Mini Physics
Part of Mini EducationSwitch subject
Search

Ripple Tank

Key idea: O Level ripple tank practical: observe wavefronts, measure wavelength accurately, and use v = fλ to find wave speed.

  • SEC G3 Physics 2027
On this page

Learning objectives

  • Describe wave generation by vibrating sources, ropes and springs
  • Describe ripple-tank waves using wavefronts
  • Explain that waves transfer energy
  • Explain that wave energy transfer does not transfer matter
  • Use amplitude, frequency and wavelength to describe wave motion
  • Define and use wave speed and period and interpret wave graphs
  • Recall and apply wave speed = frequency × wavelength
  • Compare transverse and longitudinal waves and give examples
  • Explain sound production by vibration and the need for a medium
  • Describe sound using compressions and rarefactions
  • Relate sound loudness to amplitude and pitch to frequency
  • Explain reflected-sound echoes and use them to measure distance
  • Explain ultrasound use in sonar and soft-tissue scanning
  • Use the normal, angle of incidence and angle of reflection
  • Apply the law of reflection in constructions, measurements and calculations
  • Use the normal, angle of incidence and angle of refraction
  • Apply sin i divided by sin r as a constant for a fixed pair of media
  • Define refractive index as vacuum light speed divided by medium light speed
  • Explain the critical angle
  • Explain the conditions for total internal reflection
  • Apply total internal reflection to optical fibres and state advantages
  • Describe how a thin converging lens acts on a light beam
  • Define the focal length of a converging lens
  • Construct real and virtual image ray diagrams for a thin converging lens
  • Describe lens images as real or virtual, magnified or diminished, and upright or inverted

1. Definition

A ripple tank is a shallow tray of water used to visualise water waves and wavefronts.

With a lamp above and a screen below, you can see wave patterns as alternating bright and dark regions.

Ripple-tank apparatus and observed wavefrontsA lamp shines through a shallow water tray while a motor drives a dipper. A screen below shows straight wavefronts from a line dipper and circular wavefronts from a point dipper.Side view of the apparatuslampmotorshallow watervibrating dipperscreenLine dipperPoint dipperstraight wavefrontscircular wavefronts
Ripple tank: a practical way to see straight and circular wavefronts and measure wavelength.

2. Key Ideas

  • A straight dipper produces straight wavefronts; a point dipper produces circular wavefronts.
  • You can measure wavelength by measuring crest-to-crest distance on the pattern.
  • For better accuracy, measure across several wavelengths and divide by the number.
  • A ripple tank is mainly used to demonstrate the ideas in Wave Motion, especially wavefronts.

3. Detailed Explanations

A. How a ripple tank works

  1. A motor vibrates a dipper up and down to create ripples.
  2. Light shines through the water onto a screen below.
  3. Crests and troughs focus/spread the light differently, so you see a pattern of bright and dark bands.

B. What you can observe (O Level focus)

Wavefront shape

  • line source → straight wavefronts
  • point source → circular wavefronts

Wavelength

  • measure the distance between two adjacent crests (or two adjacent troughs)

C. Measuring wavelength accurately (typical method)

  1. Choose a line across the pattern perpendicular to the wavefronts.
  2. Measure the distance across n wavelengths (e.g. from crest 1 to crest 6 is 5 wavelengths).
  3. Divide by n to find the average wavelength.
Link

Once you have f and λ, you can find wave speed using v = fλ in Waves & Properties Of Waves.

D. Video: ripple tank in action

4. Common Mistakes

  • Measuring from a crest to a trough (that is half a wavelength).
  • Measuring along a slanted line (you should measure in the direction of travel, perpendicular to the wavefronts).
  • Using one wavelength only (large percentage uncertainty). Measure several and take an average.

5. Exam Tips

  • If asked “why average?”, say it reduces percentage uncertainty.
  • Use clear keywords:
    • crests/troughs
    • wavefronts
    • wavelength is crest-to-crest (same phase)
  • In practical questions, mention how you avoid parallax and read rulers at eye level.

