Waves
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11.1 Characteristics of Waves
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Sounds bounce back
(reflect) from hard surfaces. Echoes are sound reflections. |
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When a ray of light is
reflected from a flat, shiny surface (plane mirror) the angle at which it
leaves the surface is the same as the angle at which it meets the surface. |
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Candidates should be
able to show on a diagram how rays of light are reflected by a plane
mirror. |
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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). |
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Candidates should be
able to show on a diagram what happens to rays of light when they cross
the boundary between air and glass (or 'Perspex', or water) in either
direction. |
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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. |
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Waves can be
produced in ropes and springs and on the surface of water. |
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When waves travel along
ropes or springs or across the surface of water they set up regular
patterns of disturbances: |
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* the maximum
disturbance caused by a wave is called its amplitude; |
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* the distance
between a particular point on one disturbance and the same point on the
next is called the wavelength; |
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*
the number of waves each second produced by a source (or passing a
particular point) is |
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called the frequency,
and is measured in hertz (Hz). |
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Waves transfer
energy from a source to other places without any matter being transferred. |
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Waves travelling
along a rope or spring, or across the surface of water, can be reflected. |
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Waves travelling
across the surface of water can also be refracted. |
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The change in the
speed of water waves when they cross the boundary between two different
substances causes a change in their direction (refraction), unless the
direction of travel of the waves is along a normal. |
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This behaviour of
waves suggests that light and sound: |
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* also travel as
waves; |
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* are refracted
because they travel at different speeds in different substances (media). |
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When a ray of light
travels from glass, 'Perspex' or water into air, some of the light is also
reflected from the boundary. |
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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. |
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When light travels
down an optic fibre, all the light may stay inside the fibre until it
emerges from the other end. |
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This is because light
travels down optical fibres by repeated total internal reflection. |
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Candidates should be
able to describe, using a suitable diagram, one other use of total
internal reflection. |
| H |
The
waves which travel
along ropes
and across
the surface of water are transverse
waves: the
disturbances in the substance through which the waves travel is at right
angles to the direction in which waves themselves travel. |
| H |
The waves which travel
through springs may also be longitudinal: the disturbances in the spring
are along the same direction as that in which the waves themselves travel. |
| H |
Sound waves travel
through solids, liquids and gases as longitudinal waves. |
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Light waves are
transverse waves and can travel through a vacuum, i.e. they do not need a
medium. |
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Wave speed,
wavelength and frequency are related as follows: speed = frequency x
wavelength |
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When a wave moves
through a gap, or past an obstacle, it
spreads out from the edges. This
is called diffraction. |
| H |
Electromagnetic
radiation and sound are also diffracted which supports the idea that they
travel as waves. |
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Because of diffraction: |
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* sounds can
sometimes be heard in the shadow of buildings; |
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* radio signals can
sometimes be received in the shadow of hills. |
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The extent of this
diffraction depends upon the wavelength of the waves. |
11.2 The electromagnetic spectrum
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When rays of light pass
through prisms their direction may be changed. |
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Candidates should be
able to show on a diagram how a ray of light can be deviated by a
triangular prism. |
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When white light is
used, a spectrum is produced. |
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The spectrum is
produced because white light is made up of many different colours.
