WAVE
Wave is a disturbance which moves without changing original medium.
- Energy is transmitted from one place to another through wave. Generally
waves are divided in two categories.
1. Mechanical waves:- Waves which needs medium for transmission. They are
of two types:
i. Transverse Waves – Propagation of wave is perpendicular to direction of the
particle vibration.
- They can be generated in solids and liquid surface.
- They propagates in the form of crest and trough.
1.
ii. Longitudinal Waves – Waves propagates along the direction of particle
vibration.
- They can be generated in all medium (solid, liquid, gas)
- Wave propagates through compression and rarefaction.
Example – Sound wave in air, earthquake wave.
Some Vocabulary:-
I. Amplitude:- The maximum displacement from mean position of the vibration of
the particles of the medium is called the amplitude of the wave. It is represented
by A.
II. Frequency:- The number of oscillation executed by the particles of the
medium in one second is called frequency (n) and in SI unit it is expressed in
hertz (Hz).
III. Time period:- The time taken by the vibration of the particles of the medium in
completing one cycle of vibration to pass a given point.
IV. Wave length – In Transverse wave the distance between two consecutive
crests or troughs is called its wavelength and in longitudinal wave it is the
distance between two consecutive compression or rarefactions. It is represented
by
V. Wave speed – The distance travelled by wave in unit time is called Wave
speed (V).
2. Non-Mechanical Waves:- Waves which do not need any medium to propagate
are called non-mechanical or electromagnetic waves. These waves can travel in
vaccum.
- They are transverse waves.
- Direction of wave propagation is perpendicular to electric and magnetic field.
- Their velocity is equal to velocity of light.
Following are electromagnetic waves:-
a) Gamma Waves –
Inventor = H.Becquerel
Wave length = 10-14m to 10 -10m
Frequency = 1020 Hz to 10 18 Hz
They are used for treatment of cancer.
Their penetration capacity, Energy and Frequency is high due to less
wave length.
b) X- Rays –
Inventor – Wilhelm Roentgen.
Wave length =10-10m to 10 -8m
Frequency = 1018 Hz to 10 16 Hz
Used in medical field for diagnosis.
c) U. V. Rays –
Inventor – Johann Ritter
Wave length = 10-8m to 10 -7m
Frequency = 1016 Hz to 10 14 Hz
Used in destroying bacteria photoelectric effect, producing electric discharge.
d) Visible light (spectrum) –
Inventor = Newton
Wave length = 3.9 10-7m to 7.8 10 -7m
Frequency = 1014 Hz to 10 12 Hz
Use: objects become visible due to it.
e) Infra red –
Inventor – Husserl
Wave length = 7.8 10-7m to 10 -3m
Frequency = 1012 Hz to 10 10 Hz
Used in photography, data transmission and remote sensing.
f) Microwave –
Inventor = Hertz
Wave length = 10-3 m to 1 m
Frequency = 1010 Hz to 10 8 Hz
Used – TV, mobile, Wimax, microwave oven.
g) Radio Waves –
Inventor – Marconi
Wave length = 1 to 10 4m
Frequency = 106
Hz to 10 4 Hz
Use – Mobile and Radio
SOUND WAVES
They are one kind of mechanical wave which need medium for propagation.
- They are longitudinal waves and propagated in all medium solid, liquid and gas.
- They are generated due to vibration in a thing.
- Sound waves are divided in three types according to frequency.
1. Infrasonic:
Frequency – less than 20 Hz
They are not audible.
Generated with earthquake.
2. Sonic:
Frequency – between 20 Hz to 20,000 Hz.
They are audible
3. Ultra Sonic:
Frequency – more than 20,000 Hz.
Not audible to humans.
But audible to dogs, cat, bats etc.
Galton has first generated these waves.
They are generated by flowing electricity through different cristals like Quartz,
lead, Zerconet. This effect is called Piezo electricity.
These sound waves are used in different fields.
In Medical field – Sonography.
