Demonstration

Good questions will encourage close observations of circular pulses in a ripple tank.

Apparatus and Materials

• Ripple tank and accessories

• Pencil
• Water Dropper

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

Procedure

1. Start a single ripple somewhere in the middle of the tank and then making several such ripples one after the other. Show this can be done using:
• Finger
• Pencil to touch the water
• Drop of water from an eye dropper
2. Ask:
• What is the shape of the pulse as it travels out?
• Is the speed of travel the same in all directions? How can you tell?
• Is the speed the same near to the centre of the tank and at the edges?
• Is the water moving along with the wave pattern? Drop a scrap of paper onto the water to see if it travels.
3. Let students suggest their own tests. Any materials that students suggest for a test can be fetched quickly; and if they suggest none, the problem had best be left unsettled.

Teaching Notes

• Possible learning outcomes from this experiment:
• Students become more familiar with the ripple tank.
• They find that the water does not move out with the ripple.
• They learn what is meant by a circular ripple.
• They deduce that it is circular because – if the water is of constant depth – the disturbance travels with the same speed in all directions.
• They might deduce what happens to the ripple speed if the water depth increases.
• Pulses will be circular if the tank is level. A circular wave train demonstrates that waves travel the same distance in the same time.

This experiment was safety-tested in February 2006

Demonstration

Observing what happens when straight wave pulses are produced in a ripple tank.

Apparatus and Materials

For each group of students

• Ripple tank accessories

• Wooden rod

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

Procedure

1. Lay the rod in the tank and produce a pulse by giving it a tiny but sharp roll forwards and back (see diagram}. You may find that it is sufficient just to tap the rod. You can produce continuous waves by repeating this motion periodically.
2. Ask: Do the ripples change as they move away from the rod? If so, how?

Teaching Notes

• Students should observe that the ripples are rather wide near the rod but become sharper as they move away.
• The ripples are sharpest when the filament of the lamp is parallel to them. (This is why special 48 W lamps are required.)

This experiment was safety-tested in February 2006

Demonstration

This ripple tank experiment provides a good introduction to wave reflections. Students will see a pattern in the reflections more clearly with a simple straight pulse than they might with continuous straight ripples.

Apparatus and Materials

For each group of students:

• Ripple tank and accessories

• Barrier, straight
• Wooden rod

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

Procedure

1. Ask: 'Can you see any simple story about the direction of straight waves before and after meeting a flat wall? How are the angles related?'
2. Do not, at this stage, talk about laws of reflection, measurements of angles, or urge students to remember what they saw before.
3. Make a straight-line pulse and watch what happens when it hits a straight reflecting barrier. It is easier to see what happens when there is a single pulse rather than a series of waves.
4. Try directing a pulse head-on (normally) to the barrier and then at various other angles, larger or smaller than 45°. Avoid just 45° because this produces a grid pattern and it is hard to tell the difference between incident and reflected rays.

Teaching Notes

• Most students will bring out some story about angles. It does not matter at all whether the angles are angles between wavefront and barrier or wavefront and the normal. Dragging in references to the normal in these simple studies of reflection is no help at all. Even with a curved reflector, young people can imagine a tangent to the surface just as easily as they imagine a normal. All you want here is some idea of ' equal angles'.
• At this point students could be introduced to ‘rays’ as guide lines indicating the direction in which the wave is travelling. Place a metre rule at right angles to the wave fronts to help students to ‘see’ where the rays are. This would link ripple tank experiments to ray optics.
• The template with two sets of parallel lines can be used with an OHT to simulate reflection and interference of plane waves, at a straight barrier.

This experiment was safety-tested in February 2006

Demonstration

You can use a ripple tank to introduce ideas about the relationship between an ‘object’ and its mirror ‘image’.

Apparatus and Materials

For each student group

• Ripple tank and accessories

• Straight barrier
• Water dropper
• Pencil

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

You can use either a water dropper or a pencil (dipped in the surface of the water).

