Ripple Waves

8/21/2019 Ripple Waves

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Ripple Tanks

The Ripple Tanks is a useful piece of apparatus for demonstrating wave properties.

It consists of shallow rectangular water lled tray with a glass base and an

overhead light source. When an object is placed in the water the light passingthrough the water from the overhead light is disturbed. This disturbance can be

viewed on a card placed on the oor. To see this eect more clearly! e"periments

with a ripple tank should be conducted in a dark room.

Waves can be made in the tank by oscillating an object on the surface of the

water. The waves travel away from the object in all directions.

#traight objects make waves with plane waveforms. Round objects make waves with

curved wavefronts. The inner sides of the tray have absorbing barriers to minimise

reections.


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Waves can be generated in a more controllable way by using small beam of wood

suspended from rubber bands so as to just break the water surface. $ small motor

with an eccentric cam is mounted on the beam which causes it to vibrate when the

motor is operating.

Waves travel away from the beam across the surface of the water in the tank. These

are known as plane waves as the front of each wave forms a straight line.

It is also possible to create circular waves usingthe same method. The only dierence is that

circular object! and not the beam of wood! is in

contact with the water%s surface. This is shown in

the photograph. &otice that the circular dipper is

only just in contact with the water. 'ou will also

see that we have used a crocodile clip as our

eccentric cam.

(y adjusting the speed of the motor! waves of diering wavelength may be

generated.

If the motor turns slowly! waves of longer wavelength are produced.

If the motor turns )uickly! waves of shorter wavelength are produced.


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If we add a stroboscopic light source to the apparatus! measurement of the wave

can be obtained.

If the light is ashing at the same fre)uency as the wave! the shadow will appear to

be stationary* This is because the wave travels e"actly one complete wavelengthbetween the ashes.

Therefore! If the fre)uency of the strobe is known! so is the fre)uency! f! of the

wave. We can the measure the wavelength using a ruler. These values are used in

the +rogressive Waves section to verify the relationship between. V, f,

The ripple tank can be used to demonstrate the main properties which all waves

possess. These are reection! refraction and diraction.

Refection

$ plane wave strikes an angled barrier. The reected wave

can be seen moving to the left


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Reraction

$ plane wave travels over an angled obstacle. The wave

gets refracted twice! once away from the normal when

meeting the obstacle and then once towards the normal

when leaving the obstacle.

Diraction

$ plane wave strikes a barrier with a small gap in the middle.

$ small part of the wave is allowed to pass through the hole

and diracts into a circular wave. This wave is very weak as

it only has a small part of the original wave%s energy.

(y using the ri,e tank the wave property of a reection can be demonstrated. $

straight barrier is placed in the path of some plane waves. The waves are reectedat the barrier and move in a new direction. They remain plane waves after

reection.


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$ll types of waves may also be reected. #ound waves can reect from walls or

buildings. In this way sound can reach places indirectly.

-ight waves also reect o many surfaces. In all cases the waves move o in a new

direction after striking the reecting surface.

The Terminology o Refected Waves

Waves striking a reective surface are called incident waves. Waves reected from a

surface are called reected waves. $ line perpendicular to the surface at the point

of reection is called a normal. Incident waves may meet the surface any angle. The

angle between the normal and the direction of motion of the wave at the boundary

is called the angle if incidence i). The angle between the normal and the direction

of motion of the reected wave is called the angle of reection r/.


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(y making the reecting surface curved! It%s possible to focus the incident wave.

The eect is demonstrated by using the curved reective barrier shown here in the

photograph.

The barrier is causing the wave%s energy to be concentrated at a point the

focus/.The same principle is used to focus electromagnetic waves transmitted by

television satellite.

Reraction

Refraction is the change in the direction of a wave when its velocity changes at a

boundary! this gives rise to a corresponding change in the wavelength of the wave!but the fre)uency remains unchanged.

In the ripple tank this may be shown by changing the depth if the water in one

region of the tank! and allowing waves to pass into this region at an angle.


