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6.5 Diffraction Of Waves

Diffraction of waves

The spreading or bending of waves as they pass through an aperture or round the edge of a barrier.

The amount the wave bends depends on the size of the aperture or barrier. The smaller the aperture by comparison , the more wave bends.

When the diffraction of a wave happened , the wavelength ,λ,the frequency, f and the speed , v do not change but the amplitude of the waves decrease.The direction of propagation and the pattern of the wave change.

 

Diffraction of water waves

To investigate the diffraction  of water waves an obstacle is placed at the centre of a ripple tank..

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

(a)     Size of aperture >  λ

      clip_image002

 

(b)    Size of aperture ≤  λ

        clip_image004

(c)   Edge of a obstacle

        clip_image006

(d)     Small obstacle 

       clip_image008

The experiment to investigate the relationship between the size of aperture and the angle of bent

 

Hypothesis:

The  angle of bent increases  as the size of aperture decreases

Aim of the experiment :

To investigate the relationship between the  angle of bent  and  the size of aperture

Variables in the experiment:

Manipulated variable: size of aperture

Responding variable: angle of bent

Fixed variable: frequency of vibrator

List of apparatus and materials:

Ripple tank,  lamp, motor ,wooden bar , power supply white paper , two pieces metal bar ,metre rule protractor and mechanical stroboscope.

Arrangement of the apparatus:

clip_image010

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

By using a metre rule , the width of  the slit is measured = a

The power supply is switched on to produce plane waves which propagate towards the aperture.

The waves are freeze by a mechanical stroboscope.

The waves are sketched on the screen.

By using a protractor , the angle of bent = θ

The experiment is repeated 5 times for with different widths of slit.

Tabulate the data:

 a

 

 

 

 

 

 

 θ

 

 

 

 

 

 

Analysis the data:

 Plot the graph  θ against  a

clip_image012

Diffraction of light waves

 

The diffraction of light waves occur when the light waves pass through a small slit or small pin hole.

Diffraction occurs at all edges where waves can spread round into the shadow region. Thus a narrow object like a fine wire or a human hair can show the diffraction fringes at its edges similar to those produced by a small single slit.  

The diagram shows the diffraction fringes.

 clip_image014

The wider middle bright fringe shows that the light

waves diffracted after pass through a small slit.

The experiment to investigate the relationship between the size of slit  and the wide of the  middle bright  fringe

 

Hypothesis:

The wide of the middle bright  fringe increases as the size of slit  decreases

Aim of the experiment :

To investigate the relationship the wide of the middle  bright  fringe increases and the size of slit  decreases

 

Variables in the experiment:

Manipulated variable: size of slit 

Responding variable: the wide of the middle  bright  fringe

Fixed variable: the monochromatic light (one wavelength only)

List of apparatus and materials:

Monochromatic light source, single slit plate, metre rule

Arrangement of the apparatus:

clip_image016

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

The width of  the slit is recorded  = a

The light beam from the source is directed towards the slit..

By using a metre rule , measure the wide of the middle bright fringe = x

The experiment is repeated 5 times for with different widths of slit.

Tabulate the data:

 a

 

 

 

 

 

 

 x

 

 

 

 

 

 

Analysis the data:

 Plot the graph  x against  a

clip_image018

Diffraction of sound waves

Sound diffracts very readily. This is why we can hear sound round a corner , or behind an obstacle. The reason is the sound waves have long wavelengths in air, ranging from a few centimetres up to several metres. As we have already seen, long wavelength waves diffract more readily than those with short wavelengths.

To investigate the diffraction of sound waves the following experiment  can be done:

clip_image020

A  listener is requested to stand on the other side of the corner of the wall so that the radio is beyond his vision.

The listener is able to hear the sound of the radio although it is behind the wall.

It is because the sound of the radio spreads around the corner of the wall due to diffraction of sound.

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