Procedure

1. Ask: ‘When a circular ripple (pulse} is bounced back by a straight wall, where does the wave seem to come from after that?' Do not give the answer to that, and do not, at this stage, discuss images. Instead, ask students to carefully observe this process in their ripple tanks.
2. Tell them to place the barrier somewhere near the middle of the tank (so that the image from which the reflected ripple seems to come is well inside the tank).
3. When students have seen this for themselves, suggest some further experiments:
• 'Now that you know where the ripple that bounces back seems to come from, try starting a ripple just there. Use the finger of one hand. Let the ripple spread and hit the wall and bounce back. Mark the place where the bounced back ripple seems to come from, with a finger of your other hand.'
• Then start a ripple from that place with that finger.
• 'Now start ripples with both those fingers at the same moment. Watch what happens'
4. When that succeeds, it is amusing and almost uncanny. Students may notice the geometry, but it will not matter if they fail to notice it. The main aim here is to emphasize the idea of a ' place from which the reflected wave seems to come'.

Teaching Notes

• Do not spoil the fun by doing it for students even though you can probably make the simultaneous ripples (the incident ripple and the image ripple) much more easily. Do, however, help students who are unsuccessful by encouraging them to put the second finger at the right place - without using the word image or giving the geometry. Just judge the right distance with your own eyes and point to the right place. Then the student can try again, see success and enjoy it.
• Students who find it difficult to remember the position of the place the reflected ripple comes from (the image) could put a small coin in the tank at that spot. More able students could measure the distances of the ‘object’ and ‘image’ from the barrier after marking their positions.

This experiment was safety-tested in May 2006

Demonstration

Students may have done some ray optics with light using parabolic reflectors such as those in the back of car headlights. This ripple tank experiment helps to show how a parallel beam is produced.

Apparatus and Materials

• For each group of students

• Ripple tank and accessories

• Copper wire, heavy
• Wooden rod
• Rubber tube, heavy

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

The parabolic reflector can be made with rubber tubing. To help curve and anchor the tube, put heavy copper wire (or solder) into the tube before bending.

Procedure

1. First ask, 'What happens to straight line waves when they hit a parabolic reflecting wall?’
2. Whether or not students have done so before, get them to try this.
3. Then ask, ‘Can you turn that story backwards and make straight line waves come out from the wall?'
4. This is an exercise in thinking as a scientist, so we should be very careful not to reduce it to an exercise in carrying out instructions. Simply ask the question about the reverse effect.

Teaching Notes

• Students may have done some ray optics with light, using parabolic reflectors such as those in the back of car headlights. This experiment will help to show how a parallel beam is produced
• They should find the wave concentrated after reflection into a circular ripple which closed down to a small size and then spread out again.
• After students have seen the straight line ripple reflected into a circular ripple that moves to a point, they should know where to put their finger to start the reverse experiment.

This experiment was safety-tested in February 2006

Demonstration

This experiment is best done after students have used a ripple tank to experiment with a pulse reflected by both straight and parabolic barriers.

Apparatus and Materials

For each group of students

• Ripple tank and accessories

• Barrier, semi-circular
• Wooden rod
• Water dropper

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

Procedure

Ask students to try any experiments they like, using a circular barrier.

Teaching Notes

• Do not tell students to start a circular pulse from the centre of the reflector (but many will do that of their own accord). Do not tell them to find the place where straight pulses are brought to a point after reflection; and certainly do not ask them to measure that distance and see whether it is half the radius of the mirror. It is much better to leave students to their own experimenting.
• A ripple started at the centre of curvature of the barrier will return to the same point. A circular ripple started half way between the centre of the circular barrier and the barrier itself will produce a parallel beam. The connection with ray optics is clear.
• For very able students only – you might want to:
• discuss the geometry and ask students to locate a 'focus',
• ask students to turn the reflector round and use it as a convex reflector, and look for virtual image effects.

This experiment was safety-checked in February 2006

Demonstration

A circular pulse started at one focus will be reflected as a straight pulse from the nearest part of the elliptical barrier. The straight pulse will then travel to the farthest part of the barrier to be reflected as a circular pulse centred on the second focus.

