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# Wave Motion :

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## EMPERIND

on 23 March 2018

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#### Transcript of Wave Motion :

Wave Motion :
Sound and electromagnetic radiation including light are propagated

in the form of wave . when energy is transferred from one place to another without the net transfer of matter it is known as wave motion .
*
A periodic displacement or vibration about a mean position takes place . this may take the form of a displacement of air molecules .
Eg :
(
Sound waves in air
) or water molecules
(
Waves on water
) or displacement of electric and
magnetic energy .
There are two types of waves
1 ) Transverse waves
2 ) Longitudinal waves
Transverse :
When the free end of a rope whose other end is tied to an object , is tossed up and down a wave motion sets in the rope that travels to the other end of the rope in a plane perpendicular to the propagation of the wave .

Here , the rope as a whole does not move , but the wave carry energy from the free end to the tied end. this is known as transverse wave.
In the figure above the term ' crest ' denotes the highest point of the wave and ' trough ' the lowest point .
The distance between two successive crests ( or troughs ) is called
wave length
and is indicated by distance A is the amplitude of the wave .
Longitudinal wave :
Longitudinal waves are in which the displacement takes place in the
direction of propagation of the waves.
Longitudinal waves, unlike the transverse waves , require a medium of propagation , sound waves are longitudinal waves .
The wave length of a transverse wave is the distance between two consecutive crests (
or troughs
) and in case of a longitudinal wave the
distance between two consecutive vibrations made.
The speed (
or velocity
) of a wave is obtained by multiplying its wave length by its frequency .
ELECTROMAGNETIC SPECTRUM
Electromagnetic waves - Spectrum
The waves travelling with velocity of light and consisting of oscillating electric and magnetic fields perpendicular
to each other and also perpendicular to the direction of their propagation are called the electromagnetic waves .
Such waves with different range of frequency constitute an

electromagnetic spectrum .
Visible Spectrum
The visible region of the electromagnetic spectrum is most familiar to
us , because our eye is sensitive to this region . all know sources of light emit the spectrum .
The wave length of the visible spectrum extends from a bout that is
from violet colour to red colour
Visible spectrum is emitted when the excited valence electrons in atoms jump back to their normal states
The colour of light emitted by the particular substance is characteristic of the atom s of the substance .
Example :
You are aware and colours produced by Diwali crackers
INFRARED ( IR ) SPECTRUM
Infrared radiations have wavelengths larger that those of visible light , that is from to about .
They are emitted by molecules they change their states of rotational or vibrational motion .
Heat from hot bodies travels in the form of IR radiations and therefore all hot bodies form sources of IR radiations.
IR radiations can be detected by devices like
thermometers
,
thermopiles
,
bolometer
etc.,which are sensitive in heat .
IR radiations are also used to take photographs of objects in darkness .
Micro waves
Micro waves are electromagnetic waves with wave lengths in the range

These waves are commonly produced by electromagnetic oscillators with high frequency
( 10 - 10 c/s )
in electric circuits .
9
11
Micro waves are used in

Radio detection and ranging , telemetry and microwave
ovens.
They are also used to transmit telephone conversations .
The satellite communications also utilize micro waves .
SONAR
Sonar
was invented in
1915
by professor
langevin
in
france
.
He developed the device to detect icebergs sinking of the passenger ship
Titanic
by an icebergs in

1912
Sound Navigation and Ranging
(
sonar
) ships use echo sounding to find
how deep the water is .

Sonar
is a similar device for finding underwater objects like submarines .
Doppler Effect
The
Doppler Effect
is the name for the shift in pitch of a note when
the source of the sound is moving away from or towards the observer .

When a car with a siren or a motor bike goes past a person very quickly
the siren or engine goes from the high to low as the vehicle moves
towards the person then away.

