Here we have shared the notes of chapter 8 11th class physics Waves. You can download 1st Year Physics Chapter 8 Waves Notes pdf or read online.
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Q. What are progressive waves? Give its types.
Answer
Progressive Waves
The waves which transfer energy by moving away from the source of disturbance are called progressive or travelling waves
Example
Consider two persons holding the opposite ends of the rope Suddenly one person gives a jerk to the rope The disturbance in the rope produces a pulse which moves toward other person When this reaches the other person it pushes his hand upward So the energy and momentum transferred from one person towards the other person This is an example of progressive wave
Kinds of waves
There are two kinds of progressive waves
i) Transverse waves ii) Longitudinal waves
Transverse Waves
The waves in which particles of the medium are displaced perpendicular to the direction of propagation of wave and such waves are called Transverse waves.
Example
Now take a loose spring coil (slinky spring for illustration of motion of source in generating waves in a medium Slinky is the soft spring which has small initial length but relatively large extended figure Consider a horizontal spring system with its one end fixed. When the free end is moved from side to side, a pulse of wave having a displacement pattern as shown in figure, which will move along the spring This shows that displacement of particles is perpendicular to the direction of propagation of wave, hence transverse waves are produced
Longitudinal Waves (Compressional waves)
The waves, in which particles of the medium are displaced along the direction of propagation of.
wave and such waves are called longitudinal waves
Note (Why, sound waves in air are longitudinal in nature)
Both types of waves can be set up in solid In fluids, however, transverse waves die out very quickly and usually cannot be produced at all That is why sound waves in air are longitudinal in nature
Q. What are periodic waves? Also discuss its different types?
Answer
Period Waves
The waves which are produced by continuous and rhythmic disturbances in a medium are called periodic waves.These may be transverse or longitudinal in nature A good example of a periodic waves in an oscillating mass-spring system which executes SHM
Transverse Periodic Waves
The periodic waves in which the displacement of particles of medium is perpendicular to the direction of motion of waves produced by continuous and rhythmic disturbances in a medium are called transverse periodic waves
Experiment
Let us consider a mass-spring system which can vibrate horizontally as shown in figure A long sting of uniform thickness is stretched horizontally and its one end is attached with the oscillating mass m. Due to oscillation of mass-spring system a transverse waves is produced in the string
The wave appears to be travelling on the spring from its own end to the other In this case each part of string vibrates on the spring, from its own end to the other. In this case each part of string vibrates at right angle to the length of stretched string The crest and troughs are being replaced by one another periodically and waves appear to be travelling
Crest
The portion of the wave above the mean level is called as crest
Trough
The portion of the wave below is mean level is called trough
Amplitude
The amplitude is the maximum displacement of point in a crest or a trough of the string
Wave Length
The distance between two consecutive crests or two consecutive troughs is known as wave length.
Time Period
The time for which the wave travels a distance of wave length is called time period. The time period of wave is equal to be the time period of the oscillator which produces it
Speed of Wave
When a wave progresses each particle in the medium performs SHM The time that the crest required to move a distance of one wave length is equal to the time required for a point in the medium to go through one complete oscillation. If v be the speed of wave, then Or Where is the frequency of the wave which is same as the frequency of oscillator (crest or trough) which produces it
Wave Profile
Relation between path difference and phase difference
Consider the snapshot of the periodic waves moving through the medium As any distance from the reference point then phase difference can be described as Longitudinal periodic waves
The periodic waves in which the displacement of particles of medium is along the direction of motion of waves produced by continuous and rhythmic disturbances in a medium are called transverse periodic waves
Experiment
To explain longitudinal periodic waves, we take an example of a spring which is suspended by the help of threads Longitudinal wave is produced in this spring by applying horizontal varying force at one end of the spring This force produces compression and rarefaction in the spring as shown in the figure
In this case the various parts of the spring vibrate along the length of the spring (or along the direction of motion of the waves When spring was disturbed than all the suspension threads were vertical But when the longitudinal wave is produced in the thread then these suspension threads are displaced Their displacement is same as the displacement of corresponding parts of the spring The graph of displacement of various parts of spring and corresponding values of the distances of these various parts of spring from its one end is