Apparatus and Materials

• Ripple tank and accessories

• Water dropper
• Elliptical reflector

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

The beauty of this demonstration is very sensitive to the accuracy of the ellipse. Some manufacturers supply such an elliptical reflector in the ripple tank kit as an optional extra.

To be sure of the necessary accuracy, you may prefer to make your own barrier. First draw an ellipse very carefully on paper. Someone skilful can then bend a springy brass strip to fit the ellipse, joining the ends with a butt joint and a strap outside. (This type needs very careful storage.)

Alternatively, you can draw the ellipse on plywood and then cut it out to the required shape. Great care must be taken if this method is used.

To make the best reflector of all, use a wall of plaster of Paris, drawing the ellipse with a peg moving along a loop of wire.

Coating the reflector with paraffin wax may improve the regularity of reflection by making an angle of contact with the walls exactly 90°.

Procedure

1. Place the elliptical reflector in the middle of a ripple tank with very clean water. Start a single ripple accurately at one focus and watch its progress.
2. The position of one focus of the ellipse must be located very accurately and used as the starting point of the ripples. Perhaps the best way of finding the focus is trial by ripples. Then place two small coins in the tank to mark the two focuses.

Teaching Notes

• This is an intriguing ripple tank experiment to watch. Image formation by a wide-aperture reflector like this depends on the wave path being the same from object to image by all routes, even those that use extreme portions of the aperture. An ellipse does this - though it fails to give a good image of points a little way off the focus.
• However, if part of the reflecting surface is a little off the true ellipse, the condition fails and reflection there may even harm the image instead of helping to form it. An error of 1/4 wavelength in part of the surface will do great harm. Considering how small the actual wavelength of the equivalent ripples in a pulse must be, this error is very small.

This experiment was safety-tested in February 2006

Demonstration

Some ripple tank experiments previously done with wave pulses can be repeated with continuous waves, either plane or circular. Continuous straight waves are also used in other experiment collections in the Waves topic.

Apparatus and Materials

• Ripple tank and accessories

• Motor mounted on beam, with beam support
• Rubber bands, 2
• Leads, one set, to motor
• Dipper
• Dry cells, 2
• Rheostat

Health & Safety and Technical Notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Read our standard health & safety guidance

Power for the motor:

The motor works well from a 1.5 volt cell in series with a 12-ohm rheostat. Two cells may be needed for the higher speeds but the motor then goes rather fast with the rheostat set at its minimum value. The polarity of the battery determines the direction of rotation, but that is immaterial. (If a battery with a higher e.m.f. is used, a rheostat with a correspondingly high resistance would be required.)

Some manufacturers supply special power units for use with the ripple tanks. These provide the necessary voltage for the lamps and also a variable voltage output to drive the motors. They avoid the need for a transformer to light the lamp and a separate supply for the motors and some teachers may prefer to use them despite the extra cost.

Procedure

To produce circular waves:
1. Take the wooden beam with the motor attached and hang it by two rubber bands of such length that the wood is above the water.
2. Attach a small spherical dipper to the vibrator by its L-shaped rod and adjust it so that the bottom of the sphere is about level with the surface of the water.
3. At low frequencies, it is easy to see the waves; but at higher frequencies the persistence of vision obscures them. Blinking makes them visible.
To produce straight waves:
4. Remove the dipper and re-adjust the height of the wooden beam so that the beam itself is about level with the surface of the water. When the beam is set vibrating, straight waves will travel across the ripple tank. If the beam is too deep in the water (or sitting on its glass bottom!) the ripples do not travel very far; if it is too shallow and the vibration is vigorous, the ripples are less distinct near the vibrator.

Teaching Notes

• You will find that some experiments are best done with pulses only, as the reflections from continuous waves produce confusing patterns.
• For best results, the filament of the lamp should be parallel to the ripples.
• If the wooden rod does not vibrate enough, increase the eccentric loading on the shaft of the motor.

This experiment was safety-tested in February 2006