P1.5 The use of waves

P1.5 The use of waves for communication and to provide evidence that the universe is expandingElectromagnetic radiations travel as waves and move energy from one place to another. They can all travel through a vacuum and do so at the same speed. The waves cover a continuous range of wavelengths called the electromagnetic spectrum.Sound waves and some mechanical waves are longitudinal, and cannot travel through a vacuum.Current evidence suggests that the universe is expanding and that matter and space expanded violently and rapidly from a very small initial ‘point’, ie the universe began with a ‘big bang’.Candidates should use their skills, knowledgeand understanding to:

■ compare the use of different types of waves forcommunication

■ evaluate the possible risks involving the use ofmobile phones

■ consider the limitations of the model that scientistsuse to explain how the universe began and why theuniverse continues to expand.Additional guidance:Knowledge and understanding of waves used forcommunication is limited to sound, light, microwaves,radio waves and infrared waves.

Waves transfer energy from a source to other places without any matter being transferred.

Waves

Waves set up regular patterns of disturbances:

a a’

c c’

Wavelength a – a’ the distance between a particular point on one wave and the same point on the next wave

-

amplitude

Amplitude - the maximum disturbance caused by a wave

Frequency - the number of waves passing a particular point each second, measured in “Hertz” Hz

b’b Wavelength

Wave speed, wavelength and frequency are related as follows:

wave speed = frequency X wavelength

(metre/second, m/s) (hertz, Hz)(metre, m)

v = f x λ

v

λf

metre/second, m/s

hertz, Hzmetre, m

λ; lambda, a Greek letter to denote wavelength

There are 2 types of wave, longitudinal and transverse:

Direction of energy transfer

Longitudinal waves

The disturbances in the spring are along the same direction as that in which the waves themselves travel.

compressionrarefaction

Example - Sound waves travel through solids, liquids and gases as longitudinal waves.

It is difficult to make measurements from this type of wave and as a result we often use electronics to convert it to a….

Transverse Waves

The disturbances in the substance through which the waves travel is at right angles to the direction in which waves themselves travel.

Direction of energy transfer

Example – water, electromagnetic waves (can travel through a vacuum, i.e. they do not need a medium to travel through)

Mechanical waves can be either longitudinal or transverse, eg. Earthquake waves.

Waves can be; reflected, refracted and diffracted.

Reflection

Mirror

i

Normal

Incident Ray

i = Angle of Incidence

r

r = Angle of Reflection

Reflected Ray

The normal is a construction line perpendicular to the reflecting surface at the point of incidence.

Using the table below, measure and record the angles of Incidence and Reflection. Draw one out as an example.

Angle of Incidence

(degrees)

Angle of Reflection

(degrees)

0o

20o

40o

60o

80o

Write a sentence using the scientific words from the diagram to explain what your results show.

0o

20o

40o

60o

80o

The angle of incidence is equal to the angle of reflection

The image produced in a plane mirror is; virtual, upright and laterally inverted.

E

mirror

The image produced in a plane mirror is; virtual, upright and laterally inverted.

E

mirror

The image produced in a plane mirror is; virtual, upright and laterally inverted.

We use the ability of glass to refract light in magnifying glasses.

Rays of light change direction (are refracted) when they cross the boundary between one transparent medium and another, unless they meet the boundary at right angles (along a normal). Waves are not refracted if travelling along the normal.

Refraction

The ray bends towards the normal as the light enters the denser medium and away from the normal as it leaves the denser medium.

As white light passes from air into glass it slows down. The white light is made up of different colours and each colour slows down by a different amount. The Prism uses this to separate the colours out.

Dispersion

Note: The light first begins to split up as it enters the prism.

As white light passes from air into glass it slows down. The white light is made up of different colours and each colour slows down by a different amount. The Prism uses this to separate the colours out.

Dispersion

Note: The light first begins to split up as it enters the prism.

When a wave moves through a gap, or past an obstacle, it spreads out from the edges.

If the gap (or obstacle) is of a similar size to the wavelength of the wave, the diffraction is more significant.

Diffraction

Because of diffraction:

Sounds can sometimes be heard in the shadow of buildings;

Television and radio signals can sometimes be received in the shadow of hills.

Waves having a longer wavelength are more strongly diffracted.