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The waves change direction when passing into the shallow region dierent

medium/. In this case the waves travel more slowly in the shallow region! and this

causes them to be refracted at the boundary between the two.

If the waves pass into a curve0shaped region of shallower water! then they can be

shown to converge.

This is achieved by putting a suitably shaped piece if glass into the tank.

The same eect occurs when light waves pass through a glass lens.

-ight waves slow down when they enter glass! just as water waves slow down when

they enter a shallow region. Refraction occurs in light in a similar manner.

If a beam ray/ of light passes through a rectangular glass prism with its sides

perpendicular to the beam! the light appears to pass straight through! unaected

by the glass.

If the glass block is rotated so that the light strikes the side of the block at an angle!

the beam is refracted as it enters the block and also as it leaves it.


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The terminology o Reracted Waves

Waves moving from one medium to another and meeting a boundary are called

incident waves.

Waves moving from one medium to

another and leaving a boundary are

called refracted waves.

$ line perpendicular to the boundary

between the two media is called anormal. Incident waves may meet the

boundary at an angle. The angle

between the normal and the direction of

motion of the wave at the boundary is

called the angle of incidence. The angle

between the normal and the direction of the motion of the refracted wave is called

the angle if refraction.

The light is refracted at both surfaces. There are therefore two angles if incidence

and refraction as the light passes through the block! one at the rst surface as it

enters the block and the other at the second surface as it leaves it. (oth are

measured from their respective normal.


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In this case the light is refracted

towards the normal on entering

the block! and away from the

normal on leaving it.

1n entering the glass! the angle of incidence is greater than the angle

of refraction.

1n leaving the glass the angle of

incidence is less than the angle of

refraction.

Diraction

2iraction is the property e"hibited by waves when they encounter an edge or

passing through an aperture! such as a hole or slit.

When diraction occurs! the direction of propagation and the shape of the wave

may change.

If plane water waves meet an aperture in a barrier! diraction eects are observed.

If the aperture is large compared to the wavelength of the waves! a small eectmay be seen at the edge if the wave after passing through the aperture. The edges

of the wave become very slightly curved. This is known as edge diraction.

If the wavelength of the waves is about the same si3e as the aperture! much more

noticeable diraction occurs. The waves emerge from the aperture with a curved

wavefront. They spread out in all direction with an apparent centre at the aperture.

This is often referred to as single slit diraction.


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-ight may be diracted in a similar way to water waves.

If a narrow parallel beam of light falls

on a wide aperture! the edgediraction eect in negligible. The

light continuous in its straight lines

after the aperture and can be made

to fall on a screen! with no apparent

edge diraction being visible to the

naked eye.

If light passes through very narrow

slits! diraction eects can be

demonstrated. $s the wavelength of

light is very small! around 455nanometres! the diraction eects

are not noticed in everyday

observations of the behaviour of

light. They can! however! be demonstrated in the laboratory using laser and very

ne optical slits or diraction gratings.

Intererence

When two spherical are attached to the oscillating beam in a ripple tank! both willproduce circular progressive travelling/ waves on the water surface.

$s they are created by the same vibrating source! these two sets of waves are

coherent 6 similar in amplitude! wavelength and phase.

$s they progress through the tank they overlap! When this occurs the two sets of

waves interfere with each other.


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When two wavefronts meet! at some point in the tank they are e"actly in phase with

each other. These are known as antinodes. $t this point interference occurs and the

amplitude of the wave is doubled.

$t the other point in the tank called nodes the two wavefronts meet e"actly out of

phase with each other and they cancel each out by destructive interference.

The diagram shows two circular progressive waves which are coherent.

Where two waves of the same type meet theycombine to give twice the amplitude. This point is

called an antinode. The red dots on the diagram

indicated antinode.

$t the point where a trough meets a crest!

cancellation occurs. This point is called a node.

The blue dots indicate nodes.


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The arrows show that the channels of nodes and antinodes form. The faint channels

are where nodes are placed and cancellation has occurred.

Ripple Waves

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