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Wavefronts and ripple tanks

Key idea: A wavefront joins points at the same stage of vibration. Waves travel perpendicular to their wavefronts, and neighbouring crest wavefronts are one wavelength apart.

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Drop a pebble in a still pond and rings of ripples spread out from it. Water waves are hard to watch closely in a pond, so a ripple tank makes them in a shallow tray and projects them onto a screen. Seen from above, each crest becomes a line called a wavefront, and the waves travel at right angles to those lines.

Wavefronts in a ripple tank

A ripple tank is a shallow tray of water with a light above it. A dipper, such as a straight bar or small ball touching the surface, is moved up and down by a small motor. Its repeated vibrations produce ripples that spread away from it.

The curved water surface redirects the light, producing bright and dark bands on a screen below. Each crest focuses the light like a lens, so the bright bands show where the crests are.

Ripple-tank apparatus and projected patternA lamp is above a shallow transparent-bottomed water tank. A vibrator drives a dipper touching the water surface. Dashed lines indicate illumination towards a white screen below; their spacing is illustrative, not a calibrated optical construction. The screen projection can have a different scale from the water plane.Ripple tank: side view of the apparatusLamp above the tankVibratorTransparent tank baseShallow waterDipperWhite screen below the tankScreen spacing may differ from water wavelength. Check the scale.Wavefront shapes are shown separately in plan view.
Ripple-tank apparatus and projected patternA lamp is above a shallow transparent-bottomed water tank. A vibrator drives a dipper touching the water surface. Dashed lines indicate illumination towards a white screen below; their spacing is illustrative, not a calibrated optical construction. The screen projection can have a different scale from the water plane.Ripple tank: side view of the apparatusLamp above the tankVibratorDipperShallow waterTransparent tank baseWhite screen below the tankScreen spacing may differ from water wavelength. Check the scale.Wavefront shapes are shown separately in plan view.
A vibrator disturbs shallow water. Light through the transparent base forms a pattern on the screen. The apparatus drawing explains the arrangement; it is not an optical ray calculation or a measured pattern.

The apparatus diagram is a side view: it shows the lamp, water and screen at different heights. A plan view looks down at the water surface. It shows where the ripples are across the tank, not how high the water is.

Each bright line in the plan view joins points that are all at the top of the same crest. A line joining points of a wave that are all at the same stage of their vibration is called a wavefront. We say these points are in phase. On a crest, every point is at its highest position at the same moment, so a crest is a wavefront. You could instead choose trough wavefronts, but do not mix crests and troughs in one set: they are at different stages of vibration.

  • A vibrating straight bar makes straight (plane) wavefronts.
  • A vibrating ball, or a finger dipped at one point, makes circular wavefronts that spread out from the point.
  • The wave travels at right angles to its wavefronts. Draw direction arrows perpendicular to the lines, not along them.
  • Neighbouring crest wavefronts are one wavelength apart.

The two panels below are plan views at one instant in water of uniform depth. Trace each arrow: it crosses the wavefronts at right angles. The arrows show propagation, not water flowing out from the source.

Wavefronts from line and point sourcesTwo plan views compare a vibrating straight bar and a point dipper in uniform-depth water. Parallel wavefronts propagate away from the bar. Concentric circular wavefronts propagate radially outward from the dipper. Rightward arrows are perpendicular to the wavefronts where they cross them; they do not show particle flow.Line source: plan viewPoint source: plan viewVibrating straight barStraight wavefrontsTravel is perpendicular to the lines.Circular wavefrontsPoint dipperTravel is radially outward.Each line or circle joins points in phase. Arrows show propagation, not water flow.
Wavefronts from line and point sourcesTwo plan views compare a vibrating straight bar and a point dipper in uniform-depth water. Parallel wavefronts propagate away from the bar. Concentric circular wavefronts propagate radially outward from the dipper. Rightward arrows are perpendicular to the wavefronts where they cross them; they do not show particle flow.Line source: plan viewVibrating straight barStraight wavefrontsTravel is perpendicular to the lines.Point source: plan viewCircular wavefrontsPoint dipperTravel is radially outward.Each line or circle joins points in phase. Arrows show propagation, not water flow.
At uniform depth, a straight bar gives approximately straight wavefronts and a point dipper gives circular ones. Propagation crosses the wavefronts at right angles; the wavefronts join points in the same phase at one instant.

Check your understanding 1

In a plan view, straight wavefronts run diagonally from the bottom left of the tank to its top right. The vibrating bar is in the top-left corner, lying along the same diagonal direction. A student draws the direction arrow pointing to the top right. What is wrong, and which way should the arrow point?

