Ripple Tank (Cambridge International IGCSE Physics)

Investigating waves with a ripple tank

Spec point — 3.1.5 · Describe the use of a ripple tank to show: (a) reflection at a plane surface (b) refraction due to a change in speed caused by a change in depth (c) diffraction due to a gap (d) diffraction due to an edge

Investigating waves with a ripple tank

• Ripple tanks are commonly used in experiments to demonstrate the following properties of water waves:

  • Reflection at a plane surface
  • Refraction due to a change in speed caused by a change in depth
  • Diffraction due to a gap
  • Diffraction due to an edge

Reflection, refraction and diffraction can be demonstrated using a ripple tank

• A lamp above the tank shines through the water, projecting the wavefronts onto a screen below the tank, where they can be viewed and analysed

Investigating reflection

• Wavefronts are reflected off a metal bar (plane surface) placed in the water of the ripple tank
• When the bar is placed at an angle, the wavefronts generated by the paddle reflect according to the law of reflection, which states that:

Angle of incidence (i) = Angle of reflection (r)

Diagram of reflected wavefronts in a ripple tank

Wavefronts reflect off the barrier so that the angle of reflection equals the angle of incidence

Reflected wavefronts in a ripple tank

In this arrangement, the barrier is at 45° to the incident wavefronts, so the incident and reflected wavefronts meet at right angles

Investigating refraction

• Refraction can be shown by placing a glass block in the tank

  • The glass block should sit below the surface of the water and cover only some of the tank floor

• The depth of water becomes shallower where the glass block is placed

  • Since speed depends on depth, the ripples slow down when travelling over the block

• The water surface waves slow down when passing from deep to shallow water in the ripple tank

  • The wavelength also becomes shorter in the shallow water, while the frequency stays the same

Refracted wavefronts in a ripple tank

When water waves travel from deep areas to shallow areas they slow down

Investigating diffraction

• Diffraction can be shown in a ripple tank by placing small barriers with a gap or an edge in the tank

• The amount of diffraction that occurs can be changed by changing the wavelength of the waves compared to the gap size

• Diffraction at the edge of a single barrier shows waves curving around the edge into the region behind it

  • The longer the wavelength, the more the waves bend around the edge

Changing the gap size for diffraction in a ripple tank

When the gap size is bigger than the wavelength, less diffraction occurs and the waves spread out less after passing through

Changing the wavelength of waves in the ripple tank

• The motor creates the up-and-down movement of the paddle

• The frequency of the motor affects the wavelength of the waves generated by the paddle

• The diagram below shows how the wavelengths differ with frequency in a ripple tank

  • The higher the frequency of the motor, the shorter the wavelength
  • The lower the frequency of the motor, the longer the wavelength

Wavelength and frequency of waves in a ripple tank

Ripple tank patterns for low and high-frequency vibration

Exam Hint If the wavefronts change direction at a boundary where the depth changes, the effect is refraction. Diffraction happens at a gap or an edge, not at a change of depth.