chapter 1 waves lesson 11 - 14

8/2/2019 Chapter 1 Waves Lesson 11 - 14

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LESSON 11Sound Waves

By the end of this lesson, you should be able to: Describe sound waves.

Explain how the loudness of sound relates to its amplitude. Explain how the pitch of sound relates to its frequency.

Look at the man 's fingers plucking the guitar strings.When the strings are plucked , they vibrate and sound can be heard . When the man hi ts the drum , the membrane vibrates and sound can be heard.

Sound waves are produced by vibrations. Vibrating musical instruments are examples of sourcesof sound waves.

When air is removed from the jar, there are no partic les left to trans fer sound energy. Thus , no

sound can be heard.Sound waves propagate through a medium. Sound waves cannot propagate through a vacuum-Sound waves can travel through solids, liquids a nd g a ses.


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A tuning fork produces sound waves.

The tuning fork has two prongs. When the prongs move to the right they compress air particlestogether.

This disturbance is then transmitted from one p a rticle to the next particle through the air , with theresult of a pulse of compression moving outwards.A reverse movement of the prongs give rise to the pulse of rarefaction of the air.Sound waves are longitudinal waves . The wave particles oscillate in the direction parallel to thedirection of propagation of the waves.


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Wavelength is measured from one compression to the next compression or from one rarefaction to

the next rarefaction.Turn the knob to increase the volume,

What can you say about the frequency of the waves produced?The waves displayed on the oscilloscope screen shows the same frequency even though theloudness of the sound is changed.What can you say about the amplitude of the waves produced?As the loudness of the sound increases, the amplitude of the waves also increases . The oscillation of a ir particles Is bigger far l ouder sounds . This causes the amplitude of thewave form to be larger.


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What can you say about the amplitude of the waves produced?The waves displayed on the oscilloscope screen shows the same amplitude even though the pitchof the sound is changed.What can you say about the frequency of the waves produced?As the pitch of the sound increases , the frequency of the waves increase s too.

The oscillation of air particles is faster for higher pitch sounds.This causes the frequency of the waveform to be higher.

LESSON 12Reflection of Sound Waves

By the end of this lesson, you should be able to:Describe applications of reflection of sound waves.Calculate distances using the reflection of sound waves.


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The Law of Reflection states that the angle of incidence is equal to the angle of reflection.

Comp a re the wavelength, the frequency and the speed of the sound wave before and afterreflection . A reflected sound wave is called an echo.

When you shout in front of a wall from a distance , you will hear an echo a few seconds later.For example:The time between the source and its echo is s

So, the time between the source and the wall = x -

s= s

The distance of the sound in air can be calculated by usingDistance = speed of sound in air x time

= 330 ms 1 x x - = 33 m

Thus ships use echoes to find the depth of the sea beneath them.High frequency sound waves , also known as ultrasound are used instead of using sounds in theaudible range. The system that uses echoes to detect depth is known as sonar

ultrasound. Sound with a frequency greater than 20 000 H z, approximately the upperlimit of human hearing.


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Sonar - A system us ing transmitted and reflected underwa ter sound wavesto detect and locate submerged objects or measure the d istance to the floor of a body of water.

Why are ultrasonic waves used instead of sound waves in the audible range?

Ultrasonic waves are used inste a d of sound waves in the audible range because they can beemitted as narrow beams and do not spread out as much as the audible sounds . Furthermore , water absorbs less energy from ultrasounds and stronger echoes are produced.

If the echo is detected after 0.4 seconds and the speed of sound in water is 1500 m s -1, calculatethe depth of the sea bed.The time between the pulse and its echo is 0.4 s.

So, the time between the source and the sea bed = = 0.2 s

The depth of the sea bed isDistance = speed x time

= 1500 ms 1 x0.2 s

= 300 m

Sound wave s ca n be used in a similar way to "see" things. After turning on a sound source , thepattern of reflected sound waves that are bounced back can be seen.

In hospitals, ultrasounds are used to monitor the condition of a fetus in the womb . Ultrasonicwaves are emitted by a probe.These waves are then reflected back to the probe from differentsurfaces within the womb . The

image is then seen on the screen.


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Bats emit ultrasounds and then listen for echoes. Their brains process these returning signals sothat they can navigate around objects as well as hunt for prey .

The pulse of a premature infant may be very difficult to detect with a stethoscope since the soundproduced is extremely faint.The ultrasonic echo from the moving blood can be mixed with the source frequency to produce abeat frequency.

LESSON 13Analysing Electromagnetic Waves

By the end of this lesson, you should be able fo:

Describe the electromagnetic spectrum.State that visible light is a part of the electromagnetic spectrum,List sources of electromagnetic waves.