The same type of frequency change when waves reflect off a moving object
Police use this method to tell how fast cars are moving along the road
.

police checks speed with a
'
radar
'
gun workout a motorist's speed by
detecting the echo of an electromagnetic wave
Radio Waves
Radio waves have wavelengths from
1m to about 100km
.
They are produced by the electromagnetic oscillators of low frequency
radiations are created when electrons are accelerated in a suitable electronic
circuit .they can travel long distances carrying messages.
Mapping of a radio emissions from extra terrestrial sources is known as
'
radio astronomy
' .
Ultraviolet ( UV ) Spectrum
The radiations of wavelength shorter than the visible light ranging from about
to 1nm are called
ultraviolet ( uv ) radiations
.
these radiations are

produced by the transitions of the electrons in atoms .
Since such transitions occur in the sun
, ultraviolet radiations
are received from the sun also .
Ultraviolet radiations
causes common sunburn , but long term exposure can lead to
skin diseases including cancer
.
We are protected from
UV
radiations of the sun because ozone atmosphere
obsorbs ultraviolet rays strongly .
Only a little of this radiations reaches the ground .
X-rays
X - rays
are produced in discrete wavelength in individual transitions among
the inner electrons of an atom.
X - rays
continuous wavelengths can also be produced when incident electrons are accelerated inside the target atoms .
Wavelengths of

X - rays range from 0.001nm ( 0.01 A ) to 10nm ( 100 A )
0
0
The spacing between atoms in a crystalline solid ranges from
0.1 to 10 A

.
o

X - rays
short wavelength are used to determine the structure of materials
. these X - rays are called hard X- rays .
X - rays

longer wavelength
( 10 to 100 A )
can easily penetrate through soft tissues of a human body but cannot pass through the bones and other solid
materials
o
Example
X - rays
, are used in
medical diagnosis
called '
radiography

' and also use to
cure some diseases
is called
radiotherapy .

Gamma Rays
Gamma rays are electromagnetic radiations with shortest wavelengths ranging from
0.0001 nm
or from

0.001 A to 1 A .
0
0
Natural radioactive substances like

U

emit these

radiations .
235
These radiations are also emitted when an excited nucleus comes to its ground state.
Gamma radiation leads to harmful effects on

human body .
Uses of Electromagnetic Radiation
The electromagnetic radiation is used in the propagation of radio
broadcasting , television transmission and radar .
The electromagnetic waves of shorter frequencies are less penetrative (
like radio waves
) and that of higher frequencies are more penetrative
(
like X - rays , gamma rays
)
Less penetrative power , radio broadcasting using short waves are reflected back to the earth by the ionosphere (
a layer of the atmosphere
) which can be received by radio sets where on the earth .
The radio reception during the day is weaker , but during the night ionosphere
is more stable in absence of sunlight .
The radio reception improves in clarity with less noise. the frequency of radio
waves is measured in

Hertz ( Hz )
.
Radar
Radio Detection and Ranging ( Radar
) uses electromagnetic waves ,
like
microwaves or radio waves
to detect objects .
A solid metal object in the path of the beam would reflect some of the waves back to the aerial .
The screen output from the computer is often in the form of a map showing where all the surrounding objects are.
This types of system is particularly useful for navigating ships and aircraft , especially in the fog or at night .
Long distance telecasting is possible only with the help of
geostationary satellites ( like
INSAT - 1B
) which reflect television signals back to the earth .
Long distance telephone links are also possible through geostationary
satellites .

Radar ( Radio Detection and Ranging )
also functions by using
electromagnetic radiations , a radar employs radio wave of higher
frequencies .
These waves after getting reflected by objects like aircraft and ships return to their source .
The time taken for transmission and reception of the waves is used
to find out the distance of the object from the source .
SOUND
Sound is produced by vibration of sounding body , when a material object like a guitar string is
struck , it moves back and forth rapidly .this back and forth motion of the string is called vibrating .
The number of complete vibrations, a vibrating body makes in one second is called the
frequency of
its vibration .
A sound of a higher frequency has a higher pitch .
Generally speaking women's voice is higher pitchied than that of men .

All vibrations do not cause sound. if an object vibrates at any frequency between
20 and
20,000
vibrations per second , its vibration affects the human ear so that sound is heard .
The sound waves having frequencies less than 20 per second are called '' infrasonic ''
waves
''
( infra means within and sonic means sound , and that of those beyond
20,000
frequencies er second are called ''
ultrasonic waves
'' ultra means beyond .
Human ear cannot detect infra and ultrasonic sounds , but certain animals can .
example :
Elephants can detect infrasonic sounds and bats and dogs that of

infrasonic
sounds
.
the range between

20 and 20,000
frequencies is called
''
audible range
'' .
Ultrasonic vibration uses some
dairies use them to kill bacteria in milk and to
break up milk fat intovery small particles
.