6. Worked Examples

Modelled example 1

Wavelength from multiple crests

Core

Problem

The distance from crest 1 to crest 6 is 12 cm. Find the wavelength in centimetres and metres.
Study the worked solution

  1. Count intervals, not labels

    Method

    Count five crest-to-crest spacings from crest 1 to crest 6.

    Reason

    Six labelled crests enclose five complete wavelengths.

    Working

    n = 6-1 = 5.

  2. Average and convert

    Method

    Divide the measured span by five, then convert centimetres to metres.

    Reason

    Each spacing is one wavelength.

    Working

    λ = 12/5 = 2.4 cm = 0.024 m

Guided practice 2

Wave speed in a ripple tank

About 5 min

Problem

The dipper frequency is 8.0 Hz and wavelength is 2.5 cm. Find the wave speed.

Try this before viewing the solution

Unit: m s^-1

Hints

Hint 1: make the units coherent
2.5 cm = 0.025 m.
View solution step by step

  1. Convert wavelength

    Method

    Change centimetres to metres.

    Reason

    The requested speed unit uses metres.

    Working

    λ = 0.025 m.

  2. Use the wave equation

    Method

    Substitute into v = fλ.

    Reason

    Frequency times distance per cycle gives distance per second.

    Working

    v = (8.0)(0.025) = 0.20 m s⁻¹

Common misconception 3

Why crests can look dark

Find and correct the mistake

Learner claim

A learner says every dark band must be a trough because “lower water blocks more light”. Locate the faulty assumption and explain why crests can appear dark in the observed setup.

Try this before viewing the solution

First error

View solution step by step

  1. Separate height from brightness

    Method

    Recognise that the screen pattern is an optical projection.

    Reason

    Curved crest and trough surfaces redirect and spread or concentrate the lamp light differently.

    Working

    Dark does not automatically mean trough.

  2. Explain the stated observation

    Method

    In this setup, less light reaches the screen beneath a crest region, so it appears darker.

    Reason

    The curved/thicker water region changes the light distribution; the neighbouring trough region appears brighter.

    Working

    Crest bands can therefore be dark.

Examiner practice 4

Wavelength from 8 spacings

3 marks

Examination question

The distance from crest 2 to crest 10 is 18 cm. Find the wavelength in metres. [3 marks]

Try this before viewing the solution

View solution step by step

  1. Count spacings

    1 mark

    Method

    Use eight wavelengths.

    Reason

    10-2 = 8 crest-to-crest intervals.

    Working

    n = 8.

  2. Average

    1 mark

    Method

    Divide 18 cm by eight.

    Reason

    The measured span covers eight wavelengths.

    Working

    λ = 2.25 cm.

  3. Convert

    1 mark

    Method

    Convert centimetres to metres.

    Reason

    The requested answer is in SI units.

    Working

    λ = 0.0225 m.

Challenge 5

Frequency from wave speed

Minimal support

Changed-unknown transfer

Wave speed is 0.30 m s⁻¹ and wavelength is 3.0 cm. Find frequency.

Try this before viewing the solution

Unit: Hz

Hints

Hint 1: change both unit and unknown
3.0 cm = 0.030 m and f = v/λ.
View solution step by step

  1. Prepare the data

    Method

    Convert the wavelength to 0.030 m.

    Reason

    It must match the metre-based speed.

    Working

    λ = 0.030 m.

  2. Rearrange and calculate

    Method

    Use f = v/λ.

    Reason

    Frequency is the number of wavelengths passing per second.

    Working

    f = 0.30/0.030 = 10 Hz

7. Mind Stretchers

Mind stretcher 1: Which quantity changes?Extension

In a ripple tank, the dipper frequency is increased but the water depth stays the same. What happens to wavelength?

Show Answer

For the same water depth, wave speed is approximately constant. Since v = fλ, if f increases then λ decreases.

Mind stretcher 2: Reducing uncertaintyExtension

Why is it better to measure the distance across 10 wavefront spacings than across 1 wavefront spacing?

Show Answer

The absolute ruler uncertainty is similar, but the measured distance is much larger, so the percentage uncertainty is smaller. Dividing by 10 gives a more accurate wavelength.

8. Practice and next step

Plan a wavelength measurement across at least five wavefront spacings and state the controlled depth and source frequency. Then continue to sound production and propagation.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027