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The electromagnetic spectrum is a family of continuous waves varying in wavelength andfrequency. As the wavelength increases the frequencies of the waves decrease.The electromagnetic spectrum consists of radio waves / microwaves,infra-red, visible light,ultraviolet X-rays and gamma rays.Ra dio waves have the longest wavelength whereas gamma rays have the shortest wavelength.Visible light is a part of the electromagnetic spectrum . We can see these wave s as the colours ofrainbow. Red has the longest wavelength and violet has the shortest wavelength .

Radio waves have the longest wavelength (ranging from 10 5 m to 10 -1 m) and the lowestfrequency (ranging from 10 3 - 10 9 Hz) in the electromagnetic spectrum.For bro a dcasting, among the frequencies normally used are:

i) Long Wave (LW) or Low Frequency (LF)ii) Medium Wave (MW) or Medium frequency (MF)iii) Short Wave (SW) or High Frequency (HF)iv) Very high-frequency (VHF)v) Ultra high-frequency (UHF)


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This is a satellite image of sea ice breaking off the shores Alaska.The satellite uses microwaves. Why?Microwaves can penetrate haze, light rain and snow; therefore these waves are good for viewing

the Earth from space.

MICROWAVESWavelength :10 -1 to 10 -3 mFrequency:10 9 - 10 11 Hz

Do you know how microwaves are produced?The waves are produced by the oscillation of electric current in special valves.Microwaves are used for satellite communication and also for microwave cooking-

INFRARED

Wavelength : 10 -3 to 10 -6 mFrequency ; 10 11 to 10 15 Hz

Most infrared waves are thermal. Infrared radiation can be produced by sunlight, fire or any bodythat generates heat.Infrared is used in remote controls for televisions and video recorders.Physiotherapists use heat lamps in reflexology to help heal and relax muscle injuries.


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VISIBLE LIGHTWavelength: 10 -6 10 -7 m

Frequency : 10 14 10 16 HzRed has the longest wavelength and violet has the shortest wavelength.

Red has the lowest frequency while violet has the highest frequency.Violet has the highest energy making it most difficult to diffractDuring the day, the sky looks blue because at that instant the blue colour is diffracted the most.So, we see blue sky,As the red colour has the highest wavelength, it can still be seen after thesun has set into thehorizon.Red has the lowest energy thus, it is easily diffracted and spread over the horizon

What happens when light (visible light) is directed on a prism?What happens if a second prism is placed next to the first prism?When all seven colours are seen together, is white light seen?

A fluorescent lamp Is used to identify fake money. There are ultra violet rays in the fluorescentlamp,

Do you know how ultraviolet rays are produced?


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Ultraviolet radiation may be produced by any thermal radiation of a source because of itstemperature. The sun and an ultraviolet lamp are convenient sources of ultraviolet radiation.

ULTRAVIOLET RAYS

Wavelength : 10-7

to 10-8

mFrequency : 10 15 to 10 17 Hz

Ultraviolet radiation can make a cloth glow in the dark. The glow is caused by the presence offluorescent dyes from washing powders which absorb ultraviolet radiation and then emit light

X-RAYSWavelength : 10 8 to 10- 11 mFrequency: 10 to 10 20 Hz

X-rays are very high frequency waves and carry a lot of energy. They willpenetrate most substances , and this makes them useful in medicine andindustry to see inside things.

In nature , the sun gives off X-rays. X-rays can also be produced in the lab. An X-ray machine / works by firing a beam of high velocity electrons a t an anode. When electrons with enough energy , hit the anode , X-rays will be produced.

GAMMA RAYSWavelength : 10 -11 to 10 -16 mFrequency: 10 20 to 10 24 Hz

The patient was given a slight radioactive gas to breathe , and the picture was taken using agamma camera to detect the radiation . How are gamma rays generated?Gamma rays are emitted from nuclear reac tions, atomic bombs and exp losions on the sun andother stars.

LESSON 14

Properties and Applications of Electromagnetic Waves

By the end of this lesson , you should be able to: Describe the properties of electromagnetic waves.


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Describe the applications of elec tromagnetic waves Describe the detrimental effects of excessive exposure to certain components of the

elec tromagnetic spectrum.

Electromagnetic waves are emitted whenever electrons are oscillated-These waves consist of a fluctuating electric field accompanied by afluctuating magnetic field of the same frequency and phase.The electric and m agnetic fields of an electromagnetic wave areperpendicular to each other and to the direction of propagation ofthe wave .


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The electromagnetic spectrum consists of electromagnetic waves of different frequencies andwavelengths .

Different groups of waves have different characteristics due to the

different waveleng ths and frequencies.

Radio waves are produced in a variety of ways such as alternatingcurrents in metal antennas.


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Higher frequencies of radio waves are necessary to provide enoughenergy where more information can be carried.Radio waves are used as the carrier wave widely used inbroadc a stin g and communication.

