How on Earth does diffraction work?!

Diffraction has always confused me (and probably lots of other people), but I kind of see how when a transverse wave passes through a narrow slit/gap, it spreads out.

However, why does this happen with longitudinal waves (e.g. sound waves) as well, if the oscillations are parallel to the direction of motion? Why doesn't the wave just go "straight through" the gap?

Also, why can sound waves, and long-wave EM radiation, diffract AROUND obstacles such as hills or corners? I've never understood exactly how that works - is there a "gap" in this case?

Urgh, I hate how A level physics doesn't require you to actually understand how any of this stuff works, just to know it happens and learn it for the exams... :iiam:

Thanks...

Reply 1

Kyx

6

Original post by MrLatinNerd

Diffraction has always confused me (and probably lots of other people), but I kind of see how when a transverse wave passes through a narrow slit/gap, it spreads out.

However, why does this happen with longitudinal waves (e.g. sound waves) as well, if the oscillations are parallel to the direction of motion? Why doesn't the wave just go "straight through" the gap?

Also, why can sound waves, and long-wave EM radiation, diffract AROUND obstacles such as hills or corners? I've never understood exactly how that works - is there a "gap" in this case?

Urgh, I hate how A level physics doesn't require you to actually understand how any of this stuff works, just to know it happens and learn it for the exams... :iiam:

Thanks...

Diffraction around corners:

Diffraction of longitudinal waves:

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html

Reply 2

MrLatinNerd

OP

7

Thanks for the reply! Diffraction around obstacles sort of makes sense now, but I still don't understand why longitudinal waves actually diffract - when we draw wavefront diagrams to represent sound waves, aren't we drawing them as though they were transverse waves to make things easier? They don't explain why longitudinal waves actually behave in this way...

Reply 3

MrLatinNerd

OP

7

Also, sorry for all these questions, but how would you explain why longer waves can diffract more easily around large obstacles than shorter waves?

Reply 4

AdamFlatley

1

Original post by MrLatinNerd

Also, sorry for all these questions, but how would you explain why longer waves can diffract more easily around large obstacles than shorter waves?

I don't think theres an actual answer as to WHY Slit separation/wavelength affects how much a wave diffracts by, only that it is known through experiment that diffraction tends to reach a maximum when the two approach similar values (e.g Infrared wavelength is similar to that of slit separation in Youngs double slit.

How on Earth does diffraction work?!

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