Ripple Tank

Look down on a tank of water and make ripples. With two dippers the waves add up and cancel out. Through a narrow gap they spread out in all directions.

Make waves with
Show
Wavelength
2.0 cm
Wave speed
20 cm/s
First calm line
10°

The maths, live

Wave speed v = f × λλ = v ÷ f = 20 ÷ 10 = 2.0 cm
First calm line sin θ = λ ÷ (2d)sin θ = 2.0 ÷ 12, θ = 10°

Where a crest meets a trough the water stays calm. Make the gap about one wavelength wide and watch the waves spread out in all directions.

How it works

When two waves meet, they add together. Where a crest meets a crest the water moves twice as much (constructive interference). Where a crest meets a trough they cancel and the water stays calm (destructive interference). With two dippers this makes a fan of calm lines, which spread further apart when the wavelength is longer.

When waves pass through a gap, every part of the gap acts like a new little source. This makes the waves spread out behind the gap, called diffraction. The spreading is biggest when the gap is about the same size as the wavelength. It's the reason you can hear someone talking round a corner.

Try this

  1. With two dippers, lower the frequency. Do the calm lines move closer together or further apart?
  2. Switch to "Calm and rough areas" to see the pattern clearly. Count the calm lines.
  3. Make the gap narrower than one wavelength. How much do the waves spread?