# Diffraction and waves

Sorry about my poorly worded question as i'm not to good at explaining but bare with me so here i go.

Does sound come as straight lines like ||||| and become diffracted into curves when it passes through a slit so it looks like )))))?

If this is the case (as most diagrams on the net show), my question is what causes this to curve? In the Young's experiment with the slit how does light not pass straight through as ||||| : ----- ? In diagrams it is shown like this |||| : )))).

Can someone explain diffraction a bit more clearly and easy for me to understand why it curves?

Question 2: Why do light waves look like like this? And when passed through a slit, how do they curve into ))))?

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First, some terms: The surfaces you are drawing are called "wavefronts", which are surfaces of constant phase. We usually refer to your |||| waves as "plane waves", meaning that the wavefronts are nice planes. Similarly, we call your )))) waves "spherical waves" because the wavefronts are spheres. So those are the terms I will use. Also, the basic ideas are exactly the same for sound and light, so I will try to answer both parts of your question at the same time.

If the source of a spherical wave is really far away, the sphere is really large by the time it gets to you. But you'll only be dealing with a small part of that sphere, so it looks pretty flat to you. That is, you can approximate the sphere as a plane (in a small region). For example, you might imagine a star as a source of perfect spherical light waves -- the star is roughly spherical, so the waves can be spherical. (A very crude model, but let's just go with it.) But that star is so far away that by the time the light reaches us, each sphere is enormous. And since our eyes or any telescope or whatever that we use will be tiny compared to that enormous distance, the sphere is basically flat to us; the waves are basically plane waves.

You can only have perfect plane waves if the source of the waves is infinite and acting in perfect harmony -- or if it's just infinitely far away. Since we never really have infinite sources, we never really have plane waves. But some times we have a really large source, so we can approximate the waves as plane waves.

So in the real world, you never have perfect plane waves or spherical waves, but they are frequently good approximations. And those approximations are easy to deal with (for calculations), so we use them a lot. But in reality, every wave we get is imperfect, and some sort of wiggly wave front.

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does that mean lasers give perfect plane waves? – Allen Wong Jul 23 '13 at 9:57
No, because lasers have finite diameters, so they can't be perfect plane waves. (In fact, laser beams aren't really straight and parallel; they are better understood as approximately Gaussian beams.) But, they usually do give a pretty good approximation to plane waves. It can really simplify the analysis to just assume they are plane waves. – Mike Jul 23 '13 at 19:05

When you say that sound looks like |||| or )))) you are talking about wavefronts. Very simply, wavefronts are a locus of all points of a wave which are in the same phase. For sound you can regard same phase as corresponding compressions.Wavefronts can be of many shapes. Spherical, cylindrical or planar are a few examples. The shape of the waefronts depends on the sound source.

When passing a planar wavefront of sound(i.e. ||||) through a slit, it will produce cylindrical wavefronts(i.e. )))) ). This can be said using Huygen's theory of wave propagation. Huygen's theory says that every point on a wavefront is a new point source of waves.

This diagram shows how a planar wavefront is propagated. Now if you place a slit in front of this wavefront, only the points on the slit can now propagate the wave forward(Other wavelets bounce of the board). So in effect what you get is a cylindrical wavefront(i.e. ))))) Something like this:

The same happens with light. The only difference is that the wavefronts you get with light are still planar or cylindrical or spherical ( |||| or ))))) and not like ~~~~~~. The latter is a representation of the light wave and not the wavefront

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sci-toys.com/scitoys/scitoys/radio/received_sound_wave.gif if this is what a single wave looks like, how does it look like in the wave fronts? – Allen Wong Jul 23 '13 at 5:49
nvm ive found the answer to question 2 , for those reading this, this pic helped clarify 4.bp.blogspot.com/-qDVWX9-5Y6k/TmWZEWYhz5I/AAAAAAAAAqs/… – Allen Wong Jul 23 '13 at 8:51