Science labs › Physics › Doppler Effect
A moving dipper squashes its ripples in front and stretches them behind.
Doppler Effect lab · Last updated: 16 October 2026
A dipper bobs in a water tank, making rings of ripples. When it moves, the ripples bunch up in front and spread out behind. You change the speed and see the same effect that makes an ambulance siren drop in pitch as it passes.
What you'll discover
- A moving source squeezes its waves together ahead of it and stretches them behind.
- Squeezed waves reach you more often, so sound seems higher; stretched waves sound lower.
- The faster the source moves, the bigger the change.
The science in plain words
When a still source makes waves, they spread out evenly in circles, with the same distance between crests in every direction. Now let the source move. It makes each new ripple a little further forward than the last, so the crests ahead of it are crowded together and the crests behind it are spread out.
If you stand in front, crests arrive more often, so you meet a higher frequency. Standing behind, crests arrive less often, a lower frequency. For sound, frequency is pitch, so an approaching siren sounds higher and a departing one sounds lower. The siren itself never changes. This is the Doppler effect, named after the Austrian scientist Christian Doppler, who described it in 1842.
The faster the source, the bigger the shift. If the source moves as fast as its own waves, the crests pile up into a single strong front; for sound, that is the shock wave of a sonic boom.
The Doppler effect is used in police speed guns, weather radar that tracks storms and ultrasound scans that measure blood flow. Astronomers use it with light to tell whether stars and galaxies are moving towards or away from us.
Where it fits
- India, NCERT: The Doppler effect goes beyond the NCERT syllabus for classes 6 to 10; it builds on frequency and pitch in Class 9 Chapter 11: Sound.
- United States: Grades 9 to 12
Common questions
Why does an ambulance siren change pitch as it passes?
As it approaches, its sound waves are squeezed together and reach you more often, so the pitch sounds higher. As it moves away, the waves are stretched and the pitch drops.
Does the siren really change its note?
No. The siren makes the same note all the time. Only the waves reaching you change, because the ambulance is moving relative to you.
Where is the Doppler effect used?
In speed cameras and police radar, weather radar, ultrasound scans of blood flow and in astronomy to measure how fast stars and galaxies are moving towards or away from Earth.
Related labs
- Sound Waves: A speaker squeezes the air. Watch the squeezes march across the stage.
- Wave Motion: Shake one end of a cord. How fast does the wave run, and how long is each wave?
- Sonar & Echolocation: Ping the water and listen. How long until the rock answers?
See all physics labs or every Taz science lab.
Try it yourself
The Doppler Effect lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.