Ripple Tank. Equipment:

Transcription

1 Ripple Tank Two point source interference patterns Purpose: A single vibration applied to the surface of water produces circular waves that propagate out from that point. If two identical vibrations are applied, the resulting waves will add and subtract from each other at a given point, depending upon whether the waves from the two sources are in phase or out of phase. If the waves are in phase, they will interfere constructively and produce bigger waves. If they are out of phase they will interfere destructively. In this experiment, you will show that the path difference for destructive interference is equal to odd multiples of ½ of the wavelength (1/2λ, 3/2λ, 5/2λ ) Equipment: Basic wave tank setup with bar actuator forearms (just the S-shaped wires) Water container (1000ml beaker is ideal) Ruler (cm) Long aluminum reflector Digital stroboscope

2 Preparation to set up the ripple tank should be done by TEACHER 1. Level the tank by screwing the feet in or out, using the level provided. 2. Fill the tank to a depth of about 0.5 cm. or 1000 ml of water. 3. Attach the actuator forearms to the ripple generator so they are just touching the surface of the water 2cm apart. 4. Set the amplitude to 5 on BOTH sides of the wave generator. 5. STOP read entire step before continuing. Make sure that the wave generator phase control is set to zero. If it isn t, loosen the lock on the opposite side of the wave generator, zero the control, and tighten the lock. Not loosening the phase lock will destroy the phase control and ruin the generator for your use and others. 6. Make sure that the strobe is fastened to the light and both are positioned over the ripple tank. 7. Place a sheet of white paper on the table under the ripple tank. Procedure: 1. Draw an X on the image of each of the wave sources. Label one of these A and the other B. Later on, you ll measure from these two points to determine the path lengths. 2. Set the frequency control to I. This is a fairly high frequency. 3. Turn on the light. 4. Turn on the stroboscope. 5. Now you will find the wavelength of the water waves by using the stroboscope to freeze the waves. Start at 24.9Hz on the stroboscope and decrease the frequency using the down arrow it until the waves appear stationary. 6. Measure the distance between the waves and record on the drawing. 7. Observe the wave pattern projected on the tabletop. Draw the wave pattern you see. Pay particular attention to where the waves are relatively strong or weak. This is where constructive and destructive interference are occurring. 8. Draw a line down the middle of the first minimum starting near the sources to as far as you can go on the image. Your line should follow the center of one of the quiet areas directly adjacent to the central (first) maximum. You ll have to use your judgment to determine the center of the area. There are minimums on both sides of the central maximum, draw a line marking both. 9. Draw a line down the middle of the area representing the central maximum starting near the sources (between) to as far as you can go on the image. You ll have to use your judgment to determine the center of the area.

3 10. Draw a line down the middle of the second maximum starting near the source to as far as you can go on the image. Your line should follow the center of the strongest waves just beyond the first minimum. You ll have to use your judgment to determine the center of the area. There are second maximums on both sides of the central area, draw a line marking both. 11. Draw a line down the middle of the second minimum starting near the sources to as far as you can go on the image. Your line should follow the center of one of the quiet areas just beyond the second maximum. You ll have to use your judgment to determine the center of the area. There are minimums on both sides of the second maximums, draw a line marking both. (These are sometimes hard to see or only one is visible due to poor conditions discuss with your teacher strategies in marking these lines.) 12. Turn off the lamp and turn off the ripple generator (back to setting A). 13. Mark four evenly spaced (2 cm spacing usually works well) locations along each line. Label these 1 through 4 with 1 being closest to the ripple generator. 14. Measure from each of the source points (remember you labeled them A & B) to each of the numbered points. Fill in the table below. 15. For each column, subtract the lesser of the A and B distances from the greater. Fill in the table below. 16. Now divide each difference by the wavelength that you determined back in step 6. Fill in the table below. Line Title Central Max #1 (mm) #2 (mm) #3 (mm) #4 (mm) Line Title

4 Central Max #1 (mm) #2 (mm) #3 (mm) #4 (mm) Line Title Diff. Pt. #1 Diff. Pt. #2 Diff. Pt. #3 Diff. Pt. #4 Central Max..... Line Title Diff.Pt.#1 / λ Diff.Pt.#2 / λ Diff.Pt.#3 / λ Diff.Pt.#4 / λ Central Max..... QUESTIONS 1. Explain with a diagram the two types of constructive interference encountered during this experiment. (Crest & Trough)

5 2. Explain with a diagram the destructive interference encountered during this experiment. 3. If there were another maximum and minimum beyond the pattern seen, what would the 3 rd maximum have for a ratio pattern e.g. nλ? (hint: look at the fourth data table). 4. If there were another maximum and minimum beyond the pattern seen, what would the 3 rd minimum have for a ratio pattern e.g. nλ? (hint: look at the fourth data table). 5. What are some possible sources of error that occurred in this lab?