# Wave produced in a ripple tank appear to stand still at 66.7Hz?

For an experiment involving a ripple tank and a wave monitor, the frequency was changed from 10Hz to 100Hz and it was found that at precisely 66.7Hz, the wave produced appeared (to the viewer) to be standing still. At any value below 66.7 Hz the wave moved inwards, and at any value above 66.7Hz the wave moved outwards. Is there an explanation for this (based off theory) ?

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Simon Bridge
Homework Helper
Welcome to PF;
... would there have been reflection off the far end of the tank?
... how was the tank lit (thanks to Dadface)

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Hello emie. I'm assuming that you had fluorescent lighting in the room and that the frequency of the mains was 60Hz or thereabouts. If so you were probably observing the stroboscopic effect.

Hello emie. I'm assuming that you had fluorescent lighting in the room and that the frequency of the mains was 60Hz or thereabouts. If so you were probably observing the stroboscopic effect.
The waves were lit with a hanging light that was hung (approximately) 10 cm above the waves.

Here's a photo of the experiment:

The lights in the room weren't on as it was daytime.

Does the motor vibrate the mirror slightly?
http://en.wikipedia.org/wiki/Wagon-...roboscopic_presentation_by_vibrating_the_eyes

Try with just plane waves instead of the interference setup - try without the small light on, and add damping between the mirror and it's supports.
No I don't think that the motor vibrates the mirror.
Is there an explanation for the cause of the waves to appear still? This was witnessed by a few students so it can't be the trick of the eye. But is there any theory behind the cause? Because I need to find out the reason why this occurred, rather than how to fix it.
Thank you :)

Simon Bridge
Homework Helper
The suggestions were for experimentally determining the cause.
Without the experiment, anything we suggest here would be speculation.

Do I understand you correctly that not all students see this effect?

Anyway - if the effect is that the peaks and troughs appear stationary, then it could be an interference effect... as in standing waves. However, the walls of the tank appear to be absorbtive - ruling out standing waves. Does the effect only appear for the diffraction experiment or does it appear for plane or circular waves too?

If the ripples just appear still - no motion, including vertical - then you are looking for something more like the stroboscopic effect. A slight flicker in the small lamp or small vibrations somewhere would do this.

Do not rule out a "trick of the eye" just because a large number of people see the same thing - we all have basically the same machinery in our eyes so they can all be tricked the same way.

The only way to determine the actual cause is to investigate by experiment.
We do that by eliminating possibilities... it should be a good exercise in scientific method.

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The suggestions were for experimentally determining the cause.
Without the experiment, anything we suggest here would be speculation.

Do I understand you correctly that not all students see this effect?

Anyway - if the effect is that the peaks and troughs appear stationary, then it could be an interference effect... as in standing waves. However, the walls of the tank appear to be absorbtive - ruling out standing waves. Does the effect only appear for the diffraction experiment or does it appear for plane or circular waves too?

If the ripples just appear still - no motion, including vertical - then you are looking for something more like the stroboscopic effect. I flicker in the small lamp or small vibrations somewhere would do this.

Do not rule out a "trick of the eye" just because a large number of people see the same thing - we all have basically the same machinery in our eyes so they can all be tricked the same way.

The only way to determine the actual cause is to investigate by experiment.
We do that by eliminating possibilities... it should be a good exercise in scientific method.
oh I see, my apologies! so this could be a trick of the eye.
Is there any formula's regarding the stroboscopic effect?
And if this was a trick of the eye, I think I read somewhere that us (humans) see real motion at 24 frames/second...could that relate to this matter?

Simon Bridge
Homework Helper
The wikipedia link on the wagon-wheel effect will start you out.
Also see this:
http://www.100fps.com/how_many_frames_can_humans_see.htm

I think I've found a video that shows what you are describing on this site (scroll down to the demo)
http://weelookang.blogspot.co.nz/2011/01/ejs-open-source-ripple-tank.html
... the demonstrator does not comment on it and, since it is video, we cannot rule out the wagon-wheel effect. In fact, that is the most likely cause in this example.

If your experiment seems to slow and then reverse like this one - you have your most likely candidate.

The wikipedia link on the wagon-wheel effect will start you out.
Also see this:
http://www.100fps.com/how_many_frames_can_humans_see.htm

I think I've found a video that shows what you are describing on this site (scroll down to the demo)
http://weelookang.blogspot.co.nz/2011/01/ejs-open-source-ripple-tank.html
... the demonstrator does not comment on it and, since it is video, we cannot rule out the wagon-wheel effect. In fact, that is the most likely cause in this example.