Q. Explain the reflection of waves from rare and denser mediums?
Answer
Reflection of Waves
The bouncing back of wave from the boundary of a medium is called reflection of wave When a wave produced in one medium travel to the boundary to enter into another medium, then a part of incident wave is reflected from the boundary This reflected part has same frequency and wavelength as the incident wave has But there maybe the change of phase which depends upon the nature of boundary of medium Reflection of waves from the boundary of denser medium
Let us take long slinky spring whose one end is fixed to a rigid support on a horizontal surface of a table The other end of this spring is free to oscillate A sharp jerk is given to which crest or trough is produced on it It travels on the spring from it send A towards the end B On reaching at end B spring exerts a force on the rigid support to produce similar motion in it But the rigid body has large density so it exerts equal and opposite reaction on the spring Due to which crest is converted into through and it travels back from end B to A
Reflection of waves from the boundary of a rare medium: If we attach end B of long slinky spring with a light string and keep its end A free, like before Then giving a sudden jerk to end A, a crest is produced which travels on the spring from its end A, a crest is produced which travels one the spring from its end A to end B when the crest produced reaches the boundary of string Then string being rare medium, do not give reaction to the spring So that a crest is reflected back as a crest on the spring from end B to end A
Result
If a transverse wave travelling in a rarer medium is incident on a denser medium, it is reflected such that it undergoes a phase change of 180 (path difference of if a transverse wave travelling in a denser medium in incident on a rarer medium, it is reflected without any change in phase (no path difference)
Q. Differentiate between echo and reverberation?
Answer
Difference between echo and reverberation
1) Echo is a single reflection of a sound wave off a surface Reverberation is the sound or the pattern created by the superposition of such echoes
2) An echo can be heard only when the distance between the source of sound and the reflecting also
body is at least 17 m. A reverberation can occur when sound wave is reflected by a nearby wall also.
3) An echo is usually clear and can be clearly distinguished A reverb is not a clear replica of the original sound sample
4) Echo can be used to determine the distance of a reflecting object such as a large building or a mountain of the ambient temperature is known Reverberation cannot be utilized for distance measurement applications
5) An echo can be heard both in open and closed spaces Reverberation is usually experienced in closed spaces with multiple reflecting objects
Q. How can we change the frequency of string on a musical instrument? Also discuss resonance of air column in resonance tube?
Answer
The frequency of a string on a musical instrument can be changed either by varying the Tension in string Length of string
For example
The tension in guitar and violin strings is varied by tightening the pegs on the neck of the instrument Once the instrument is tuned, the musicians vary the frequency by moving their fingers along the neck By doing so that change the length of the vibrating portion of the string resonance of air column in resonance tube:
Vibration of air column can be set up in a resonance column apparatus It consists of a long metal tube held vertically in a tall jar containing the water The tube can be fixed in vertical position The length of the air column can be varied by raising or lowering the tube Here, the surface of water will act as the closed end When a vibrating tuning fork is held above the open end, longitudinal waves are sent down the air column.
These waves are reflected at the water surface and thus produce standing waves Nodes are produced at the water surface and antinodes are produced at the open end When the frequency of waves in the air column becomes equal to the natural frequency of tuning fork, a loud sound is produced in the air column, It is the condition for resonance it occurs only when the length of air column is proportional to one-fourth of the wavelength of sound waves having frequency equal to frequency of tuning fork
Q. Find the frequencies produced in organ pipe when it i) Open at both ends. ii) Closed at one end.
Answer
Stationary Waves in Air Column
Stationary waves can be set up in air column inside a pipe or tube A common example vibrating air column is an organ pipe
Organ Pipe
An organ pipe is a wind instrument in which sound is produced, due to setting up of stationary waves in air column. It consist of a hollow long tube with both end open or with one end open and the other closed There are two types of organ pipes
Open Organ pipe: It is that organ pipe whose both ends are open.
Closed Organ pipe: It is that organ pipe whose one end is closed Modes of vibrations in organ pipe open at both ends
Let us consider an organ pipe of length which is open at both ends As tat the open ends air molecules have complete freedom of motion so it acts as antinode Longitudinal waves set up inside the pipe have been represented by transverse curves which represent the displacement and amplitude vanations of air at vanous points
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