# P1.5 The use of waves.

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P1.5 The use of waves

P1.5 The use of waves for communication and to provide evidence that the universe is expanding
Electromagnetic 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, knowledge and understanding to: ■ compare the use of different types of waves for communication ■ evaluate the possible risks involving the use of mobile phones ■ consider the limitations of the model that scientists use to explain how the universe began and why the universe continues to expand. Additional guidance: Knowledge and understanding of waves used for communication 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: Wavelength b’ b c
amplitude Wavelength a – a’ the distance between a particular point on one wave and the same point on the next wave - Amplitude - the maximum disturbance caused by a wave Frequency - the number of waves passing a particular point each second, measured in “Hertz” Hz

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 metre/second, m/s λ; lambda, a Greek letter to denote wavelength v f λ hertz, Hz metre, m

There are 2 types of wave, longitudinal and transverse:
Longitudinal waves The disturbances in the spring are along the same direction as that in which the waves themselves travel. Direction of energy transfer compression rarefaction 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 r Reflected Ray Incident Ray Normal
The normal is a construction line perpendicular to the reflecting surface at the point of incidence. i = Angle of Incidence r = Angle of Reflection

Angle of Incidence (degrees) Angle of Reflection 0o 20o 40o 60o 80o
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 0o 20o 40o 60o 80o 0o 20o 40o 60o 80o Write a sentence using the scientific words from the diagram to explain what your results show. The angle of incidence is equal to the angle of reflection

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.

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

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

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

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

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

Diffraction 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.

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.

The Electromagnetic Spectrum
Electromagnetic 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, m/s.

The waves cover a continuous range of wavelengths called the electromagnetic spectrum.
We can use these different types of waves for communication.

Most energy Highest Shortest Frequency Wavelength (about 10–15 metres) gamma rays X-rays ultra violet rays light infra red rays microwaves radio waves Least energy Lowest Longest Frequency Wavelength (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.

One of the two main endoscope fibre-optic cables carries light into the body, illuminating the cavity where the endoscope has been inserted. The light bounces along the walls of the cable into the patient's body cavity. The diseased or injured part of the patient's body is illuminated by the light. Light reflected off the body part travels back up a separate fibre-optic cable, bouncing off the glass walls as it goes. 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.

Sounds result when objects vibrate.
The number of complete vibrations each second is called the frequency (hertz, Hz).

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.

Sound waves can be shown on an oscilloscope.
The pitch of a sound is determined by its frequency … … and loudness by its amplitude. The higher the number of the vibrations (frequency) the higher the pitch. The greater the size of the vibrations (amplitude)the louder the sound.

Sounds bounce back (reflect) from hard surfaces
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.

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

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. (There is a red-shift in light observed from the most distant galaxies and the further away galaxies are the bigger the red-shift.) The speed with which these other galaxies appear to be moving away from us suggests how the Universe might have begun.

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.