# Waves aperture: cylindrical or spherical?

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#1
I am reading from the Daniel Fleisch waves book: in the book he says,
http://storage4.static.itmages.com/i...06bae21d63.png

He's discussing waves going through and aperture and points out that: if the aperture size is comparable to wavelength then there is 'spreads out slightly' but if the aperture is much smaller than wavelength, 'spreads out greatly'

I wanted to know what he means by spreading out.. as in:
1. is a plane wave entering the aperture?
2. is the plane being bent into a cylindrical surface or a spherical surface?
0
4 years ago
#2

if it's introductory (presuming it is because the text talks about 'spreading out' rather than using technical vocabulary) it's probably talking about surface waves in water which are easy to visualise and demo in the classroom.

surface waves have to stay on the 2d surface so they're not spreading out into 3d space.
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#3
He's talking about plane waves - i've uploaded the page as an image:

'Waves, however, behave quite differently. The right portion of Fig. 6.2 shows the effect of openings of various sizes on a plane wave incident from the left.

In the top right portion of the ﬁgure, the aperture is many times larger than the wavelength of the wave, and the wave passes through the opening without signiﬁcant change. But, if you reduce the size of the opening so that the size of the aperture is similar to the wavelength of the wave, the portion of the wave that passes through the opening is no longer a plane wave.

As you can see in the center portion of the ﬁgure, the surfaces of constant phase are somewhat curved, and the wave spreads out after passing through the opening. And, if you reduce the size of the opening even more, such that the aperture is smaller than the wavelength of the wave, the wavefront curvature and spreading out of the wave becomes even greater. This effect is called “diffraction”.

He's talking about curvature of a plane wave - but that can be a cylindrical sort of curvature or a spherical one. So i was wondering which it was.
0
4 years ago
#4
The demo is 2D, so not cylindrical or spherical. But I see your point- if you were to send a 3D plane wave through a slit that is infinitely long in the out-of-screen dimension, then it would produce a cylindrical-type wave.

In terms of your other question, nothing is actually entering the slit. Remember a wave is not a tangible thing that travels around, it is a disturbance in some underlying medium/field. The lines that you see in the sketches are just lines along which the amount of disturbance is constant (e.g. showing peaks and troughs).
1
#5
ah - thank you!
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