Ultrasonic vibrations are used in controlling smoke and in clearing fogs.

paints and face cream can each be mixed more evenly by using ultrasonic
vibrations
.
Speed of Sound
As we have seen in the previous chapter , sound waves are longitudinal cannot travel in vacuum.
These media , sound travels faster in

solids
,
followed by in
liquids
and
gases .
the
speed of sound in different substances
.

They are two factors that influence the speed of sound in a medium ,
a)

elasticity
and

b
)
density
.
Higher the density and elasticity of a substance , faster the speed of sound in the
substances .

The speed of sound in steel is over

15
times faster than that in air and over
3

times than that in water
.
Example :
A
train
can be detected earlier , by placing the ear on the steel rail track

than hearing its noise through the atmosphere .
SPEED OF SOUND IN DIFFERENT MEDIA
Noise
Noise

also sound , but it is unwanted sound . loud sound is noise.
The nature of human ear , to a certain extent loudness depends
on frequency of the sound waves .
A whisper is of

20 db
,
an ordinary conversation 65 db and the

noise of an aircraft taking off is

135 db
.
prolonged exposure to
noise level of

110db

and above can impair hearing permanently .
Echo
The sound heard after being reflected from a rigid obstacle is called an
echo
.
like , sound is also reflected from the plane of curved surface
Thus an echo cannot be distinguished from the original sound if it reaches the
observer within the interval of one-tenth of a second .
Hence
,
to hear an echo clearly , it is necessary that it reaches the observer after
one-tenth of a second or later , than the production of the original sound .
At original temperature
,
the speed of sound in air
340 meters per second ,
an
observer must stand at least
17 meters
from the source of reflection , like a
cliff or wall .
Example :
Bats use a system called location system for navigation . if there is any
obstruction in their path , the sound waves are return back to them .
Uses of Echo
The bats detect these returning waves by their ears and avoid such obstruction
and fly freely in dark .
The same echolocation system is used in radar , but it uses electromagnetic waves .
The dolphins and whales also use this system to locate their respective groups under water .
Echo is also used in an apparatus called
Sonar
(
sound navigation ranging
) . it
is used for locating submerged objects by transmitting ultrasonic waves and
collecting reflected waves .
The time for waves to travel to the object and return , give an indication of the
depth.
This system is know as echo dept sounding or sound ranging .
Reverberation
If on other hand , the distance between the observer and the reflecting surface is
less than

17meters

an echo cannot be distinguished as a separate sound and gets
mixed up with the original sound , in such cases it gives an impression , as if the original
sound is being prolonging sound reflection is called

reverberation
.
If there are more than one reflecting surface , reverberations can be caused even when the observer stands beyond

17 meters

from the reflecting surfaces , in such cases reverberations is caused by the series of echoes produced by the reflecting
surface and the consequent mixing up of echoes .

Example
Theatres , concert halls and public places .
They have (echo less ) by providing a special material on the inner walls , made of
light substances like sponge and
Free Vibrations
: When a body which can oscilate (
e.g., a simple pendulum
) is displaced from its position of rest and released it begins to vibrate with a frequency of its own , such vibrations of the body are called free or natural vibrations
Forced Vibrations
: When an external force is applied , the body continues to vibrate , not with its natural frequency but with the frequency of the external force , this is known as forced vibrations .
Resonance
:
When the frequency of the external force coincides with the natural frequency of a system , then the system is said to be in resonance
.
Example
:
the resonance is child's swing
.
Doppler Effect
Change in frequency of a wave due to motion of the source or observer .
The frequency of approaching source higher than the actual and when source recedes from him it is lower than the actual .
Whistle of approaching train appears shrilly due to Doppler effect .
Mach Number
:
It is generally used to denote the speed of an air-craft , when an aircraft files at a speed less than that of sound in air , then it is said to fly at subsonic speed and if its speed is more than that of sound in air , then it is said to fly at supersonic speed , mach number 1 for aircraft indicates the speed of sound in air , the speed of an aircraft is said to be super sonic , if its Mach number is greater than unity
.
_______________________
Speed of any object in air
Speed of sound in air
Mach Number =
LIGHT
Speed of light 2.99 x 10 m/s
8
Rays and Beams of Light
:
A collection of rays is said to constitute a beam of lit , a beam of light may be converging , diverging or parallel
.
Rectilinear Propagation of Light
: Light travels in straight lines , this is known as the rectilinear propagation of light .
Reflection of Light
:
When light strikes the surface of a material object , some part of the light is sent back to the medium from which is coming , this is known as reflection of light
.
Image formation in inclined Mirrors
:
When an object is placed between the reflecting surfaces of two mirrors inclined to each other at an angle , several images are formed .
Number of images = - 1
__________________________
360
0
Angle between mirrors
Spherical Mirrors
:
There are two types of spherical mirrors viz , concave and convex , a concave mirror is curved inward i.e., the focussing surface is bent inward , a convex mirror is curved outward
.
LENSES
:
A lens is a device which causes a beam of rays to converge or diverge on passing through it
.
Uses of Lenses
:
A convex lens is used to connect long sightedness (

