Purpose
This interactive allows you to explore the principle of superposition for waves and the associated interference patterns.

Using the Interactive

• Configure the interactive using the MODE, CONTROLS, and INTERFERENCE PATTERN templates on the right-hand side.
• The large white area is the "tank" where waves are created. The dark blue region below it is where graphs of waves can be displayed.

• Add Dippers: When selected you can place a dipper (up to 3 at a time) in the tank.
• Frequency: Sets the frequency of the waves created by the next dipper added.
• Add/Change Cross Section: When selected, you can add a cross sectional line (a "screen") on which waves will be recorded, or change an existing cross section. There only can be one cross section in the tank.
• Presets: Provides two preset scenarios for your use.
• Show Grid: When selected, displays a grid for more accurate placement and evaluation.
• Snapshot: Pauses the animation and shows a contour plot of the interference pattern of the waves.

1			Choose PRESET 2. Note that the sources are coherent and that the lines are wave fronts: blue represents crests and red represents troughs. Click PAUSE. Which of the following correspond to places where there is always constructive interference?
			A)	Along a horizontal line either left of the left source or right of the right source
			B)	Along a vertical line passing through either source
			C)	Along a vertical line that passes directly through the center of the two sources
			D)	Along a slanted line extending away from just outside the central region between the sources
2			Click PLAY to restart the animation. Note the nodes in the wave to the left and right of the center point. Identifying the location corresponding to those nodes on the cross section can help you answer the next question. Click PAUSE. Which of the following correspond to places where there is always destructive interference?
			A)	Along a horizontal line either left of the left source or right of the right source
			B)	Along a vertical line passing through either source
			C)	Along a vertical line that passes directly through the center of the two sources
			D)	Along a slanted line extending away from just outside the central region between the sources
3			Click SNAPSHOT. Note that dark blue regions are high amplitudes and light blue are lower positive amplitudes, while dark red regions are large negative amplitudes and light red regions are smaller negative amplitudes. White regions have zero amplitude. Select ADD/CHANGE CROSS SECTION. Drag then end balls of the cross section (one at a time) so that the cross section lies in one of the white regions. Click PLAY and note the graph of the wave. Move the cross section so that the ends exactly cover the sources. What best describes the interference pattern that will result from these waves?
			A)	An oscillating straight line indicating constructive interference everywhere.
			B)	A zero amplitude line indicating destructive interference everywhere.
			C)	standing wave pattern.
			D)	A randomly appearing pattern with no easily identifiable structure.