Different colours of light are refracted different amounts; red light is
refracted least and violet light most. |
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Candidates should be
able to draw a diagram to show how a prism disperses white light into a
spectrum. |
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Light is one type of
electromagnetic radiation. |
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All types of
electromagnetic waves travel at the same speed through space. |
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The various types of
electromagnetic radiation form a continuous spectrum extending far beyond
each end of the visible spectrum: |
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Different wavelengths
of electromagnetic radiation are reflected, absorbed or transmitted
differently by different substances and types of surface. |
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When radiation is
absorbed, the energy it carries: |
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* makes the
substance which absorbs it hotter; |
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* may create an
alternating current with the same frequency as the radiation itself. |
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The uses and effects of
different types of radiation depend on these and other properties. |
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Radio waves are
used to transmit radio and
TV programmes between
different points on the
Earth's surface. Longer wavelength radio waves are reflected from an
electrically charged layer in the Earth's upper atmosphere. This enables
them to be sent between distant points despite the curvature of the
Earth's surface. |
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Microwave radiation of
wavelengths which can pass easily through the Earth's atmosphere is used
to send information to and from satellites. Microwave radiation, with
wavelengths strongly absorbed by water molecules, is used for cooking. |
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Infra red radiation is
used in grills, toasters and radiant heaters, in optical fibre
communication and for the remote control of TV sets and VCRs. |
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Light is not only used
for seeing but can also be sent along optical fibres, for example in
endoscopes used by doctors to see inside patients' bodies. |
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More information can be
carried than by sending electrical signals through cables of the same
diameter. There is also less weakening of the signal in optical fibres. |
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Ultraviolet radiation
is used in sunbeds. Special coatings which absorb ultraviolet radiation
and emit the energy as light, are used in fluorescent lamps and security
coding. |
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X-radiation is used to
produce shadow pictures of materials which X-rays do not easily pass
through, including bones and metals; |
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Gamma radiation is used
to: |
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* kill harmful
bacteria in food; |
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* sterilise surgical
instruments; |
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* kill cancer cells. |
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Different types of
radiation have different effects on living cells: |
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* microwaves are
absorbed by the water in cells, which may be damaged or killed by the heat
released; |
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* infra red
radiation is absorbed by skin and is felt as heat; |
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* ultra violet
radiation can pass through skin to deeper tissues. The darker the skin,
the more ultra violet it absorbs and the less reaches deeper tissues; |
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* X-radiation and
gamma radiation mostly passes through soft tissues, but some is absorbed
by the cells. |
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High doses of ultra
violet radiation, X-radiation and gamma radiation can kill normal cells.
Lower doses of these types of radiation can cause normal cells to become
cancerous. |
11.3 Sound and Ultrasound
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Sounds are produced
when objects vibrate. |
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The greater the size
(amplitude) of vibrations the louder the sound. |
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The number of complete
vibrations each second is called the frequency (hertz, Hz). The higher the
frequency of a sound the higher its pitch. |
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Candidates should be
able to compare the amplitudes and frequencies of sounds as they would
appear on an oscilloscope trace. |
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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. |
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Ultrasonic waves can
be used: |
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* in industry for
cleaning and for quality control; |
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*
in medicine for pre-natal scanning. |
| H |
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 industry to detect flaws in metal castings and in medicine for
pre-natal scans. Information about the time taken for reflections to
travel is usually processed to produce a visual display. |
| H |
Ultrasonic waves in
liquids can also be used for cleaning delicate mechanisms without having
to disassemble them. |
11.4 Seismic Waves
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| H |
Our knowledge
of the structure of the Earth comes
mainly from studying how the
shockwaves from earthquakes (seismic waves) travel through it. These waves
are detected using seismographs. |
| H |
There are two types of
shock waves: |
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* faster travelling
P waves, which are longitudinal and travel through liquids as well as
solids; |
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* slower travelling
S waves, which are transverse and travel only through solids. |
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Both types of wave
travel faster through more dense materials. The waves travel in curved
paths when the density of changes gradually through a material. When the
density changes abruptly e.g. when moving from solid to liquid, the wave
direction also changes abruptly. |
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Observations of the
path of these waves suggest that the Earth has a layered structure: |
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* a thin crust; |
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* an extremely
viscous mantle whose density increases with depth and which extends almost
halfway to the centre of the Earth; |
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* a core, with just
over half of the Earth's diameter, the outer part of which is liquid and
the inner part of which is solid. |
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Candidates should be
able to interpret diagrams of the paths of seismic waves inside the Earth
in terms of: |
| H |
* the liquid nature
of the Earth's outer core: |
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* refraction at the
boundaries between layers; |
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* refraction due to
change in density within a particular layer. |