Waves required practical

Aim: To measure the frequency, wavelength and speed of waves in a ripple tank and of waves on a stretched string, and choose suitable equipment for each.

Variables

• Independent: Frequency of the vibration (set on the signal generator for the string; motor speed for the ripple tank)
• Dependent: Wavelength of the waves
• Control: Depth of water in the ripple tank; Tension in the string (same hanging mass); Length of the vibrating string; Type of string

Equipment

• Ripple tank with a motor-driven dipper
• Lamp above the tank and white paper or screen below
• Ruler
• Stopwatch or stroboscope
• Vibration generator and signal generator
• String or elastic cord
• Pulley and bench clamp
• Hanging masses
• Metre rule

Method

1. Ripple tank: pour water to a depth of about 5 mm and level the tank so the depth is the same everywhere.
2. Switch on the lamp and the motor so a wave pattern appears on the paper below the tank.
3. Measure the length across as many wave shadows as possible, for example 10, and divide by the number of waves to find the wavelength.
4. Count the waves passing a fixed point in 10 seconds and divide by 10 to find the frequency.
5. String: attach the string to the vibration generator, pass it over a pulley and hang masses on the end to keep it taut.
6. Switch on the signal generator and adjust the frequency until a clear stationary wave is seen.
7. Measure across as many half-wavelengths (loops) as possible with a metre rule and work out one full wavelength (one loop is half a wavelength).
8. Record the frequency from the signal generator and calculate the wave speed for each set of readings.

Safety

• Mop up spilt water from the ripple tank, as water and mains-powered lamps are a hazard.
• Place a tray or foam under the hanging masses in case the string breaks.
• Wear eye protection in case the stretched string snaps, and do not touch the vibration generator while it runs.

Results and calculations

Record frequency, number of waves, total length and wavelength in a table. Calculate wave speed using v = f λ (speed in m/s, frequency in Hz, wavelength in m); the speed on a given string, or in a given depth of water, should stay about the same when frequency changes.

Common mistakes and improvements

• Waves are hard to measure while moving: use a stroboscope set to the wave frequency to freeze the pattern, or take a photo with a ruler in view.
• Measuring one wavelength gives a large uncertainty: measure across many waves and divide.
• Uneven water depth changes the speed across the tank: level the tank and keep the depth the same.
• Confusing a loop with a whole wavelength on the string: remember one loop is half a wavelength.

Exam tips

• Always show v = f λ, and convert units such as cm to m before calculating.
• Explain why measuring across several waves improves accuracy: it reduces the percentage uncertainty in each measurement.
• Be able to say why a stroboscope or a photograph helps measure moving waves.
• Questions may ask you to judge which set-up is best for measuring each quantity; give a reason tied to the size of the wave or ease of measuring.
• Know that speed depends on the medium (water depth or string tension), not on the frequency.