Two dippers in a water tank

In the picture

1. 1The left dipper: one source sending out circular ripples
2. 2The right dipper: a second source sending ripples in step with the left one
3. 3A tall bright ridge: crests from both dippers arrive together
4. 4A flat calm strip: a crest from one dipper meets a trough from the other
5. 5The central bright ridge: water here lies the same distance from both dippers

Examples

• Two loudspeakers playing the same note make loud patches and quiet patches across a room.
• Noise-cancelling headphones add an opposite sound wave and flatten the noise.

Quiz: Two dippers in a water tank (Age 17)

1. 1

   For two sources that are in step, what decides whether a point in the water is on a bright band or a calm band?

   • The amount of time since the sources first started
   • The distance from that point to the nearer source only
   • The difference in distance from that point to the two sources
2. 2

   What happens where crests from both in-step sources arrive at the same point?

   • They reinforce each other and make a high ridge
   • They cancel each other and leave the water flat
   • They switch quickly between a high ridge and a flat strip
3. 3

   What happens where a crest from one source meets a trough from the other source?

   • The water rises into a bright ridge because the crest is stronger
   • The water stays almost flat because the waves cancel
   • The water makes a new crest halfway between the two sources

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.