Purpose In this investigation you will study the behaviour of waves in two dimensions by observing water waves in a ripple tank. A ripple tank is a shallow, glass-bottomed tank. Light from a source above the tank passes through the water and illuminates a screen on the table below. The light is converged by wave crests and diverged by wave troughs, as shown in Figure 1, creating bright and dark areas on the screen. The distance between successive bright areas caused by crests is one wavelength (λ). light from source Student Worksheet 6.5A (Supplementary) Investigation 6.5.1 Waves in a Ripple Tank water glass bottom dark area bright area bright area screen l Figure 1 Bright lines occur on the screen where light rays converge. Circular waves may be generated on the surface of the water by a point source, such as a finger or a drop of water from an eyedropper. Straight waves may be produced by moving a dowel in the water or by using a wave generator. You may wish to recall your studies in optics on the reflection and refraction of light. (See Chapter 10 for a review.) In this activity you will also look at the interaction of periodic waves with obstacles, which produces diffraction. Diffraction is the bending effect on a wave s direction of travel as it passes through an opening or by an obstacle. Questions (i) How are waves transmitted in two dimensions? (ii) How do barriers reflect waves? (iii) How does the wavelength and direction of transmission of a wave change when the speed of the wave changes? (iv) How are waves refracted? (v) How are waves diffracted around barriers and through openings, and how does diffraction depend on wavelength? Copyright 2002 Nelson Thomson Learning Unit 3 Waves and Sound SW-7
Materials ripple tank and related equipment wooden or metal dowel wax blocks ruler rubber tubing (50 cm) glass plate glass plate supports (optional) wave generator Procedure Transmission and Reflection 1. Level the tank to ensure that the depth of the water is uniform. Pour water into the ripple tank to a depth of approximately 1 cm. 2. Touch the surface of the water lightly at the centre of the tank with your finger. What is the shape of the wave produced by this type of point source? Sketch the source and the waves. At four equally spaced points on the crest of the wave, draw arrows indicating the direction of wave motion. How can you tell, by the shape of the wave, if its speed is the same in all directions? 3. Generate a straight wave with the dowel by rocking it back and forth on the bottom of the tank or by using the wave generator. Does the wave remain straight as it travels across the tank? Does its speed change? In what direction does the wave move relative to its crest? SW-8 Unit 3 Waves and Sound Copyright 2002 Nelson Thomson Learning
4. Generate periodic straight waves. What happens to the wavelength if you reduce the frequency? Does the speed change? What do you predict the effect will be on the wavelength and on the speed of the waves if the frequency is increased? Test your prediction in the ripple tank. 5. Prop up the tank so that the water on one side is only 1 mm deep, ensuring that the water does not spill out of the other side. Send straight waves from the deep end to the shallow end. Note any changes to the speed and the wavelength as the waves move to the shallow end. Reflection 6. Form a straight barrier using a wax block on one side of the tank. Send straight waves toward the barrier so that the wavefronts are parallel to the barrier. Note the appearance of the incoming, or incident, wavefronts compared to that of the reflected wavefronts. Note any changes in the speed or the wavelength of the waves after they have been reflected. 7. Arrange the barrier so that the waves strike it at an angle. How does the angle between the incident wavefronts and the barrier compare to 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 on the screen below. 8. Place some rubber tubing in the tank, allowing it to fill with water. Bend the tubing into the approximate shape of a parabola. With the dowel, generate straight waves toward the open side of the parabola. Observe how the wavefronts move before and after they strike the curved barrier. Copyright 2002 Nelson Thomson Learning Unit 3 Waves and Sound SW-9
9. Using your finger as a point source, generate circular waves that are reflected from the parabolic barrier as straight waves. Refraction 10. Support the glass plate on the spacers so that the longest edge of the glass plate is parallel to the wavefronts. 11. Pour enough water into the tank so that the glass plate is uniformly covered to a depth of approximately 1 mm. 12. Adjust the frequency of the vibrator so that it produces waves with a long wavelength. Note the appearance of the waves as they pass from deep to shallow water, including any changes in the direction of motion and the wavelength. 13. Set the edge of the glass plate at an angle of approximately 45 to the incoming or incident waves. In a sketch, record the changes that occur in the direction of travel of the refracted waves in the shallow water, showing the wavefronts and the direction of wave travel. Diffraction 15. Set up the wave generator to produce straight waves. Place a wax block about 3 cm or 4 cm in front of the source. Generate waves with a high frequency (short wavelength), noting the diffraction pattern as the waves move around the edge of the barrier. 16. Gradually reduce the frequency used by increasing the wavelength. Note any changes in the pattern. SW-10 Unit 3 Waves and Sound Copyright 2002 Nelson Thomson Learning
17. Predict the shape of the diffraction pattern that will occur when short-wavelength waves and long-wavelength waves interact with barriers. Test your predictions in the ripple tank. 18. While generating a wave with a constant frequency, slowly decrease the size of the opening by moving one of the blocks. Note any changes in the direction of motion of the diffracted waves, recording your observations in two sketches, one for a wide opening and one for a narrow opening. 19. Predict the relationship between the size of the opening and the wavelength that will result in minimum diffraction. Test your prediction in the ripple tank. Evaluation (a) Make a summary statement based on your observations for each step in the investigation. (b) Answer the Questions, using diagrams as appropriate. Copyright 2002 Nelson Thomson Learning Unit 3 Waves and Sound SW-11