How Do You Count the Number of Waves in a Ripple Tank Experiment?

[roam]

Homework Statement

When you shine a light on the ripples in a tank you get dark and bright patches like this:

http://img687.imageshack.us/img687/4343/rippletank.jpg

So my question is: how do we count the number of waves?

In my experiment I am trying to find the wavelength by dividing the distance traveled by a train of waves by the number of waves in it. Do we have to count the number of bright fringes or the shadows?

The Attempt at a Solution

We are not trying to find the distance from one trough/crest to the next (that would be the wavelength). So, what is "one wave" in this case? How many waves are there in the picture I've posted (I can see 7 bright patches and 7 shadows)?

 

[spacelike]

Well it's not as simple as just counting bright or dark fringes.

Because an entire wavelength includes both, so in your picture you actually have a fractional amount of wavelengths.

Unless you want to estimate that there are 6.9 wavelengths or something like that then the best thing to do would be to adjust your experiment such that you are measuring a region that you are fairly sure contains an integer number of wavelengths.

But 1 wavelength which is what I would say counts as having "one wave" extends from crest to crest, or trough to trough.
If I start at the first bright spot on the left and go to the next crest that is 1.
So I see 6 full wavelengths in the image.
Then there is a very tiny little sliver on the very left, and also about half a wave on the right side.
So idk... 6.7 waves?

Also, I think you should avoid calling them "waves", and use "wavelengths" instead. That entire thing is one wave, which spans several wavelengths. (of course this last sentence is just semantics, I could be wrong, but that's how I think it goes)

 

[roam]

Well it's not as simple as just counting bright or dark fringes.

Because an entire wavelength includes both, so in your picture you actually have a fractional amount of wavelengths.

Unless you want to estimate that there are 6.9 wavelengths or something like that then the best thing to do would be to adjust your experiment such that you are measuring a region that you are fairly sure contains an integer number of wavelengths.

But 1 wavelength which is what I would say counts as having "one wave" extends from crest to crest, or trough to trough.
If I start at the first bright spot on the left and go to the next crest that is 1.
So I see 6 full wavelengths in the image.
Then there is a very tiny little sliver on the very left, and also about half a wave on the right side.
So idk... 6.7 waves?

Also, I think you should avoid calling them "waves", and use "wavelengths" instead. That entire thing is one wave, which spans several wavelengths. (of course this last sentence is just semantics, I could be wrong, but that's how I think it goes)

Thanks for the input. They call them "waves" in the pamphlet that describes the experiment. The aim is to find the wavelength by using a ruler to measure the length of as many waves as possible, and then dividing them by the number of waves.

So "one wave" would be just one wavelenght? So, the start of a bright patch to the end of a dark patch would count as one wave?

 

[spacelike]

Thanks for the input. They call them "waves" in the pamphlet that describes the experiment. The aim is to find the wavelength by using a ruler to measure the length of as many waves as possible, and then dividing them by the number of waves.

So "one wave" would be just one wavelenght? So, the start of a bright patch to the end of a dark patch would count as one wave?

Sorry I didn't notice you responded sooner, for some reason it didn't notify me.

But "one wave" should be measured from bright spot to bright spot.

Or if you prefer, you can go from dark spot to dark spot. That might seem more intuitive since you are including the entire bright region.

 

[asteriatic]

As a scientist, it is important to first define what is meant by "one wave" in this context. In the case of a ripple tank, one wave can be defined as the distance between two consecutive bright patches or two consecutive dark patches. This means that there are 7 waves in the picture provided, as there are 7 bright patches and 7 dark patches.

To accurately count the number of waves, it is important to have a clear understanding of the experiment and its setup. In this case, it would be helpful to have a measurement of the distance between the light source and the ripple tank, as well as the distance between the light source and the screen where the pattern is projected. This information can help in determining the scale of the image and the distance between each wave.

Additionally, it is important to note that the wavelength can also be calculated by dividing the distance between two consecutive bright patches or two consecutive dark patches by the number of waves. Therefore, it is not necessary to count the number of waves, but rather to accurately measure the distances involved in order to calculate the wavelength.

In conclusion, the number of waves can be counted by identifying the distance between two consecutive bright or dark patches and dividing the total distance by the wavelength. It is also important to have a clear understanding of the experiment setup and accurate measurements in order to obtain reliable results.