Sonar System in ships – to measure depth of ocean and identify icebergs.
To reduce fog in atmosphere.
To destroy bacteria.
SPEED OF SOUNDIt is a mechanical wave – needs medium to propagate sound has different speed in
different mediums
Speed of sound depends upon elasticity and density.
Speed of Sound increases with increase in elasticity and decreases with
increase in density.
Solids have highest elasticity and gases have lowest hence sound has maximum
speed in solid and lowest in gas.
Speed of sound: In air = 332 m/s, water = 1450 m/s, iron = 5400 m/s and
Aluminium = 6100 m/s
Sound has more speed in low density wet air than high density dry air that’s why
we can hear sound from long distance in rainy season but not in summer.
With increase in temperature speed of sound get increased. Pressure do not
have any effect on sound speed.
Characteristic of Speed:-
- Three Characteristic of Speed.
1. Intensity of Sound – That characteristic of sound according to which sound is
identified slow or fast.
- Intensity depends on Amplitude of Sound.
- Intensity is measured in decibal. Sound reading meter is used for this purpose
Whispher – 15-20db
Conversation – 40-50 db
Loud Voice – 50-60 db
Sound Pollution – More than 80 db.
- Maximum intensity of sound is of sirens used for navigation in ships (180 db –
200 db)
2. Pitch of Sound – That characteristics of sound which tells about loudness of
the sound. Pitch depends on frequency as frequency increases sound becomes
shrill like in women.
3. Quality of Sound – Sounds of similar amplitude and frequency are
distinguished on the basis of quality.
Reflection of Sound Waves:-
When sound returns back after colliding the surface of another medium
then this phenomena is called Reflection.
For reflection of Sound big reflective surface is needed as it has higher
wave length.
When speed of sound in air is 332 m/s and our ear are not able to
distinguish among sounds coming during 0-1 sec, therefore for echo to be
produced there should be at least 17 m distance between source and
reflecting surface.
Refraction of Sound waves:-
Sound wave gets deflected from its path when it moves from one medium to
another medium. This phenomenon is called refraction.
When it moves from dense medium to rare medium then it deflects away from
normal and when it moves from rarer to denser than sound wave move towards
the normal.
Because of refraction sound is heard to large distance in night in comparison to
day.
Interference of Sound:-
When two sound waves of equal amplitude and frequency are met at a point
then sound energy gets redistributed, at that point. This is called interference of
Sound.
1. Constructive Interference – When two sound waves of equal amplitude and
frequency are met in same phase at a point then its amplitude get increased and
its intensity is maximum.
2. Destructive Interference – When waves meet in opposite phase. Amplitude
get reduced and intensity is minimum.
In ocean at some places intensity of sound is minimum, that’s why silence zone
are constructed.
- In Radio at some places intensity is more and some places it is less.
RESONANCE:- When a body is freely vibrating and an external force is imposed
with equal frequency then amplitude of oscillation get increased. This
phenomena is called Resonance.
Example: Radio works on the principle of resonance.
- Army personnels are not advised to have similar steps while walking on a
bridge.
- Tansen cracking glasses (In History it is said) can be explained through
resonance.
Defraction of Sound:-
- Quality of Sound because of which sound gets diverted in different direction
after colliding at edges of an obstacle.
Example: Person sitting in room can listens sound of moving train.
DOPPLER’S EFFECT:-
When there is a relative motion between source of sound and listener then
frequency of sound appears to be changing. This effect is called Doppler Effect.
If distance is reducing then frequency gets increased and when distance is
increased then frequency get reduced.
Example: When train approaches platform, frequency of sound appears to be
increasing for a person standing on platform, and appears lowering if train is
moving away from the platform.
-Radar decides velocity of aircraft on the basis of Doppler Effect.
- Doppler Effect is also seen in light waves.
- In laboratory, if light received from a star is displacing towards violet colour
then that star is moving towards earth but its getting displaced towards red
colour then that star is moving away from the star.
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