Place a flat, linear barrier parallel to the shorter side of the tray, but somewhere in the middle of the tray. (We built our barrier out of Lego bricks about 30 studs long altogether. We made sure the underside of the Lego wall was wet so it’d be less likely to float, and we added levels as additional weight to help keep it in place.)
Tap or roll the rod once to make a single linear wave pulse.
What happens to the wave after it hits the flat wall?
Tap or roll the rod rhythmically to make a series of linear wave pulses.
What happens to the waves after they hit the flat wall?
Students should notice that the waves reflect.
This is a reflected wave.
Use a stiff wire, pen or pencil, eyedropper, or your finger to tap the surface near the side of the tank to send a circular ripple to the flat barrier.
What happens to the circular ripple when it bounces off a straight wall?
Where does it seem to come from?
Note the location of the imagined starting place with a coin. Try starting two ripples at the same time, one from the imagined starting place of the bounced ripple and one from the original place.
Now, angle the barrier at about 45°. Send another single linear wave pulse to the barrier. Then a circular one. Try several other angles. For each angle and wave form, ask for the students’ observations.
Use curved barriers in the shapes of parabolas and half-ellipses (including semicircles). If you are building the shapes out of clay, it might help to print out the shapes and place them temporarily under the tray to “trace” the shape with the clay.
What do you think will happen when a straight, linear wave hits this curved wall?
Try tapping the water surface from multiple locations. Then use the rod to roll linear waves at the curved barrier.
A fun trick to try
A single linear wave will reflect into a circular wave that converges to a single point. Mark the point with a coin.
Is there a way to make straight line waves bounce off a curved wall?
What do you think will happen if we tap the surface of the water just once from this point?
When you tap the surface of the water just once from this point, it should produce linear waves.
The place where you put the coin is the focal point. A single tap to the surface at this point should reflect off the curved barrier as a linear wave.
The curved barrier reflecting the waves acts as a concave mirror would if the waves hitting it were light waves.
Change the distances and angles from which you send linear waves to the curved barrier.
Does the point where the waves meet change?
The answer should be no.
Solar cookers, telescopes, satellite television antennas, and flashlights all use curved reflectors.