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6.4 Refraction Of Wave

Refraction of waves

Refraction of a wave occurs when the wave moves from one medium to another  which causes to travel at different speed.

The refraction occur when a water wave moves from deeper water to shallower water, a light wave moves from air to water , and a sound moves from air to carbon dioxide gas.

 

When the refraction of a wave happened , the frequency, f does not change but the wavelength ,λ, the speed , v and the direction of propagation of the wave change.

Refraction of water waves

The refraction of water waves occur when the water waves move into different depth of water.

Medium

v

l

f

Shallow water

decrease

decrease

constant

Deep water

increase

increase

constant

If the waves meet the boundary between deep and shallow water at an angle , then the direction of the waves changes.

The plane wavefronts will change  to the curved wavefronts if the waves move into the  curved boundary.  

To investigate the refraction of water waves a  Perspex plat  is placed on the bottom of a ripple tank to make  a shallow region of the water on the Perspex  plat.

The  following  figure shows two examples pattern of the refraction of the water waves.

(a)               

    clip_image002

Conclusion:

l become smaller at shallow region

V become smaller at shallow region

f does not change at any condition.

(b)

    clip_image004

(c)

     clip_image006

(d)

 

clip_image008

(e)

  clip_image010                           

(f)

clip_image012                                 

The experiment to investigate the relationship between the depth of water  and the wavelength of a water wave.

Hypothesis:

The depth of water increases as the wavelength of water waves increases.

Aim of the experiment :

To investigate the relationship between the depth of water and the wavelength of water waves.

Variables in the experiment:

Manipulated variable: depth of water

Responding variable: wavelength

Fixed variable: frequency

List of apparatus and materials:

Ripple tank,  lamp, motor ,wooden bar , power supply white paper , protractor ,plane reflector , Perspex plate , metre rule and mechanical stroboscope.

Arrangement of the apparatus:

clip_image014

The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

The depth of water on a perspex  is measured by using a metre rule = d

The power supply is switched on to produce plane waves which propagate onto  the perspex plate .

The  waves are freeze by a mechanical stroboscope.

The  waves are sketched on the screen.

The distance between  11 successive bright bands is measured by the metre rule = x

The wavelength is calculated i.e. clip_image016       

The experiment is repeated 5 by increasing the number of perspex plate to change the depth  the water.

Tabulate the data:

 d

 

 

 

 

 

 

 λ

 

 

 

 

 

 

Analysis the data:

 Plot the graph  λ against  d

 

clip_image017

Refraction of water wave of the sea water

            clip_image019

 

When the water waves propagated  from the sea to the beach  , the water waves  follow the shape of the beach.

It is because the water waves propagate from the deep water to the shallow water. The speed and  wavelength decrease as the waves approach the beach and hence  the water wave  is refracted towards  the normal.

The water in the bay stationary compared to the water at the cape.

It is because the depth of water varies slowly across the area  of the bay and the energy of the water wave spreads to a wider area compared to the region near the cape. The amplitude of the water wave near the bay is low and hence the water at the bay is comparatively still.

 

Refraction of light waves

 

Refraction of light  waves is a phenomenon where the direction and speed of light  are  changed ( change in velocity) when it crosses the boundary between two materials of different optical densities

When a light  waves travels from optically less dense medium to optically denser medium , the ray is bent towards  the normal.

After entering the  denser medium the speed of light  decreases.

    clip_image021

 

When a light  wave travels from optically denser medium to optically less dense medium , the ray is bent  away from the normal.

After entering the less dense medium the speed of light  increases.      

       clip_image023

Experiment to investigate the relationship between the angle of incidence and the angle of refraction.

 

Hypothesis:

The angle of refraction increases as the angle of incidence increases.

Aim of the experiment :

To investigate the relationship between the angle of incidence and the angle of refraction.

Variables in the experiment:

Manipulated variable: Angle of incidence

Responding variable: Angle of refraction

Fixed variable: Refractive index

List of apparatus and materials:

Glass block, ray box, white paper , protractor, power supply .

Arrangement of the apparatus:

     clip_image025

 

 

The procedure of the experiment which include the method of controlling the manipulated variable and the method of measuring the responding variable.

The glass block is placed on a white paper.

The outline of the sides of the glass block are traced  on the white paper and labeled as ABCD.

The glass block is removed.

The normal  ON is  drawn.

By using a protractor , the angle of incidence is measured = i

The glass block is replaced again on its outline on the paper.

A ray of light  from the ray box is directed along incidence line.

The ray emerging from the side CD is drawn as line PQ.

The glass block is removed again.

The point  O and P is joined and is drawn as line OP.

The experiment is repeated 5 times for the other angles of incidence.

Tabulate the data:

i

 

 

 

 

 

 

 r

 

 

 

 

 

 

Analysis the data:

 Plot the graph r against  i

clip_image027

Refraction of sound waves

Refraction of sound waves occurs when the sound passes through layers of air at different densities or temperatures.

Sound waves can be refracted by a balloon filled  with carbon dioxide.

clip_image029

 

When the stopwatch is started , the listener  can detects at a point only beyond the balloon where the sound wave is strongest.

It is because a balloon of carbon dioxide( denser than air)  acts like a convex lens ,focusing the sound wave.

Effects of refraction of a sound wave at night

               clip_image031

Distant sounds often appear louder and clearer at night.

At night  the air near the ground is often cooler than the air higher up, because it is close to the cold ground. The sound waves travel faster through the warmer , less dense layers of air , and this refracts the sound wave back toward the ground.

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