Ripple Tank

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The ripple tank creates wave patterns to demonstrate various wave mechanics principles.



Background

A ripple tank is a small, shallow, transparent based tray used to illustrate surface wave properties in water. Light is shone through the tank onto a screen. The waves create changes to the path of light so that the image of the surface has different intensities depending on the origin of the rays.

What can be shown?

Plane wavefronts, point sources, refraction (lenses), reflection (mirrors), diffraction and even Doppler shift.


Reasons for inclusion of topic

Technician involvement is usually to set up the equipment for demonstration purposes. New staff may require the demonstration of the procedures to illustrate various properties. It is useful to know what the equipment is capable of showing and how a teacher may deliver these examples.

Safety Notes.

Sticking a tank of water above a live electrical device is a self evident hazard. Use care when filling and clear any spillages. If possible go for a gantry option with display screen/mirror combination.

Equipment set-up and description.

There are two types of ripple tank. Those that require light be projected from a gantry over the top of the tank and those where the light is shone through the bottom of the tank, usually by an overhead projector (OHP). Some are designed for both modes of display.

If an OHP is to be used it should have reasonable optics set correctly. See end of posting.

Clean the bottom of the tank using glass cleaner and soft cloth.

The most important aspect in setting up the ripple tank is to get the base of the tank as level as possible. Use a spirit level lengthwise and then across the breadth of the tank. Repeating this process if alterations are made. Note that the spirit level may not be level! When a level is obtained turn the spirit level through 180 degrees an see if it is still level. Levelling mechanisms are usually rubber wedges placed under the edges of the tank on OHP models or screws on the three legs of the gantry type. For the OHP type it is often easier with three large blobs of “blutac” or similar in a triangular arrangement on the edges and pressed flat until a level is obtained. There are various methods used to reduce reflections from the sides. Sloping side or foam absorbers have produced the most effective solutions in my experience.

Gantry models should have the filament lying in the horizontal plane and parallel to the back edge of the tank. This provides a better contrast in the image. More refined (expensive) units have a built in stroboscope.

Various accessories are used depending on property to be demonstrated. These should be put out with the tank to accommodate the lesson plan diverging from topic. Stroboscopes if not part of the kit can be made available to “freeze” waves.

There are three methods that waves can be produced. Manually a rod can be moved to and fro to produce waves of a long period. This is the simplest method but is the most limited. The second method has an eccentric cam on a bar that is suspended from a gantry using either elastic bands or springs. Because of inertia (or “sticktion”) the motor is run fast and the speed reduced until the desired rate is obtained, stalling is frequent. The speed is controlled by varying the voltage to the motor. The third type has a pivoted arm linked to a vibration generator. As slower rates of wave production produce clearer images I would recommend trying to adapt you own version of the vibration generator system.

Water should be clean with the addition of a few drops of detergent. Keep a beaker of this solution to hand for topping up.

The most problematical demonstration is that of refraction. If multiple demonstrations are being used set out for this first. Water will show refraction as ripples pass over a submerged object and slow down. This effect is only noticeable if the height of the ripples is comparable to the clearance. The transparent object is usually a rectangular block. [Tip: If the water tends to run off this too easily rub with ordinary toilet soap whilst wet]. Pour over enough water to just cover the block and then increase the depth by about 1 to 2mm. Place one corner of the block pointing at the bar. The wave producing bar should be lowered into the water so that, at rest it is parallel to the surface and JUST below the surface of the water.

Start the bar oscillating and reduce the speed so that the waves are clearly visible.


Demonstrations

  • Refraction - Covered initially as above, the use of lens shaped objects may be inserted into the water to show how lenses work. The convex lens usually needs to be close to the bar as the tank may only just show the focal length.
  • Reflection: - Any solid object should reflect waves provided that its height extends above the surface of the water. The demonstrations on reflection are a plane, concave and convex mirrors. The plane mirror should be placed at 45degrees to the wave fronts. The reflected waves are at 90degrees to the original fronts. The concave /convex mirror is placed centrally to the fronts. The converging or diverging waves can be shown by reversing the orientation of the mirror.
  • Diffraction - A short straight edge barrier is placed parallel to the wave fronts. The waves in the “shadow” of the barrier will show an end effect as it bends around the edge.

This is then shown with a second barrier so that there is a wide gap between the two edges. Parallel lines are seen to be between the two edges but both ends bend in an elongated C shape. The two barriers are then brought closer together and the waves form a more curved C. As the separation becomes close to the wavelength of the ripples, the waves become more circular. The next variation is “Young’s slits”. A very small barrier is placed between the two barriers so that there is about a wavelength between the each of the two outside barriers and the central barrier.


  • Interference: - The “Young’s slits” demo above shows interference.
    Interference pattern generated using a school riple tank
    This should be compared to two point sources. The bar of the oscillator is removed from the water and a dipper is inserted. This gives circular waves from a “point source”. A second dipper is inserted and the waveforms interfere. If many dippers are used “Huygens wavelets” can be demonstrated.


  • Dispersion: - The convex lens can be used to show dispersion, as with any refractive demo this is difficult to get right. Using a low frequency the focus will be in one place. A higher frequency will give a focus further away. A very high frequency will be even further from the lens.


  • Doppler shift - An eye dropper can be used to drop regular drops to produce concentric rings (as a point source). If the dropper is moved from left to right the ripples appear closer together at one end and further apart at the other.


  • Speed of a water wave: - If the frequency of the oscillator is known, or measured and the length of tank is known the speed of the wave is simply found from the product of these two quantities.
  • See also:

OHP Maintenance

and if all else fails a virtual ripple tank applet is available through external Falstad link

--D.B.Ferguson 16:56, 18 March 2007 (GMT)



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