Show answer

The student’s arrow lies along the wavefronts, but the wave does not travel along a wavefront: every point on it is at the same stage of vibration. The wave travels at right angles to the wavefronts and away from the bar, so the arrow should point towards the bottom right, crossing the wavefronts at 90°.

Check your understanding 2

Crests lie 2, 6 and 10 cm from a vibrating bar, with troughs halfway between. A learner marks wavefronts at 2, 4, 6, 8 and 10 cm. Which set contains neighbouring crest wavefronts, and how far apart are they?

Crest wavefronts
Unit: cm
Show answer

The learner has mixed crests and troughs, which are at different stages of vibration, so they are not in phase. Draw the crests at 2 cm, 6 cm and 10 cm (or the troughs at 4 cm and 8 cm), not both. Neighbouring crest wavefronts are 4 cm apart: one wavelength.

Check your understanding 3

Two plan views show straight crest wavefronts 2 cm apart and 4 cm apart. A student concludes that the second wave has twice the amplitude. What has actually doubled, and what further view or measurement is needed to compare amplitudes?

Show answer

The wavelength has doubled, from 2 cm to 4 cm. A plan view shows crest positions across the water surface, not their height. To compare amplitudes, use side views or measurements of the maximum vertical displacement from the rest level. The wider wavefront spacing alone gives no amplitude comparison.

Drawing wavefronts and travel arrows

Worked example 1

A straight bar at the left of a tank

Problem

A straight bar vibrates up and down along the left side of a ripple tank. The water has uniform depth. Sketch a plan view of the waves travelling into the tank and add a direction arrow.

Show full solution

  1. Use the source to choose the shape

    Method

    Draw wavefronts at one instant, then choose the travel direction.

    Reason

    A straight vibrating bar produces approximately straight wavefronts. Propagation is perpendicular to them and away from the source.

    Working

    Answer

    1. Draw the bar as a vertical line at the left. You are looking down, so you do not draw the up-and-down vibration as a wavy line.
    2. Draw several straight, parallel crest wavefronts to the right of the bar. Keep their spacing equal for a steady source in uniform-depth water.
    3. Add a horizontal arrow pointing right, across the wavefronts. Check that it crosses them at 90° and points away from the bar.

    This sketch matches the left panel above. An arrow along a crest would show the wrong direction: points along that crest are at the same stage of vibration; the crest travels across the tank.

Check your understanding 4

Replace the bar with a point dipper at the centre of the tank. Draw the new plan view on paper. What changes in your sketch? Add a travel arrow towards the top edge, then compare your sketch with the answer.

Show answer

Compare your sketch: draw concentric circles centred on the dipper instead of parallel straight lines, with equal spacing for a steady source in uniform-depth water. Label these as crest wavefronts, not water height. At the top of each circle, the direction of travel is upwards, along a radius. The arrow crosses the circles at right angles. It shows the wave travelling outwards, not water particles flowing to the top edge.

Check your understanding 5

A projected screen pattern has neighbouring bright bands 4 cm apart. What two things should you check before saying that the wavelength in the water is 4 cm?

Show answer

Check that the bright bands are neighbouring crests, and whether the pattern on the screen is enlarged. The light spreads out on its way to the screen, so the pattern is usually larger than the waves in the water; use the scale of the projection to find the real wavelength.

Check your understanding 6 Use a measured span and projection scale

Five consecutive crest bands span 12 cm from first to fifth on a screen. The projection doubles every distance in the water. How many crest-to-crest gaps are there? Find the screen spacing and the wavelength in the water.

Show the steps Optional working
Unit: cm
Show the steps Optional working
Unit: cm
Unit: cm
Show answer

Five consecutive crests have four gaps. The screen spacing is 12/4 = 3.0 cm per gap. The projection doubles distances, so the wavelength in the water is 3.0/2 = 1.5 cm (0.015 m). Dividing by five would count crests rather than gaps; using 3.0 cm would omit the projection correction. You can instead correct the whole span first: 12/2 = 6 cm in the water, then 6/4 = 1.5 cm.

Common mistakes

  • Drawing the direction arrow along a wavefront. The wave travels at right angles to its wavefronts, away from the source.
  • Joining a crest to a trough as one wavefront. A wavefront joins points at the same stage of vibration.
  • Drawing a plan view as a wavy line. Looking down, each crest is a line; the up-and-down shape belongs to a side view.
  • Reading arrows on a wavefront diagram as water flowing away from the source. They show the direction the wave travels.

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