What are the bad effects of excessive exposure to radio waves?Excessive exposure to radio waves such as those emitted by theradio wave transmitters and power lines may cause leukaemia andcancer.Telephone signals are also said to inhibit the melatonin hormoneand can cause insomnia , headaches , nausea and vertigo / ear acheand tinnitus , frequent nosebleeds and raised white blood cell count,

Microwaves

The frequency of microwaves is higher than that of radio waves asmicrowaves have shorter wavelength. Thus / microwaves have higherenergy and penetrating power than radio waves.Microwaves are widely used in satellites , radar and cellular telephones.

TV s ignals are beamed to satell ite using microwaves as carr ier waves .

S atellite links make broadcasting of live events on TV possible acrossthe world .This can happen because microwaves have shorter wavelength thannormal TV carrier frequencies. Thus / the waves can be directed in anarrow beam from earth to the satellite and back to another part of theearth.What is the effect of overdose expo s ure of microwaves?It has been reported th a t prolonged e a ting of food cooked or heated up inmicrowave ovens can cause cancer cells to increase in human blood , lossof memory and concentration / emotional instability and decrease of

intelligence.This may be because nutrition In the food cooked or heated up in


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microwave ovens is decreased when food molecules are rotated bymicrowaves to generate heat

Infrared waves

The surfaces of all bodies emit infrared radiation in a continuous range of wavelength.The hotter the object is / the greater the energy per second is carried away by infrared radiation.

Infrared penetrates smoke and dust better than visible light.It widely used in household appliances and night vision devices such as rifles, cameras andbinoculars.Infrared radiation is used in sauna treatment to treat cancer .Infrared lamps are used for the treatment of muscular comp laints and to dry the wet paint on cars.Remote control keypads for television and video recorders use infrared signalling.Very sensitive infrared detector such as photoelectric devices is used to detect the infrared emitted

by a person buried alive in a collapsed building and to locate victims of earthquakes.Infrared radiation causes an increase of internal energy which usua lly results in a temperature risewhen absorbed by matter. This is why the skin experiences heat irritation when exposed toinfrared rays.

Visible light

Light is detected by cells in the eye retina . This enables us to see things.

Visible light c on also be detected chemically with photographic filmand electronically by usingsemiconductors . The high frequency of light makes it ideal for transmitting information,


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An optical fibre communication system can carry telephone conversations , internet links and videoconferencing signals .

Bundles of optical fibres are used to carry images back from previously inaccessible places,especially inside the human body using an endoscope. Chlorophyll in green plants absorbs most of the violet, blue, orange and red light that falls on themand uses the energy for photosynthesis.Excessive exposure to sunlight can lead to adverse health effects , such as the pain of sunburn , headache, lumbago, insomnia, leukaemia or skin cancer.Thus, one should not focus directly at high intensity light sources such as welding sparks or thesun .

Ultraviolet rays

Ultraviolet rays are produced by sunlight and mercury vapourlamps. The very short-wavelength of ultraviolet rays has very high energy and penetrating power.It is widely used in fluorescent lamps and sterilisation.Ultraviolet stimul a tes vitamin D production in the skin and causes tanning of the skin.

UV spectroscopy is used in pharmaceutical industry to analyse drugs and in dentistry topolymerise materials used in some fillings.


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It is also used to detect forged banknotes.Bees can detect UV light and flowers attract the bees with beautiful patterns that are invisible tohuman eyes.Long - wavelength ultraviolet radi a tion or UV-A pene trates the skin very deeply and c a uses skin tosag and wrinkle prematurely.Excessive exposure to short wavelength ultraviolet radiation or UV-B is harmful and can causeskin cancer.The ozone layer in the e a rth 's atmosphere absorbs almost all of the very short -wavelengthultraviolet radiation or UV-C.

An ultraviolet lamp should never be viewed directly as ultraviolet ray is harmful to the eyes.

X-rays

X-rays are generated by the rapid deceleration of high speed electrons projected against a metaltarget.X-rays have high kinetic energy , thus the penetrating power of X- rays are also high .

X-rays are used in medical Imag ing and secur ity systems at a irports.As bones absorb X-r ays mare strongly than flesh, medical doctors can see the conditions of bonesin the human body with the help of the X-ray photograph. Dentists use the same technique to treattheir patients.In the industrial world . X-ray scanners are used to detect minute flaws in heavy metal equipment.X-rays are harmful as they are a form of ionising radiation .

Excessive exposure to the X-rays can break DNA chains and cause cells to die or mutate theDNA. Mutated cells may become cancerous.


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Gamma rays kill microorganisms in fruitsGamma rays are produced by excited nuclei releasing energy during radioactive decay-Gamma rays have very high energy and penetration power.

Gamma rays are used ta trace the flow of fluids, to kill cancer cells and microorganisms in food-They can also be used to sterilise medical equipments.Gamma rays are destructive to living cells.As they are used to kill cells of organisms, normal exposure togamma rays can also damage thecells.Excessive exposure to gamma rays can cause genetic mutations especially in reproduc tive cells.

chapter 1 waves lesson 11 - 14

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