• The descending lithosphere enters the hot mantle and partially melts to form magma
• At subduction zones, increased temperature and pressure can cause metamorphism, producing new rocks without any melting - by recrystallisation
• The continental crust is compressed, causing folding, faulting and metamorphism. Thus explain the formation of mountain ranges, island chains, mid-ocean ridges and rift valleys in terms of moving tectonic plates. This forms offshore trenches and and parallel mountain chains with volcanoes.
• Earthquakes are produced and magma may rise through the continental crust to form volcanoes. This is happening along the western side of South America (along the Andes mountain range)
• Explain why scientists cannot yet accurately predict when earthquakes will occur (using information provided in the exam about the complex probable causes of earthquakes and volcanic eruptions and the difficulty of making measurements of many of the factors involved).

     - move away from each other, resulting in volcanoes and/or formation of new rocks.

• Explain why Wegener’s theory of crustal movement (continental drift) was not generally accepted until more than 50 years after it was proposed (using information provided in the exam). (At one time it was believed that the major features of the Earth’s surface were the result of the shrinking of the crust as the Earth cooled down following its formation.)
• The separation of tectonic plates causes fractures that are filled by magma, producing new rock: basaltic, oceanic crust. This is known as sea floor spreading
• Rising magma can crystallise deep below the Earth's surface to form coarse-grained rocks (e.g. granite) or rise to the surface in volcanoes to form fine-grained rocks (e.g. basalt lava or volcanic ash)
• Sea floor spreading is happening along mid oceanic ridges, including the mid-Atlantic ridge.
• The iron-rich minerals in the magma record the direction of the Earth’s magnetic field at the time when the rising magma solidified. Magnetic reversal patterns in oceanic crust occur in stripes parallel to oceanic ridges, matching the period reversals of the Earth’s magnetic field and so supporting the concept of sea floor spreading. In the diagram, a represents the spreading ridge about 5 million years ago; b about 2 to 3 million years ago; and c present-day.