If your experiment seems to slow and then reverse like this one - you have your most likely candidate.
Thank you!
And when you say, if my experiment seems to slow then reverse like in that video, do you mean when I change the frequency? Because that's what occurred for me! Though I'm still wondering why it was at precisely 66.7 Hz that it seemed to be still.
If it were the wagon-wheel effect, and it occurred by 'vibrating the eyes', none of the examples presented by Wikipedia occurred. None of the witnesses were humming, or eating.
Are you referring to the 'vibrating the eyes' part of Wikipedia when you say that it is most likely for the reason to be the wagon wheel effect? Or are you referring to the fact that the light source may be at the same frequency as the wave monitor (which vibrates the dippers)- this is mentioned in the 'Danger' section of Wikipedia.
Quote from Wikipedia:

Because of the illusion this can give to moving machinery, it is advised that single-phase lighting be avoided in workshops and factories. For example, a factory that is lit from a single-phase supply with basic lighting will have a flicker of 50 or 60Hz (depending on country); thus, any machinery rotating at 50 or 60Hz (3000 or 3600rpm) may appear to not be turning, increasing the risk of injury to an operator. Solutions include deploying the lighting over a full 3-phase supply, or by using high-frequency controllers that drive the lights at safer frequencies

Simon Bridge
Homework Helper
when you say, if my experiment seems to slow then reverse like in that video, do you mean when I change the frequency?
That is correct.

The effect on the video is almost certainly due to the wagon-wheel effect in the recording.
That the same thing occurs for you (you should watch the video to check) is suggestive.
Your task remains to work out what effect is causing it for you.

I'd start out by using a different light-source.
Use a battery-powered lamp instead, say.

Then look for other sources of vibration.
Also simplify the experiment ... see what you have to do to remove the effect - then you have your cause.
I've been wondering if there is a stationary solution for two point sources - it would be simpler to eliminate the possibility than to calculate so I'd want to do that if it were me.

Note 1 - it won't be "precisely" 66.7Hz ... your equipment will have some uncertainty.
Note 2 - you have said that "[t]his was witnessed by a few students" which suggests that not all students witnessed this effect (only "few"). So - just to be clear - does everybody watching see this effect for the same frequency?

What I meant to say was that everyone who was watching the experiment, saw the waves standing still. It was just that not everyone was looking at the experiment in the first place, but the few who did look, saw the same thing.

So if I used a battery powered lamp (at a different frequency), and the waves were not standing still at 66.7 Hz, would that mean that the light source that I used before can be taken into account for being a part of the matter?

Simon Bridge
Homework Helper
If a new lamp removes the effect then it is sensible to conclude that the old lamp had something to do with it.
If nothing changes, then we can rule out the lamp.

A mains-powered lamp usually has a slight flicker close to the mains frequency (or twice that).
A DC lamp - i.e. run off a battery - should not have this flicker.
If removing the flicker removes the effect, then it is sensible to conclude that the flicker was the cause.
Thus the change to a battery-powered lamp. A flashlight should do.

This kind of control of variables is core to the scientific method: you try to change just one thing, and see what else changes. We also try to design the experiment to disprove our favorite theory.

Note: if you want to use a DC power supply instead of a battery, check the outputs with an oscilloscope.
Common school/lab PSUs don't properly rectify the current, or they don't smooth the signal, leaving a significant ripple. You can smooth out any ripple by putting a big capacitor across the terminals.

I would think that 66.7 Hz is the natural frequency of vibration for the volume of water/setup. The volume of water vibrates as a unit at this frequency. The problem is then to explain why the waves seem to move inward or outwards when the frequency is offset a bit. I think one sees the same effect with waves in a tea cup.

Simon Bridge
Homework Helper
You get standing waves in a tea-cup due to the reflection from the sides.

In the ripple tank, the sides have an absorptive layer.
Put some absorber around the inside of your teacup and you won't get standing waves there either.

Standing waves still have the transverse motion - the antinodes bob up and down.
We have report that no motion is observed - though it could be that the motion is too fast to spot.

The body vibrates as a whole at its natural frequency. Reflections occur at its boundaries. In this case the bottom, sides of the tank and the top surface of the water. So I suspect that these are not waves or oscillations that involves just the surface of the water.

Simon Bridge