Relationship between frequency, wavelength, and speed

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  • #1
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Hi there,

I am in need of another's thoughts on this topic. I at first thought frequency is something that doesn't change and is inherent to the source, so then increasing the wavelength shouldn't lead to an increase in frequency, but an increase in speed. I realized later that this only applied to situations where there is a change in medium, since speed is a property of the medium. I concluded that in situations with change in medium and constant source, the frequency will stay constant and the relationship holds for velocity/wavelength (directly proportional) = constant.

However, if you were to change the source's frequency (a laser for example), in the same medium, the velocity is now constant and the relationship holds for f*wavelength (f and wavelength being inversely proportional) = constant. Please let me know if my way of thoughts are correct. Thanks.
 

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  • #2
nasu
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The relationship is always the same.
Which parameter changes and which is constant depends on the situation. As for any relationship.

Your analysis seem correct in principle. The velocity depends on the medium and is approximately constant usually. There is a weak (again, usually, in visible range) dependence of the velocity on frequency (see dispersion). This dependence is not covered by the above relationship, it is more complicated.
 
  • #3
davenn
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I at first thought frequency is something that doesn't change and is inherent to the source, so then increasing the wavelength shouldn't lead to an increase in frequency,
it doesn't
at a constant velocity ....
increasing wavelength = decrease in frequency, and visa versa, it's inversely proportional

eg
10 MHz = 30 metres, 100MHz = 3 metres, 1000MHz = 0.3 metres ( 30cm) etc

However, if you were to change the source's frequency (a laser for example), in the same medium, the velocity is now constant and the relationship holds for f*wavelength (f and wavelength being inversely proportional) = constant.
the velocity of the laser will decrease in a medium ... this is the norm for all EM radiation

Dave
 
  • #4
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it doesn't

increasing wavelength = decrease in frequency, it's inversely proportional



the velocity of the laser will decrease in a medium ... this is the norm for all EM radiation

Dave
Dave,
I am not sure if I follow your logic. You said f doesn't change, and then followed by stating it does change with changing wavelength.

Anyhow, I don't think the frequency will change with wavelength change in an example of a typical thin film problem:

monochromatic light goes from lets say air to soap bubble and wavelength of the wave changes as it enters the slower medium. In this case, the frequency is constant and change in velocity leds to change in wavelength of wave.
 
  • #5
davenn
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I am not sure if I follow your logic. You said f doesn't change,
I never stated that .... quote me
 
  • #6
sophiecentaur
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I am not sure if I follow your logic.
This is a very common sort of question from someone who hasn't read around a subject in order to increase understanding. If you read something that someone has posted - off the cuff- on a PF forum and try to hang your complete understanding on it then you can well get the wrong idea. Read a text book (or on-line equivalent) that has been written and formatted with many hours of effort to get the right message across. Compare what it says with what Dave said. Dave is in no way wrong but he may have put it 'the other way round' from what you expected. You have drawn the wrong conclusion from his statement of and inverse relationship - that's all. Frequency cannot change when objects are not moving relative to one another. He did not claim that it does and, if you look at other sources, that is quite clear.
Lesson: do not try to learn a subject by question and answer methods. Learn it by reading a properly structured source and ask the occasional question when things are not quite clear. The speed/wavelength/frequency relationship is perfectly clear as it is a simple three-term equation.
 
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  • #7
sophiecentaur
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Reading through the above, I can see it is a bit on the grumpy side. I actually meant it to be more positive but the message, nonetheless, is that no one can get a good grasp of any Science by Q and A on a Forum. The path is too random for the questioner to get anywhere fast.
"Is this right?"
"No"
"Is this right?"
"No "
"Is this right?"
"No."
etc. Maybe somewhere, after half a dozen questions, we may get somewhere with the problem.

Alternative
Page xyz of book "abc" tells you what you need to know directly. All you need to learn is how to use an Index or how to Search on line.
 
  • #8
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Reading through the above, I can see it is a bit on the grumpy side. I actually meant it to be more positive but the message, nonetheless, is that no one can get a good grasp of any Science by Q and A on a Forum. The path is too random for the questioner to get anywhere fast.
"Is this right?"
"No"
"Is this right?"
"No "
"Is this right?"
"No."
etc. Maybe somewhere, after half a dozen questions, we may get somewhere with the problem.

Alternative
Page xyz of book "abc" tells you what you need to know directly. All you need to learn is how to use an Index or how to Search on line.
Thanks for the tip- Jack
 
  • #9
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The velocity is your constant... It's an electromagnetic propagated wave.... it moves at the speed of light. The variables that can be changed are wavelength
and frequency.
 
  • #10
sophiecentaur
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The velocity is your constant... It's an electromagnetic propagated wave.... it moves at the speed of light. The variables that can be changed are wavelength
and frequency.
Careful about that statement. Once the wave has been launched, the frequency is not a variable. The other two variables are then interrelated as you go from medium to medium. (I am ignoring Doppler, here, of course)
 
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  • #11
sophiecentaur
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It is always important not to confuse the relationships between the variables in a Mathematical expression and the Causal (and often 'one way') relationships between Physical Variables in Science.
Science is a subset of Maths.
 

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