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### Course: Class 11 Physics (India)>Unit 19

Lesson 7: The Doppler effect

# Doppler effect formula for observed frequency

The Doppler effect is the change in frequency of a wave as the source moves relative to an observer, and explains why the pitch of a sound sometimes changes as it moves closer or further to or from an observer. Learn how to derive the formula for perceived frequency of a sound using an equation that accounts for a sound and observer’s velocities relative to one another. Created by Sal Khan.

## Want to join the conversation?

• But, isn't the velocity of the wave for moving particle the sum Vw and Vs, because the speed of the wave would also include the speed of the object emitting the light?
• The speed of the sound wave is not the speed of the particles moving, but the speed at which the peaks (compressed air) propagate through the medium that the sound is travelling in. It is not related to the speed of the emitting source. If you think this is unclear, you should watch the earlier videos in this series.
• im still really confused... can someone explain the Doppler effect in simple terms??
• When the sound source is approaching the observer or when the observer is approaching the sound source, the apparent frequency of the sound source increases. You notice this when a train whistling is approaching you. The apparent frequency of the whistling train is larger compared to the frequency of the whistling when both you and the train are stationary. However if the train is approaching you, then you'll notice an apparent increase in frequency, which is perceived by the higher pitch of the train's whistling sound, keeping in mind that higher pitch is associated with higher frequency sound.
• What happens if the wave source is moving faster than the waves themselves?
• Since the source is moving faster than the sound waves it creates, it actually leads the advancing wavefront. In the case of Sound, The sound source will pass by a stationary observer before the observer actually hears the sound it creates. " SONIC BOOM" is heard and seen when source or object breaks the speed of sound
• What would happen if the source travels faster than the wave?
• this can only happen with sound waves in which one experiences a sonic boom by the pressure difference at the front and rear of a aircraft. since other electromagnetic waves travel at the speed of light,no source can travel faster thus making such a situation impossible
• What happens when things approach the speed of light
• When an object approaches the speed of light very closely (Very little difference), it's mass increases and goes up, and so does the energy it is carrying (Kinetic, Mechanical, and Potential), at one point both reach infinity when it hits the exact same speed of light. This arises a lot of complications, such as dealing with the Laws of Conservation of Energy and Matter, Time Dilation, and much more, which is why he wanted to avoid it. Traveling at such a speed (or very close to it) means that waves can't travel as fast enough, including sound and light, causing them to stay in front of the object, and they would be unable to leave the object, and instead pile up on the front. (Fun fact, we don't actually know the true speed of Light in one direction, only an mean of light travelling in two directions. It is also impossible to travel at the speed of light.)
(1 vote)
• Can't we write To = Ts [ ( Vw - Vs ) / Vw ] as
To = Ts [ ( 1 - Vs / Vw ) ] ?
• In fact, this is the best way to express this equation
• I didn't exactly understand how Sal represented the crest of the waves in the form of circles. Please help..
• It's like when you throw a stone in a pond. It makes circular ripples that spread out from the stone.
• Sal says that in the last video he showed the formula for doppler effect when the object is moving towards you. Where is that video
• Is Sal saying that the time taken by source and time taken by wave both are equal? because in calculating its distance he has used the same time(T sub S). And if the time is different how is the calculation going to differ?