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A LITTLE THING ABOUT WAVES 

Ripple Tank Wave Phenomena

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Print [L | t+ | —] | keywords: Huygen's Principle,transverse waves, interference, diffraction,refraction, reflection, standing waves, totalinternal reflection, ripple tank Interference

patterns of water waves generated by differentsources at adjustable frequency.

What It Shows

Various wave phenomena demonstrated withwater waves: circular waves from pointsources, plane waves from an array of point

sources, change of wavelength with frequencyand/or speed, reflection of waves, refraction of 

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waves, focusing of waves, standing waves,interference from point sources, interference

and diffraction from apertures, obstacles, and

barriers, phased arrays of sources (directionalplane waves), beating phenomenon, dopplershift, and shock waves.

How It Works

A shallow, one square meter glass-bottomedtank is filled with a couple inches of water. Adipper, suspended from an 8 Ohm speaker

driver, is partially submerged. When the driver

is driven by a function generator, the dippercauses ripples in the water. To show theseripples, bright light from a xenon arc lampshines through the tank's glass bottom,reflects off a one square meter front surface

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mirror, and illuminates a large, thin screen thathangs vertically.

The height of the driver above the water canbe adjusted to accommodate different dippers.A sphere can be used as a point source. Ahorizontal dowel can produce coherent parallelwavefronts. Several variations exist.

Obstacles can be placed to interact with therippling water; a single barrier can show edge

diffraction; multiple barriers can show sinlge ormulti-slit patterns; a triangluar peice of plexiglass acts as a prism; a convex peicebrings the wavefronts to a focus.

Setting It Up

The apparatus is large and heavy. Extra timeshould be allowed for wheeling it into a hall.The mirror and screen can block a significant

portion of the blackboard, so if one plans touse the boards the apparatus should be placedoff to the side. The angle of the mirror should

be adjusted so that the entire tank is visible onthe screen. Make sure the entire class can viewthe image on the screen.

Once the apparatus is set in position, fill thetank with about an inch of water. Placeabsorbent rags along the perimeter of the tank

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to prevent unwanted reflections. Whenpreparing for a demonstration of refraction,

use the leveling scews near each edge of the

tank to make sure it is perfectly level, and becareful to add just enough water to cover theinserted plastic. After use the water can be

drained through a hose attached to one of thecorners of the tank.

The dipper should be only partly submerged.Adjust the height of the driver as necessary.Start by driving the dipper at a small amplitude

and slowly increase it until distortion orsplashing occurs, and then back it off from thatpoint. Higher frequencies will generally cause

distortion with less amplitude, so sweeping upin frequency should be compensated byreducing the amplitude slightly.

The xenon arc lamp requires a fan to keep itcool, and should always be running when thelamp is onIn physics and engineering, a ripple tank is a shallow glass tank of water used in schools andcolleges to demonstrate the basic properties ofwaves. It is a specialized form of a wave tank. The

ripple tank is usually illuminated from above, so thatthe light shines through the water. Some small rippletanks fit onto the top of an overhead projector, i.e.they are illuminated from below. The ripples on thewater show up as shadows on the screen underneath

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the tank. All the basic properties of waves, includingreflection, refraction, interference and diffraction, canbe demonstrated.

Ripples may be generated by a piece of wood that issuspended above the tank on elastic bands so that itis just touching the surface. Screwed to wood is amotor that has an off centre weight attached to theaxle. As the axle rotates the motor wobbles, shakingthe wood and generating ripples.

Close-up of the oscillating paddle

Demonstrating wave properties [edit] 

A number of wave properties can be demonstratedwith a ripple tank. These include plane waves, reflection, refraction, interference and diffraction. 

Circular waves [edit] 

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Ripple tank with a spherical source producing circularwaves

When the rippler is attached with a point spherical balland lowered so that it just touches the surface of thewater, circular waves will be produced.

Plane waves [edit] 

Image of plane waves.When the rippler is lowered so that it just touches thesurface of the water, plane waves will seen to beproduced. (In the illustration, the brown rectangle is

the rippler).Reflection [edit] 

Demonstrating reflection and focusing of mirrors[edit] 

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 By placing a metal bar in the tank and tapping thewooden bar a pulse of three of four ripples can besent towards the metal bar. The ripples reflect fromthe bar. If the bar is placed at an angle to thewavefront the reflected waves can be seen to obeythe law of reflection. The angle of incidence and angle

of reflection will be the same.

If a concave parabolic obstacle is used, a plane wave pulse will converge on a point after reflection. Thispoint is the focal point of the mirror. Circular waves

can be produced by dropping a single drop of waterinto the ripple tank. If this is done at the focal point ofthe "mirror" plane waves will be reflected back.

Refraction [edit] 

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If a sheet of glass is placed in the tank, the depth ofwater in the tank will be shallower over the glass thanelsewhere. The speed of a wave in water depends onthe depth, so the ripples slow down as they pass overthe glass. This causes the wavelength to decrease. Ifthe junction between the deep and shallow water is atan angle to the wavefront, the waves will refract. In

the diagram above, the waves can be seen to bendtowards the normal. The normal is shown as a dottedline. The dashed line is the direction that the waveswould travel if they had not met the angled piece ofglass.

In practice, showing refraction with a ripple tank isquite tricky to do.

• The sheet of glass needs to be quite thick, with thewater over it as shallow as possible. Thismaximizes the depth difference and so causes agreater velocity difference and therefore greaterangle.

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• If the water is too shallow, viscous drag effectscause the ripples to disappear very quickly.

• The glass should have smooth edges to minimise

reflections at the edge.Diffraction [edit] 

If a small obstacle is placed in the path of the ripples,and a slow frequency is used, there is no shadowarea as the ripples refract around it, as shown belowon the left. A faster frequency may result in a shadow,as shown below on the right. If a large obstacle is

placed in the tank, a shadow area will probably beobserved.

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Numerical approximation of diffraction pattern from aslit of width equal to five times the wavelength of anincident plane wave in 3D spectrum visualization

Numerical approximation of diffraction pattern from aslit of width equal to wavelength of an incident planewave in 3D spectrum visualization

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 See also: Huygen's principle 

Diffraction from a grid [edit] 

A phenomenon identical to the x-ray diffraction of x-rays from an atomic crystal lattice can also be seen,thus demonstrating the principles of crystallography. Ifone lowers a grid of obstacles into the water, with the

spacing between the obstacles roughly correspondingto the wavelength of the water waves, one will seediffraction from the grid. At certain angles between thegrid and the oncoming waves, the waves will appearto reflect off the grid; at other angles, the waves willpass through. Similarly, if the frequency (wavelength)of the waves is altered, the waves will also alternatelypass through or be reflected, depending on theprecise relationship between spacing, orientation andwavelength.

Interference [edit] 

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Ripple tank with source1 and source 2 interference inwhite and black visualization

Interference can be produced by the use of twodippers that are attached to the main ripple bar. In thediagrams below on the left the light areas representcrests of waves, the black areas represent troughs.Notice the grey areas: they are areas of destructiveinterference where the waves from the two sourcescancel one another out. To the right is a photograph

of two-point interference generated in a circular rippletank.

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See also [edit] 

• Wave tank • Capillary wave 

• Shallow water equations 

External links [edit] 

• Open Source Physics Ripple Tank Model 

• Virtual ripple tank ( java applet)