Wave Motion and Definitions

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"A" Level Physics.                        Wave Motion and Definitions                2.2

2.2.1                What is a wave ?

Waves involve a disturbance from an equilibrium position and the propogation of that disturbance from one region of space to another. Eg a brick thrown into a pond.

(1)         Pond showing water in equilibrium position.

(2)        Pond showing a disturbance from an equilibrium position.

        (ie brick thrown into pond)

(3)         Disturbance is propogated.


2.2.2                Wave Definitions

Progressive Wave 

is one  that  carries  energy and momentum from one place to another, without there being a transfer of matter  (ie the  material  that  the wave is moving through eg the water in a water wave).   

Stationnary Wave 

is the superposition of two waves of equal wavelength travelling towards each other at the same speed. Unlike progressive waves there is no net transfer of momentum or energy by the wave (see later for details about superposition).   

Mechanical Wave  

is one in which  the  wave is carried by material substance eg water for water waves, air for sound waves or vibrating string or slinky !      

 

Electromagnetic Waves 

may travel through a vacuum and require no medium eg waves all the waves in the electromagnetic spectrum. The waves are caused by vibrations of electric and magnetic fields.

2.2.3                Wave Nomenclature

  • Period               T

  • Frequency            f

  • amplitude            A

  • displacement         x

  • wavelength        λ


2.2.5                Wavespeed and the Wave Equation.

        Tells us how fast the "energy" moves from one place to another by a progressive wave or how fast the wave is moving.

                        Speed         =         Frequency         x         Wavelength 

                                v        =        f  λ

Hutchings: Q 11.1 and 11.2 p 164.

2.2.6                Transverse and Longitudinal Waves.

Waves may be produced by two distinct forms of vibration. We may thus classify any wave as being a TRANSVERSE WAVE  or a LONGITUDINAL. 

Tranverse Wave.

eg.  Water waves, some Seismic waves and ALL Electromagnetic waves.

Vibration causing disturbance is at right angles to the direction in which the energy travels

Longitudinal Waves.

eg.  Sound waves and some Seismic waves.

Vibration causing disturbance is in the direction of energy  propogation.


2.2.7                Polarisation  Occurs only for Transverse waves.

Transverse waves may frequently occur in more than one plane at any one time. Eg the wave below may be light from a bulb, note that although two directions are shown there could infact be many more. If a suitable material called a POLARISER is placed in front of the wave it will limit the wave to propogating in one plane only...it has said to have POLARISED the wave.

A polarised wave is one in which the vibrations causing the wave are confined to one plane alone.

Uses.

(a)         Use of Crossed Polaroids.

        If two polarising filers have there planes of polarisation at

        right angles to each other then then can be completely cut out.         

        This is most easily seen in a diagram.

(b)         Stress Analysis. if a perspex object  is  placed  between the

            crossed polaroids and stressed (force applied) a  variety  of

            colours  may  be  seen  through  the  polaroids  each  colour

            highlighting a different degree of stress.  This is useful in

            engineering ...small scale models are made out of perspex and

            stressed between crossed polaroids and examined  for areas of

            potentially  hazardous  high  stress  (where  a fault is most

            likely to occur).

 

(c)          Polaroid Lenses (sunglasses)  to reduce glare from water or

             snow etc.  (see Huthcings p173 for details)

(d)          TV Signals.

             Most TV and Radio signals are polarised  in one direction or another

             hence careful positioning of the aerial is needed for good reception.

             Q:  Why then can you still pick up signals in other directions ??

             Ans... a wave may be resolved into two directions at right angles

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             to each other (see work on vectors)..hence a receiving aerial will

             pick up some of the signal.

2.2.8                 The PRINCIPLE OF SUPERPOSITION

When two or more waves meet two things are observed to happen.

  • They will pass through each other and be unaffected see fig 11.3 Hutchings. (this is just as well practically)

  • At the point(s) where they meet the RESULTANT amplitude will be the VECTOR SUM of the individual waves...

eg..consider the two waves below meet and superpose.

...

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