Ripple tank with two sources

In the picture

1. 1Left loudspeaker: sends out one set of circular water ripples
2. 2Right loudspeaker: sends out a second set of circular water ripples
3. 3Bright crossing crest: two crests meet and reinforce
4. 4Calm patch between bright lines: a crest meets a trough and cancels

Examples

• Two stones dropped together in a pond create areas of strong and weak ripples
• Noise-cancelling headphones add an opposite sound wave to cancel unwanted noise

Quiz: Ripple tank with two sources (Age 17)

1. 1

   When two wave crests from coherent sources meet, what happens?

   • They cancel each other and produce a calm patch.
   • They keep a constant phase difference but do not overlap.
   • They reinforce each other and produce a bright line.
2. 2

   When a crest from one coherent source meets a trough from the other, what is observed?

   • A calm patch, because the waves cancel.
   • A permanent change in the phase difference between the sources.
   • A bright line, because the waves reinforce.
3. 3

   Why does the interference pattern stay in the same fixed positions when the sources are coherent?

   • Because the sources keep a constant phase difference.
   • Because each source emits ripples at a different steady rate.
   • Because the ripples from one source are blocked before they can overlap.

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.