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Do You Have a Science Question for our Community?

Do You Have a Science Question for our Community?

One of our members Miz Abynaah wanted to know more about the Doppler Effect ( Mike Aben  wrote this terrific explanation below as well as sharing this great video by +Alt Shift X.

Mike Aben: The Doppler Effect

The Doppler effect occurs whenever an object emitting waves is moving.  The effect is observed with any waves (water, sound, light, etc).  The waves being emitted ahead of the object’s motion are compressed, which means they will reach an observer at a higher frequency than if the object was stationary.  The waves emitted behind the object are stretched out and will reach an observer at a lower frequency.

With sound waves, this affects the pitch of the sound heard.  For example, the sound of a car traveling towards you will be at a higher pitch than the sound of a car moving away from you.  With visible light, it is the colour of the light that is affected.  Lower frequency light is redder, while higher frequency light is bluer.  For the effect to be noticeable with light, the relative velocity of the object to the observer has to be very high.


This post was chosen by Zuleyka Zevallos  for #SoG+CuratorsChoice  #physics   #science


Join the Conversation


  1. Zamokwakhe Sishi The speed of light can be measured in the laboratory rather easily – no fancy equipment needed, just some spinning mirrors, a longish baseline, and some careful observation (read up on Fizeau’s mirrors) and you’ll get answers to a few % of the real value in vacuum.

    The distances to stellar objects are calculated in a number of ways. You see, there are, in the universe, certain events that always yield a predictable result. For example, there are stars known as Cepheid variables. These stars slowly pulse – and their speed of pulsing depends on their mass only. So if you see a star, pulsing at a certain speed, then it must be a certain size (and thus, brightness). If the star seems rather faint, then it has to be farther away than if it seemed more luminous. Other examples are novae – they, we think, go ‘bang’ every so often, and the size of the explosion is expected to be constant. So if a nova goes off, and it looks rather faint, then it’s probably far away.

    Check out the ‘cosmic distance ladder’ on wiki. Plenty of other examples there, but a great question! Thanks!


  2. Dan Corby  

    Okay, try this. The good ol’ double-slit experiment. Weak light source, two slits, screen: the usual.

    The wavefunction that describes the photon, as it leaves the source, has a non-zero value at both slits. It’s just a function – a mathematical contrivance that depends on time, location, etc. This function is complex (has a real part and an imaginary part). So the function has a perfectly legit value inside either slit – but when you look at the screen, treating the two slits as sources, the complex parts of these two new functions leads to interference. And if you don’t muck with the slits, then a lovely interference pattern is seen.

    But the minute you try to figure out where each photon went, the pattern is destroyed. Put a 50:50 mirror in one slot and a detector and you’ve constrained the wavefunction either to be zero valued or not. In either case, the pretty pattern goes to the single-slit blur.

    I did this 25 years ago in my 3rd year physics practicals – awesome stuff.


  3. Zamokwakhe Sishi my guess is that on a large scale, you can calculate it, but of course, across a room, it is too tought to gague the speed. It takes 8 seconds for light to travel from the earth to the moon, so (distance in 8 seconds) can be raised to a LCD of one minute, or one hour for MPH.


  4. fratam junior i heard that when you think over a question, and find 2 answers, the first one is more likely to be right. Test this out with difficult questions, it might be true.

    also, Sherlock Homes quote- eliminate the impossible, and that which remains, however improbable, must be true

    good luck, and eat some brain food. fish is good for staying mentally sharp


  5. There is a change in frequency and wavelength of waves with respect to an observer. Waves infront will be compressed hence high frequency.

    While those at the back are stretched hence low frequency and high wavelength.

    This explains why d car approaches an observer with high note of pitch than wen leaving.


  6. I like the Doppler Effect! I am a physicist, and former/retired college professor at Wilberforce University, Ohio. I wrote a computer program for a problem on this subject. I could give velocities of a sound  source and a listener, stationary or moving, in given directions, along a straight line. Could print the two dimensional wave fronts of the source heard by the listener. I was getting beautiful two dimensional printouts, Loved It!

    Let me hear from you about your views!



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