Radio wavelengths are long enough to be diffracted by hills, allowing the signal to be received in the shadow of the hill.

Electromagnetic radiations travel as waves and move energy from one place to another.

The Electromagnetic Spectrum

They can all travel through a vacuum and do so at the same speed, 300 000 000 m/s.

The waves cover a continuous range of wavelengths called the electromagnetic spectrum.

We can use these different types of waves for communication.

gamma rays

X-raysultra

violet rayslight

infra red raysmicrowav

esradio

waves

Highest ShortestFrequency Wavelength

Lowest LongestFrequency Wavelength

Least energy

Most energy (about 10–15 metres)

(more than 104 metres)

Radio waves Radio waves are used to transmit radio and TV programmes between different points on the Earth's surface.

Radio Caroline

When radiation is absorbed, the energy it carries may create an alternating current with the same frequency as the radiation itself.

We can use this alternating current to transmit data, audio and video information.

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.

Microwaves

Microwave radiation consists of wavelengths which can pass easily through the Earth’s atmosphere is used to send information to and from satellites,

and within mobile phone networks.

Infra red

Infra red radiation and light are used in optical fibre communication…

…and for the remote control of TV sets and VCRs.

Light

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.

1. One of the two main endoscope fibre-optic cables carries light into the body, illuminating the cavity where the endoscope has been inserted.

2. The light bounces along the walls of the cable into the patient's body cavity.

3. The diseased or injured part of the patient's body is illuminated by the light.

4. Light reflected off the body part travels back up a separate fibre-optic cable, bouncing off the glass walls as it goes.

5. This light shines into the physician's eye so he or she can see what's happening inside the patient's body. Sometimes the fibre-optic cable is directed into a video camera (which displays what's happening on a television monitor) or a CCD (which can capture images like a digital camera or feed them into a computer for various kinds of image enhancement).

Visible light can also be used for taking photographs.

Sounds result when objects vibrate.

The number of complete vibrations each second is called the frequency (hertz, Hz).

Sounds result when objects vibrate.

Sound waves are longitudinal waves and cause vibrations in a medium (material).

Sound waves cannot travel through a vacuum because there are no particles to pass on the vibrations.

The greater the size of the vibrations (amplitude)the louder the sound.

Sound waves can be shown on an oscilloscope.

The higher the number of the vibrations (frequency) the higher the pitch.

The pitch of a sound is determined by its frequency … … and loudness by its amplitude.

Sounds bounce back (reflect) from hard surfaces. These sound reflections are called echoes.

Colorado canyon echo cliffs c.1890

The Refraction of Sound

In the daytime, the ground is heated. The air is warmest nearest the ground, getting cooler higher up.

The sound is refracted upwards and the sound does not travel as far.

At night-time, the ground is cooling. The air is coolest nearest the ground, getting warmer higher up.

The sound is refracted downwards and the sound travels further.

Musical instruments

The note in a wind instrument is produced by a vibrating column of air.

The note in a string instrument is produced by a vibrating string.

The note in a percussion instrument is produced by vibrating the instrument.

If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency.

The Doppler Effect

When the source moves away from the observer, the observed wavelength increases and the frequency decreases. (Red shift)

When the source moves towards the observer, the observed wavelength decreases and the frequency increases. (Blue shift)

Red ShiftWhen objects move away from Earth at high speed, the light waves from them become “stretched out” so that the wavelengths are shifted towards the red end of the spectrum. This is called red shift.

The speed with which these other galaxies appear to be moving away from us suggests how the Universe might have begun.

(There is a red-shift in light observed from the most distant galaxies and the further away galaxies are the bigger the red-shift.)

Light waves from most distant galaxies are stretched out, suggesting that these galaxies are moving away from us. The further away the galaxies are, the faster they are moving, and the bigger the observed increase in wavelength.

The whole Universe is expanding and reversing time suggests that it might have started, billions of years ago, from one place with a huge explosion ('big bang'). The Universe began from a very small initial point, we think about 13.7 billion years ago.

The ‘Big Bang’ theory is currently the only theory that can explain the existence of CMBR.

Cosmic microwave background radiation

A further piece of evidence is the cosmic microwave background radiation.

CMBR is a form of electromagnetic radiation filling the universe. It comes from radiation that was present shortly after the beginning of the universe.