Purpose:
Use the ripple tank to investigate wave properties of wave speed, reflection and diffraction.
Websites:
Go to Waves links and use the Virtual Ripple Tank, and others to answer the following questions:
Note that these questions are for a real ripple tank, and that you will have to play with the simulation to recreate the effects.
Procedure:
Record a detailed line drawing for each of the following procedures. (A1, A2, B1, B2 etc.) Do not include walls. (Your ripple tank is infinitely large!) Only put four diagrams per sheet of paper please. If there are questions within a procedure, put the answers immediately under the corresponding sketch.
A. Transmission and speed of a wave
1. Set up the tank as instructed by the teacher. Put water in the tank to a depth of approximately 7 mm ensurng that the tank is level. Touch the surface of the water once lightly at the centre of the tank with your finger or a pencil tip. Draw NEAT sketch showing the wave. Although water waves are not the simplest waves, they are familiar to everyone. Observing the behaviour of water waves in a ripple tank will introduce you to the analysis of wave motion.
- What is the shape of the pulse?
- Does the speed of the pulse seem to be the same in all directions? How can you tell?
2. Generate a straight wave with the dowel by pushing it forward 1 cm with your hand.
- What is the shape of the pulse?
- Does this shape change as it travels across the tank?
- Draw a straight wave showing its direction of motion.
B. Reflections
1. Place a wood block in the tank. With the dowel, generate straight pulses that strike the barrier head-on (wave crests should be parallel to the barrier).
- What do the pulses do when they reach the barrier?
- After the pulses strike the barrier, what is the new direction of the reflected pulses?
- Does the speed of the waves change after they have been reflected?
2. Produce circular wave pulses with your finger.
- How do the pulses reflect from the barrier? Sketch your observation.
- From what point do the reflected pulses appear to be originating?
3. Arrange the barrier so that straight waves strike it at an angle.
- How does the angle between the incident (initial) wavefronts and the barrier compare with the angle between the reflected wavefronts and the barrier? To help you judge the angles, align rulers or other straight objects with the wavefront images.
- Draw a diagram showing the incident wavefronts and the reflected wavefronts and their directions of travel.
4. Place the parabolic curve into the tank. With the dowel, generate straight waves towards the open side of the parabola.
-Sketch your observation of the reflected waves.
5. Continuing to use the parabolic curve, find the point at which the straight pulses reflected by the barrier meet. This is the focal point of the parabola. Generate a circular pulse with your finger centred at the focal point.
- What is the shape of the reflected pulse?
C. Changing frequency
Start generating straight waves at a consistent frequency. The distance between the bright bars in the wave is the wavelength. Adjust the frequency of your wave generation.
- What effect does increasing the frequency have on the wavelength?
D. Changing water depth
Prop up the tank so that there is a strip of “beach” with no water on it. Send straight waves from the deep end to the shallow end. Sketch the way in which the speed and the wavelength change as the waves move to the shallow end.
E. Diffraction (waves bending around corners!)
1. Place a straight barrier across the wave tank, fixed to one side of the tank and open at the other side. Generate periodic waves with a long wavelength and observe how the wavefronts pass by the open end of the barrier. Slowly increase the frequency of the waves.
- What change in wavelength occurs?
- Is the amount of diffraction greater or smaller than before?
- Record your observations in one sketch for smaller frequency and one sketch for larger frequency, showing a series of wavefronts on both sides of the barrier.
2. Using two barriers, create a barrier with an opening in the centre of about 4cm. Generate waves with a long wavelength, observing the amount of bending that occurs. Increase the frequency gradually.
- Was the amount of diffraction greater or smaller with the increased frequency?
- Record your observations in the form of sketches, one for smaller frequency and one for larger frequency
3. While your partner generates waves with a constant frequency, slowly decrease the size of the opening to 0.5 cm by moving one of the blocks.
- How does the size of the opening affect the diffraction? Record your observations in the form of sketches, one for smaller opening and one for larger opening.
Discussion:
1. What is the direction of the wave motion relative to the crest of a wave?
2. How does a decrease in frequency of the source affect the wavelength of the wave?
3. Does a change in frequency affect the speed of a wave?
4. If the depth of the water decreases, what happens to the speed and the wavelength of the wave?
5. Using your observation of reflected waves, how does the angle between the barrier and the incident waves compare with the angle between the barrier and the reflected waves.
6. How does wave diffraction change as the width of the opening is changed?
7. As frequency increases, what happens to wave diffraction?
8. How are straight waves reflected by a parabolic reflector?