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I Why the Frequency of Light Does Not Change Through a Medium

  1. Jun 1, 2018 #1
    My question is about why the frequency of light or another EM wave does not change while passing through a medium. We know their Speed decrease and wavelenght change but think about this analogy for ex i am 4 meters high and drop 5 balls in 5 seconds and my friend waits at ground he will receive 5 balls in 5 seconds too the frequency is 1 ball for second now in second example i drop same 5 balls from 4 meters high but at this time 2 meters of this lenght is water (Pool) and my friend is waiting at the bottom of pool. When the balls touch the water they are gonna slow down and my friend will her 5 balls in later time of seconds for ex 20 seconds and now the frequency has changed as 0.25 balls for second. The speed decreased and the frequency also decreased but why this does not valid for light. You can think this for WiFi signals when you put a wall between router and your laptop the signal level will fall. I know the some of the signal will be emitted by the wall but at the same time the speed of wifi signals will decrease and my laptop will not receive the same number of signal at same time period that is to say the frequency will decrease. Can anyone explain this please
  2. jcsd
  3. Jun 1, 2018 #2
    First you may want to re-think your example.
  4. Jun 1, 2018 #3


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    Try to imagine a way of actually making it change and you will see that not changing is the only way a wave can propagate on a totally passive path. How could one part of the wave be oscillating faster (or slower) than the part just before it? Where would the energy be coming from to make this happen at every point on the path?
    It may be easier to consider a wave travelling along a string, rather than light because there really is an 'up and down' to think about. All the same arguments apply and it is much more tangible.
  5. Jun 1, 2018 #4
    I only want to understand if speed of EM wave decreases through medium, how a receiver gets that same wave same per time
  6. Jun 1, 2018 #5


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    The front end of wave 2 leaves the emitter 1 sec after the the front of wave 1. The light travels at c in a vacuum until it hits a medium where the speed at which the light travels is 1/2 c. If we assume that the width of this medium is 1 light sec then when the front of wave 1 will take 2 sec to cross the medium and thus reaches the end of the medium 2 sec later. The front of the second wave hits the medium 1 sec after the front of the first wave. It also takes 2 sec to cross the medium, and thus also reaches the the end of the medium 2 sec later. Since the time taken by each wave to cross the medium is the same, they will reach the end of the medium with the same separation in time (1 sec) as they entered it, and there is no change in frequency.

    The same holds for your example of dropping balls in water. If the balls hit the water 1 sec apart, and each ball takes a equal amount of time to travel from the top of the pool to the bottom, then your friend at the bottom will receive balls at 1 ball per sec.
  7. Jun 1, 2018 #6
    I was referring to the balls in the water, they will not be 4 sec apart they will remain 1 Sec apart.

    As for the WiFi - the lower data transfer rate, has nothing to do with the frequency of the signal at the receiver, the walls (and everything) absorbs some of the energy, affecting the error rate. This is a more complex issue - more factors.

    How do we define frequency, consider the units. Does it have any thing to do with speed? What is the common factor in both frequency and speed.

    Can you think of a case where the receiver DOES receive a different frequency?
  8. Jun 1, 2018 #7


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    It doesn't. The interval between the arrival of successive wave crests does not change, but the time for any given crest to move from source to receiver does increase.
  9. Jun 1, 2018 #8


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    As others have hinted, this is not correct. The balls will still arrive at a frequency of 1 ball per second. You should work out the math on this for yourself.
  10. Jun 1, 2018 #9


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    And I have given an alternative form of wave (for which the constant frequency criterion must also apply) which would probably be easier to get your head around. It is a basic function of all waves so why not consider a simple form to get started with?
    The example of a stream of balls is not hard to appreciate and you could also consider a constant stream of traffic on a motorway. If twenty cars a minute are moving down the road then twenty will enter one end and exit the other but when they vary their speed on the strip, it is the separation that changes (wavelength). If the rate of cars passing different sections of road were to be different then there would have to be extra cars injected into the system or taken from it. You could achieve this with extra intersections but a wave on a string or a beam of light has no such facility available.
  11. Jun 2, 2018 #10
    Frequency has nothing to do with what medium the wave is traveling through, it's only related to the source.
    See it this way: a EM full wave is produced when a charged particle goes up, down and once again up to the previous position... Now when it reaches a higher density medium the wave doesn't disappear, it just moves forward more slowly.
    In the image I think you can see it more clearly:
    On the top is when frequency is unchanged and on the bottom is when frequency is changed. In the same period of time part of the wave is not produced yet

    I hope I made sense :)
    PS: I know, I messed up and wrote whater first then fixed it XD
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