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Progressive Wave
Light, Sound and Waves
Measuring waves in a ripple tank
for 14-16
Using the stroboscope to ‘freeze’ waves in a ripple tank, and to confirm the relationship between wave speed, frequency and wavelength.
Light, Sound and Waves
Using a stroboscope to 'freeze' continuous ripples
Practical Activity for 14-16
Demonstration
A stroboscope makes it easier to see patterns of wave behaviour with continuous ripples in a ripple tank, especially with ripples at higher frequencies.
Apparatus and Materials
- Motor mounted on beam, with beam support
- Paper, white
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.
Photo-induced epilepsy
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments.
Read our standard health & safety guidance
You will also need an appropriate power supply for the motor (special, or 1.5 V cell with 12Wrheostat)
If there are not enough hand stroboscopes to go round, with a little practice students can wave a hand, with fingers spread, in front of their eyes to freeze the wave motion.
See also:
Vibrator to generate continuous waves
Procedure
- Generate continuous circular ripples and then continuous straight-line ripples, possible with reflections at a straight barrier.
- Ask students to use the stroboscopes to ‘freeze’ the motion of the waves.
Teaching Notes
- Students should already be familiar with hand stroboscopes. They may take a little time to develop skill in
freezing
the motion. - Point out how patterns easily seen with the unaided eye at low frequencies become impossible to see as the frequency is increased. Stroboscopes make visible phenomena at higher frequencies.
- To make this exercise more interesting, you might ask students to observe:
- Whether wavelength changes when waves are reflected.
- Whether (and how) wavelength changes with frequency.
- What happens to frequency and wavelength as waves enter shallow water.
- See also:
This experiment was safety-tested in February 2006
Up next
Estimating wavelength, frequency, and velocity of ripples
Light, Sound and Waves
Estimating wavelength, frequency, and velocity of ripples
Practical Activity for 14-16
Class practical
Students derive the wave equation, by closely observing ripples in a ripple tank.
Apparatus and Materials
For each group of students
- Motor mounted on beam, with beam support
- Metre rule
- Paper, white
- Stopwatch or stopclock
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.
Photo-induced epilepsy
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments.
Read our standard health & safety guidance
You will also need an appropriate power supply for the motor (special, or 1.5 V cell with 12Wrheostat)
If there are not enough hand stroboscopes to go round, with a little practice students can wave a hand, with fingers spread, in front of their eyes to freeze the wave motion.
Procedure
-
Estimating wavelength
- Generate continuous straight waves, running the motor-driven beam as slowly as possible to produce low frequency ripples of long wavelength.
- Freeze the wave pattern using a stroboscope.
- Find the wavelength by measuring a batch of wavelengths (say ten) on the paper on the floor and finding the average. Estimating frequency
- Count the number of vibrations in a given time interval (perhaps in groups of four). One way to do this: let a thin piece of paper just touch the spindle of the vibrator, to make audible sounds. This should not slow the motor much.
- Calculate the number of waves per second.
- For higher frequencies, observe the rotation of the motor using a hand stroboscope. Estimating velocity
- One student should run a pencil along the paper, keeping it level with one wave. Another student measures the time taken to travel between the two points marked on the paper.
- Calculate speed = distance / time
- NOTE: The velocity of wave pulses is easier to measure but they do not travel at the proper wave speed, so is not appropriate.
Teaching Notes
- It should be possible to calculate whether frequency x wavelength is near to the measured value for the velocity.
- Estimating wavelength: Make clear that the idea is to get a rough estimate rather quickly, and not to try to achieve great precision with a technique which cannot really support it.
- An easy way to introduce the wave equation is the following. Walking at 50 strides a minute, with each stride 0.5 metres long, then a distance of 25 metres can be covered each minute. The number of strides per minute is the frequency of the stride and when it is multiplied by the length of the stride then the velocity of the walker has been calculated. Similarly with wave motion so that velocity = wavelength x frequency.
- The wavelengths and speeds that are measured are not those of the ripples but those of their shadows on the floor. This does not matter if measurements on the floor are used consistently; but measurements on the water can be calculated by proportion, if students prefer that. Only able students, who bring it up, need to discuss it.
- NOTE: Wave speed is a function of the wavelength for water waves. In other words, water is a ‘dispersive medium’. The wave speeds that students measure will differ, depending on the frequency of their vibrator.
- You could go on to look at other waves such as radio waves: work from the published wavelengths and frequencies to calculate the velocity of the waves.
This experiment was safety-tested in February 2006
Up next
Using ripple tanks
Students should whenever possible experience and experiment for themselves using real equipment, rather than using software which shows ripples. They need to try things out for themselves rather than just following instructions.
Practical tips: See apparatus note "Ripple tank and accessories" for important details.
Asking questions – an activity which may help with discipline in a half-dark room – encourages students to think and extend their observations. When you ask whether the water moves along with the pattern, you could leave the students to devise their own tests and to think and experiment on their own, rather than giving detailed instructions.
It is worth considering where the dark and bright ripples come from. The convex and concave surfaces on the top of the wave make perfect lenses. When the light falls on the surface in the ripple tank then light is either focused by a convex surface or spread out by a concave surface. The concentrated light produces bright bands.
It takes time to set up ripple tanks properly. If you are going to use a set of ripple tanks for a class experiment, you may want to leave them on a side bench between successive lessons. Or, if the lesson follows lunch or morning break, you could ask a few students to come early and help set them up.
For demonstration purposes, you can now use a compact ripple tank designed to sit on an overhead projector. This produces a large image on screen, which the whole class will easily see.
Safety 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.
Photo-induced epilepsy:
In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments. The danger is obviously greater for xenon stroboscopes than for hand ones.
Up next
Asking questions
Asking questions
Teaching Guidance for 11-14 14-16
Contrary to what is popularly believed, physical phenomena do not in themselves reveal theories. Interpreting what is seen often depends on knowing what you are looking for. There are many examples from the history of science either where a discovery was made as a result of the prepared mind of the scientist or where no progress was possible for a time because of theory-laden observation.
Avoid giving students instructions that tell them what they are going to see. With patience and care, even demonstration experiments can usefully model the questioning process basic to science. Students should have many opportunities for experiencing how a series of fruitful questions leads to understanding. A first question leads to an observation, which in turn provokes a new question, etc. Encourage students to discuss what they see.
This approach does take time, but is far better than simply giving dry answers before there is any grasp of a question. Students like to think for themselves and deserve to enjoy this pleasure. Passive learners are more likely to disengage.
Up next
Classroom management in semi-darkness
There are some experiments which must be done in semi-darkness, for example, optics experiments and ripple tanks. You need to plan carefully for such lessons. Ensure that students are clear about what they need to do during such activities and they are not given unnecessary time. Keep an eye on what is going on in the class, and act quickly to dampen down any inappropriate behaviour before it gets out of hand.
Shadows on the ceiling will reveal movements that are not in your direct line of sight.