Ejs Open Source Ripple Tank Interference Model java applet a remix from Interference Model: Ripple Tank written by Andrew Duffy http://www.compadre.org/osp/items/detail.cfm?ID=9989

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Description adapted from the original by Andrew Duffy
Interference in Two Dimensions of 2 source
With this simulation, you can explore the interference pattern that results from the superposition of two sources of waves. The simulation models what happens
a) two speakers, emitting sound waves;
b) two oscillating bobbers or dippers in a water tank, producing water waves;
c) two light sources, so the interference is with two light waves.

In the simulation,
1) red regions (depending on the type of visualization selected) are areas where the net displacement is positive (such as when two peaks overlap)
2) blue regions are areas where the net displacement is negative (such as when two troughs overlap).
3) In the black regions, the net displacement is zero, or close to zero.


To understand this pattern, we use the idea of the path-length difference, S1P - S2P. There is a movable blue-point P in the simulation. The path-length difference (?L) for this point is the distance the point is from one of the sources minus the distance the point is from the other source, or  simply S1P - S2P. These distances are expressed in units of the wavelength ?.
When the sources send out waves that are in phase with one another, the waves will interfere completely constructively when the path-length difference is an integer number of wavelengths, and they will interfere destructively when the path-length difference is an integer number of wavelengths, plus half a wavelength. We can express this in the form of equations.
When the sources are in phase
condition for constructive interference: ?L = m ?, where m = 0, 1, 2, ...
condition for destructive interference: ?L = (m + 0.5) ?, where m = 0, 1, 2, ...

Activities by Andrew Duffy
1 First, press the Play button to start the simulation running. One thing the simulation can help with is in understanding how this interesting pattern is formed, from two sources that put out identical single-frequency waves. Click-and-drag the blue dot P to change its location on the screen. You should observe that, whenever the blue point P is at a position in which there is a large amplitude displacement, that the path-length difference for that point (shown in blue bar at the bottom right) is an integer number of wavelengths (or close to it). In contrast, whenever the blue point P is at a position in which there is generally no displacement (it's always dark there), the path-length difference for that point is an integer number of wavelengths plus half a wavelength (or close to it).

2 Explore different points on the line joining the two sources. In between the sources, along the line joining the sources, you should observe a standing wave, with nodes (zero displacement points) and anti-nodes (maximum displacement points) that are fixed in position. For a node, you should measure a path-length difference that is close to an integer number of wavelengths plus half a wavelength. For the anti-nodes, you should observe a path-length difference that is close to an integer number of wavelengths. Is this what you find? Explain why you will always find an anti-node at the place that is halfway between the two sources.

3 Let's keep exploring what happens along the line joining the two sources, but now place the blue point P on this line, to the right of the source on the right. Adjust the frequency of the waves, or the x-coordinate of one or both of the sources, until the path-length difference is 3 wavelengths. Is constructive interference taking place at the point, or is it destructive interference? What is the distance between the sources (expressed in terms of wavelengths) in this case? Repeat, when the path-length difference is 3.5 wavelengths.

4 The wave speed has a constant value in the simulation. With the wave speed constant, what happens to the wavelength when the frequency is increased? In general, what happens to the pattern when the frequency is increased?

5 In general, what happens to the pattern when the two sources are move closer together?