1. ### Blood flow velocity via Doppler effect

Homework Statement A Doppler flow meter is used to measure the speed of red blood cells. The frequency of the apparatus is f = 12 MHz. The sensor in the apparatus measure 1.8 kHz beats between the emitted frequency and the frequency of the ultrasound reflected back by the blood cells. The speed...
2. ### Insights Coordinate Dependent Statements in an Expanding Universe - Comments

Greg Bernhardt submitted a new PF Insights post Coordinate Dependent Statements in an Expanding Universe Continue reading the Original PF Insights Post.
3. ### I New possible explanation for Hubble redshift

I just saw this news article on the home page of the university I'm working in: http://www.aalto.fi/en/current/news/2017-06-30/ It seems to be about a study that can potentially explain the redshift of distant stars with a mechanism that's related to the interaction of photons with the...
4. ### Insights Relativity Variables: Velocity, Doppler-Bondi k, and Rapidity - Comments

robphy submitted a new PF Insights post Relativity Variables: Velocity, Doppler-Bondi k, and Rapidity Continue reading the Original PF Insights Post.
5. ### Doppler shift derivation from Lorentz Transformations

Homework Statement Show how one can obtain the Doppler transformation for the frequency of a receding source just using the Lorentz transformations for the energy (where E=h). Homework Equations Relativistic transformations for momentum and energy: E = γ(E' + vp'x) pc/E = v/c = β The Attempt...
6. ### On the definition of width in Doppler broadening problems.

The following question has stumped me. I am not getting the answer marked. Instead I am getting option a). I have produced my attempt below. Also I have no idea which formula to use when the question says Doppler width. Since frequencies form a distribution, there are multiple notions of width...
7. ### I Doppler Effect Question

Let's say you have a sound emitter and receiver a fixed distance apart on a moving object but open to the air, such as on the hood of a car. The emitter is near where the windshield wipers are, and the receiver is closer to the grill. They are in aligned along middle of the car, longitudinally...
8. ### Can we hear a supersonic plane?

A very clever first-year physics student I know, who had just been learning about the Doppler effect, asked me a question. If a plane were flying a straight trajectory at Mach 2, playing a song on its speakers very loudly, and an observer with incredibly sensitive recording equipment were to...
9. ### I Is it possible to image individual air molecules?

I was wondering if it is possible to image the motion of individual air molecules? What I am picturing is using a laser to illuminate a volume of air, and using scattered light to measure the velocity of individual air molecules through the doppler shift of the scattered light. It seems that...
10. ### I Doppler effect with moving medium but no relative motion

I would like to be sure about one case of the use of Doppler effect with sound waves. If the medium (in the case of sound air) is moving, but there is no relative motion between the observer and the source there is no Doppler effect at all. (And the absence of relative motion is...
11. ### Problem on Doppler effect and sound level

Homework Statement A source emits a spherical sound wave at frequency ##f=110Hz## with power ##P=1 W## in a solid angle of ##2 \pi sr##. An observer moves with velocity ##v_{oss}=108 km/h## towards the source. Determine the sound intensity level received by the moving observer. Homework...
12. ### I Doppler shift and change in intensity of a sound wave

How are the intensity of a sound wave and the Doppler shift of frequency related togheter? That is, if the source or the observer are in relative motion, how does the intensity change? For a sound wave $$I=\frac{1}{2} \rho \omega^2 A^2 c=2 \pi^2 \rho f^2 A^2c$$ (##c## is sound speed, ##\rho##...
13. ### B Sound waves and speed of it in different reference frames

Think at a cop car with a siren that moves with a velocity Vc, it emits a sound with a velocity C Now think about a person that doesn't move, in front of the cop car, shouldn't he register that the speed of the sound emited is Vs= Vc+ C? According to the galileian equations (true for v <<c...
14. ### B Photon red shifting and Doppler effect

When thinking of sound waves (or more commonly seen waves) the doppler effect is fairly obvious. A car is moving away from you, it's horn's membrane is fluctuating, increasing & reducing air pressure over time. So if it's moving away from you the sound's peak would occur further away...
15. ### Deriving E=mc^2 from relativistic doppler effect

How is E=mc^2 derived from relativistic doppler effect? I looked up on wikipedia but it seems to have some errors. https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#Alternative_version
16. ### Doppler effect for accelerated source

Homework Statement At the initial moment source and detector are located in same point and are both in rest. Source is released into free fall and detector remains in rest. Source has frequency fs=512 Hz and speed of sound is v=340 m/s . When detector detects frquency f =485 Hz , what...
17. ### On the usual confusion among what it is and what is measure

I have found a book from my father epoch of undergraduate student in Physics. It was apperently Physics 101, and was written with a typewriter and the formulae with pen. I have gone straight to Special Relativity section and, to my dismay, and even if all formulae were indeed correct, the...
18. ### Expansion is a compounded error based on a wrong assumption?

I accept that what I write below will be unpopular and argued with vehemently, however I think it should be considered with an open mind and recognised for the sense that it has. Originally the concept of an expanding universe (and therefore by extrapolation the concept of the Big Bang that...
19. ### Measuring speed

Dear PF Forum, I'd like to know how to measure speed only by doppler effect. What if we don't know the frequency? What if we don't know the distance? If we look this diagram on our computer screen, we'll know everything. Distance, speed, time dilation event relative simultaneity of event. But...
20. ### Doppler, mirror vs lamp

Dear PF Forum, I have a question again about Doppler effect in relativity. Before I ask about mirror and lamp, I want to refresh the discussion again as in my previous thread: https://www.physicsforums.com/threads/doppler-for-light.823942/ The formula for Doppler factor for light is. ##f =...
21. ### Lorentz and Doppler and signal pick up

Dear PF Forum, After so many questions about Twins Paradox and Universe Frame of Reference, I'd like to know about Lorentz and Doppler. First. Here V is ##\sqrt{0.75}## ≈ 86.60%. If we put V in Lorentz Transformation formula as speed, we'll have Gamma = 2. Okay here's the question. Two probes A...
22. ### Rearrange Doppler Equation

Hi, So i know that the doppler equation is as follos; F(observed)= (Vsound + Vdetector) / (Vsound +- Vsource) x F(emitted) Does anyone know how to reaarange the equation to solve for the other variables?
23. ### Stretching Light

Is it possible to create a doppler shift of EM radiation in a lab? or is it only feasible on universal scales? I was just thinking that high energy x-rays have a sub-atomic wavelength,could these be transformed to visible light to let us 'see' an atom?
24. ### Doppler effect - Apparent change in frequency or wavelength?

Hello, I've been thinking about what exactly is the doppler effect and I'm not sure I understand it because I thought it meant a change in frequency due to relative motion while at the same time its defined in terms of changing wavelength such as in red shift or blue shift. Case 1 The...
25. ### Doppler effect w/ percentage frequency shift?

Homework Statement As you stand by a railroad track a train passes with its whistle blowing. As it passes, you hear a frequency shift equal to 21 % of the frequency of the whistle. How fast is the train moving. Express your answer in km/hr.(Speed of sound is 331.4 m/s) Homework Equations f’...
26. ### Doppler effect in modulated signals

Hello, If been for a while trying to find information about how Doppler effect affects modulated signals. When a transmitter is moving relative to a receiver, the receiver will have a shift in the carrier frequency that will vary as the transmitter gets closer or moves away from the...
27. ### Doppler effect speed of sound

Homework Statement Problem: a car is moving away from a stationary observer at 25ms^-1. It emits a frequency of 810Hz and the observer hears an apparent frequency of 762Hz. What is the speed of sound in air at that time? Homework Equations The equation for a source moving away is f' =...