Properties of waves — Required practical 8

How do you actually measure the properties of waves? AQA's required practical 8 covers two methods: a ripple tank for water waves and a vibrating string.

Setup 1 — water waves on a ripple tank

• Shallow water in a clear tank, lit from above with a strobe (or below with a lamp casting shadows on a screen below).
• A vibrating bar or motorised dipper creates waves at a measurable frequency.

To find:

• Frequency: read off the dipper motor.
• Wavelength: photograph the surface or place a metre rule across the water; count waves in 1 m of length, divide.
• Speed: $v = f\lambda$.

Setup 2 — waves on a stretched string

• String fixed at one end; the other connected to a vibration generator (signal generator + speaker-style coil).
• Adjust frequency until standing waves form (clear nodes and antinodes).
• Wavelength: $\lambda = 2L/n$ for a string of length $L$ with $n$ "loops" (half-wavelengths).
• Speed: $v = f\lambda$.

Standing waves on a string

When the string is plucked at the right frequency, energy bounces between the ends and forms a standing wave with:

• Nodes — points of zero displacement (at fixed ends and intermediate).
• Antinodes — points of maximum displacement.

The fundamental: 1 antinode in the middle. Higher harmonics: more loops.

Sources of error

• Wavelength of moving waves: hard to measure if waves are fast — a strobe synchronised to wave frequency makes them appear stationary.
• Frequency: signal generator may be slightly off; check with an oscilloscope.
• Standing waves: too low or too high amplitude makes nodes hard to see.