Properties of waves that change from moving from one medium to another

[reising1]

When waves move from one medium to another, I was taught that the only thing that changes is the wavelength (and hence the speed).

What about waves that reflect? Is this the same rule? I just did a question saying that when sound waves travel through air and and bounce off an imperfect reflector, the speed stays the same before and after. I thought wavelength changed and hence speed?

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[catzzz88]

Hi there! Just wanted to clarify--when waves travel from one medium to a denser medium, the speed decreases and thus the wavelength decreases, but frequency always remains the same. Waves with a higher frequency are refracted more strongly than waves with a lower wavelength (thus we see diffraction). For the level of physics on the MCAT, frequency of light can be changed only from the sense of relativity (think doppler effect).

Waves that reflect off of a surface never change in speed, wavelength, or frequency. However, the amplitude can decrease due to incomplete reflection. Also, the wave can be flipped when traveling from a denser medium and reflecting off of a less dense surface (as in reflecting from water to air). In this case, the wave flips 180 degrees.

Hope this helps! 🙂

 

[Ron1x]

If it's reflecting, then the medium before and after is the same, hence no change.

 

[reising1]

Thank you for the replies, but I think there is some level of confusion happening. See this question from an old exam:

Question:
Sound of a known frequency, wavelength, intensity, and speed travels through air and bounces off an imperfect reflector which is moving toward the source. Which of the following properties of the sound remains the same before and after reflection?
A. Speed
B. Intensity
C. Frequency
D. Wavelength

The answer is A.

Why is this? Doppler should be affected the frequency but doesn't it also affect the speed?

 

[QrtrLifeCrisis]

Thank you for the replies, but I think there is some level of confusion happening. See this question from an old exam:

Question:
Sound of a known frequency, wavelength, intensity, and speed travels through air and bounces off an imperfect reflector which is moving toward the source. Which of the following properties of the sound remains the same before and after reflection?
A. Speed
B. Intensity
C. Frequency
D. Wavelength

The answer is A.

Why is this? Doppler should be affected the frequency but doesn't it also affect the speed?

Yes, that is correct. The Doppler effect changes both the frequency and the wavelength of the wave reflecting back so that the speed of sound itself remains constant. You could not use the Doppler formula otherwise if the speed of sound changed. The only time the speed of sound changes is if it goes from one medium to another. In this case, that is not happening. The reason why Intensity is incorrect is because of the fact that it reflected off an imperfect reflector, losing some of its intensity coming back.

 

[SSC2]

Thank you for the replies, but I think there is some level of confusion happening. See this question from an old exam:

Question:
Sound of a known frequency, wavelength, intensity, and speed travels through air and bounces off an imperfect reflector which is moving toward the source. Which of the following properties of the sound remains the same before and after reflection?
A. Speed
B. Intensity
C. Frequency
D. Wavelength

The answer is A.

Why is this? Doppler should be affected the frequency but doesn't it also affect the speed?

Also note: Speed is determined by medium. Medium before and after the wave hits the "imperfect reflector" is the same. Air, as noted in the question, is the medium.