hyper metropia
) , a person suffering from the defect of long sight can see clearly objects placed at a distance , but cannot see objects placed nearer to his eye
.
A concave lens is used to correct near sightedness (
Myopia
) , in such cases an individual is not able to see clearly objects placed at a distance and the vision is blurred .
Refraction of Light
:
when a ray of light travels obliquely from one medium to another , it is bent at the surface separating the two media , this bending of light is known as refraction of light , the refraction occurs because light travels at sightly different velocities in different media , the ray before refraction is termed incident ray and on being refracted it becomes refracted ray
.
To what extent a medium refracts the light rays entering it obliquely , is given by its refractive index and is equal to .
_______________________
Velocity of light in vacuum
velocity of light in the medium
The refractive index of diamond is the highest .
Effects of Refraction
a .

Real and Apparent Depth
:
when we look into a clear pond or lake , the bottom surface of the pond appears to be elevated and looks shallower than it actually is
.
b .
Twinkling of Stars
:
it is the atmosphere of the earth that causes the twinkling of stars , the atmosphere is not of uniform density throughout , it is denser near the surface of the earth and rarer at high altitude , because of this fact , the atmosphere prides a number of air layers of varying densities , but because of air currents and winds , these atmospheric layers are not stationary and their densities keep changing almost constantly
.
c .
Lengthening of the day
: when sunlight enters the atmosphere it passes through various layers of air of increasing densities , because of this it is bent towards the normal and thus takes a curved path .
d .
Elliptical Shape of the Sun
: At drawn and dusk , when the sun is near the horizon it appears as if its horizontal diameter is larger than its vertical diameter and gives an impression that it is an elliptical body .
e .
Occurrence of Mirage

: Mirage's are frequently witnessed on hot summer days in deserts .
f .
Total Internal Reflection
: Light always passes from a rarer medium to a denser medium , whatever be the angle of incidence , but it cannot always pass from a denser medium to a rarer medium .
Refraction Through a Prism :
When a light ray enters a prism deviates the incident ray , in a prism a light ray is refracted twice -once while entering and once while leaving the prism .
Primary and Secondary Colours
:

Blue
,
green
and
red
are called primary colours
, they are called so , because when they are mixed in equal proportion they form white light and neither of them can be obtained by mixing any other two colours
.
Pigment Colours
:
A pigment is a colouring matter Dyes and paints are pigments , Pigment colours are different from spectral colours , whereas spectral colours are pure , pigment colours are impure
.
thus ,
Red + Green = Yellow
Red + Blue = Magenta ( purples red )
Green + Blue = Cyan ( Greenish blue colour .
Also
Green + Magenta = White
Red + Cyan = White
Blue + Yellow = white .
T.V Remote Control
:
When a key on the remote control is pressed , it translates it into infra-red signals which are received by the electronic circuit in the TV , and the desired operation , such as switching on/off , varying volume , etc , are performed
.
Scattering of light
: when a beam of light traverses a material medium , scattering of the beam takes place , in the atmosphere the air molecules present , act as tiny mirrors and scatter the sunlight in all directions .
DEFECT OF VISION
Type of defect can see cannot see reason ratification
1 .
Long - sightedness
of Hypermetropia
far objects Near Objects
Eyes ball becomes smaller or focal
length f the eye lens is large
Convex Lens
2 .
Short - sightedness
or Myopia
Near Objects far objects
Elongation of eye ball or focal
length of the eye lens is small
Concave
lens
3 .
Astigmatism ( images
are formed at varying)
_ -
Eye ball is not curved equally in
all directions
Cylindrical
lens
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