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## High school physics

### Course: High school physics>Unit 10

Lesson 4: Doppler effect

# Doppler effect introduction

Learn about the Doppler effect and how it explains the change in frequency of a wave when its source and an observer are moving. Created by Sal Khan.

## Want to join the conversation?

• At I don't understand how the observer further away from the moving object has a wave every 1.5 seconds if the object is moving 5m/s and the wave is 10m/s.. Wouldn't it be 2 seconds between each wave? How did you get 1.5 seconds? Is there a calculation that was not shown?
• Hello, desrosierskloe. I read your question. At first it confused me to but later I came up with the solution. As you know the formula for time that is Distance/Speed. Here we know that the waves are travelling at the speed of 10m/s and distance between crest is 5m( towards which the train is coming).Here if you want to know the time period, you simply apply the fomula given above you get to the answer..
The wave is travelling with speed 10m/s and the distance between the crest is 5m. So Time Period = 5m/10m/s
= 5m * 1/10s/m
= 1/2 s
That is what that you get above the answer and hence the frequency 2 hertz.
If you apply this to the observer 2 (from which the train is moving away) you will get the Time Period = 15m/10m/s
= 15m*1/10s/m
= 1.5 s
That's how you got this answer. And in hertz if you reverse this you would get 10/15 that is equal to 2/3. Same as given in the answer.....
Hope you will find that useful..................
• I don't understand this, what is the Doppler Effect?
• The Doppler effect is the change in frequency that you hear when a source and an observer are moving with respect to each other. The classic case is when you have an ambulance racing by you: the pitch of the siren is higher when it is moving toward you and lower as it goes away. The Doppler effect is the mathematical description of this variation as a function of the velocities of the source and of the observer.
• Correct me if Im wrong sal messed up the direction of the wave sources saying it goes to the right but then saying later it's going left. At and
• Since he is going back in time, every time Sal goes back in time, the source will have been more and more to the left, since it is moving to the right, as time progresses positively the other way.
When time progresses positively, the source moves right.
When time progresses negatively, the source moves left.
(this is kind of like slope of a line, if you go forward, it goes one way, but if you go the other way, the slope is opposite too)
The source is moving right, but in the past it was more to the left.
• I am confused- when a source emits waves at a certain frequency, will the observed frequency continually increase as the source (moving at a constant velocity) approaches the observer (stationary), or will the observed frequency be higher but constant?

Thanks!
• Higher but constant if the source approaches the observer at a constant speed. Note that the source being at a constant speed does not mean that it approaches the observer at a constant speed. For example, imagine a car is moving on a straight road at 10 meters per second, and the observer is not on top of this road (so he doesnt get ran by), instead he is 10 meter by the side of the road. The car's approaching speed is not 10 m/s, because the car is not going towards the observer, the approaching speed changes according to the car's position on the road, therefore the observed frequency changes with the position as well.
• What exactly does 'pitch'mean ? I mean i understand frequency period or amplitude but how is pitch different ? ( ı am not a native english speakr ?) thanks
• Pitch is another word for the frequency of sound.
• What if the source moves faster than the wave?
• so how do we know the wavelength of the sound either stationary and moving object?
• Well, if the perceived frequency is higher or lower than the actual frequency, than the sound is moving and if the perceived frequency remains the same than its stationary.

Also, wavelength is inversely proportional to frequency.
(1 vote)
• i dont understand it when sal draws circles and he says they are crests.... makes no sense to ME
(1 vote)
• The circles represent the locations of the crests of a wave. The sound wave travels in all directions, so the crests are equidistant from the center.
Imagine that the sound is just beeps and don't worry about the waves.
• how come he is drawing the waves for the stationary point forward in time and the moving point backwards in time
• Hello... I have a few questions:

Say for example that there is an ambulance with a siren on, and it is not moving. There is also an observer standing nearby, and he is not moving either. Does this mean that the observer will not here the classic siren sound (the changing pitches) since neither he nor the ambulance are moving?

Is the true sound of an ambulance siren actually just one long, straight single pitch?

What does the siren sound like to the ambulance driver / other people in the ambulance?