Question

Diffraction question

Answer 1

This is not an area of knowledge for me. What's the question?

Answer 2

oh oops forgot to put ill be right back ok heres the question ....

Answer 3

i have three points that i need clarification on

first point: diffraction happens when waves appear to fill in the missing spots after hitting or going (slits or openings) through the object. ok so this confuses because what does fill in the spots even mean...

second: i have an example which backs up this point but since i dont get the first point i wont get the example
EX: if water waves go through a small opening then they will spread out on the other side to eventually be wider then the opening they went through

example involving HITTING: water waves fill in the missing area behind the obstical so that it doesnt look like anything was ever blocking the wave

Answer 4

actually theres only two points

Answer 5

Good, this isn't beyond me.
Lets look at the waves of water example. Say you put a barrier across a small pool that had just one opening, say about 3 inches wide. The barrier is high enough so that no wave we create will go over it and the lower edge of the barrier extends down enough in the water to block any surface motion from effecting the other side.
We go to one end of the pool and drop a rock in, large enough to make a good size set of ripples. As they approach the barrier all of the ripples will be blocked from effecting the water on the other side except for where the opening is. At this point, assuming the pool was large enough to start with, the ripples look almost straight as they hit the barrier, that is they are not small circles like they were at the point the rock was dropped in.
The ripples will go through the opening but on the other side, they will start fanning out in circles as if they were produced by drooping a rock at the opening in the barrier. This might seem normal, especially if you were to think about what light would do with the same geometry. If a light bulb was placed where the rock was dropped, the light would be blocked by the barrier - except for at the opening. The light though would not fan out like the water did. It would stay in a narrow path (a very small pie shape) and light up only a few inches of the other side of the pool. If the water did this, it would remain a a narrow set of waves, with abrupt edges on either side. It would hit the other end of the pool with a set of waves only a couple of inches wider than the opening. That would look pretty strange - but why doesn't light do the same thing?
Does you understand the picture so far?

Answer 6

Also, you may be confused by the wording of the example ... "water waves fill in the missing area behind the obstical so that it doesnt look like anything was ever blocking the wave"
If you had multiple opening, the water going through each opening would fan out and would run into other waves that had fanned out from each of the other openings. This effect is called Wave Interference. Where two wave peaks collide, they intensify. Also for two wave troughs, they intensify the depth. Where a peak and a trough meet, they will cancel if they are of the same, but opposite amplitude.
If you had pictured that the waves on the other side of the barrier would self generate as if there were no barrier (not starting as small circles at each hole), then I understand your confusion. It isn't worded in the best way.

Answer 7

Diffraction is a phenomenon that is edge, opening and wavelength dependent. This is not an area I can do the math in but I can say that when the opening gets down to a width that is proportional to the wavelength of the energy (water waves, light, etc.) the effect is for the energy to spread out behind the opening instead of just going straight thru. When I say proportional, I mean that the opening size will be no more than a couple of times the wavelength.
This effect can be seen in light by looking at some examples of the Double Slit experiment. This experiment was one of the initial problems that scientists and mathematicians had with defining light as just a wave or just particles. When a narrow beam of light was projected on a barrier with two "wide" slots, the image on the screen behind was of two slots - no problem. As the slots were narrowed to the point that they were only a few thousands of an inch wide, more than two bars of light started to appear. As the slits got more narrow, many bars appeared on the screen. If light was just photons (particles), then only two bars should appear behind the two slits.
What was happening was like the waves of water going through to openings in the barrier. The waves of water would produce Interference patterns when they got to the wall in the pool. Where crests of the waves met, the water would be high on the edge of the pool. Two troughs meeting would produce a low point on the edge. and where a crest and a trough met, the water should be a its normal height.
Does this help?

Answer 8

"the ripples look almost straight as they hit the barrier, that is they are not small circles like they were at the point the rock was dropped in"

so when you say they are straight when hitting the barrier, your talking about it being straight for both the opening and the other parts of the barrier which does not have an opening right?

Answer 9

And what did I mean by Edge, Opening and Wavelength? Back into the pool.
EDGE: If the barrier was about 6 feet in horizontal depth, and the opening was very rounded and not sharp, the waves of water would first have to compress into the opening which would cause those crests near the widest part to lag behind the crests going down the middle. By the time a single wave crest made it to the middle of the opening, it would be flattened out from the difference it time the waves crests had arrived there. If there were any crests at that point they would start fanning back out but would be confined by the shape of the opening.
OPENING: If the width of the opening was very wide, most of the wave as it went through would be supported by the wave to the side so very little of the wave would fan out. Also, because the shape of the wave is not that of a radiating circle, it could not meet other waves from other openings and cause interference patterns.
WAVELENGTH: If we go back to the thin barrier and make it really tall, we can then drop a very large rock into the pool. The large waves with proportionally large wavelengths would go through the opening and not spread out in even circles. If there were two openings, it would be impossible to tell one wave from the other.

Answer 10

Straight? The waves would be circles of very large radius. When the wave hits the barrier, it will hit any other nearby part of the barrier at nearly the same time (it a long pool). For practical purposes, with in a small area of the barrier, the wave will act as if it were a straight line.

Answer 11

ohhh ok

Answer 12

There are wave pools in which the waves are created from a straight source, no radius to worry about.

Answer 13

If you used a wave pool, the results would be more refined but look pretty much the same.

Answer 14

Is this helping at all?

Answer 15

yep i just have to take time to take in all this information. but one question what you said about wave interference where troughs and troughs or peaks and peaks meet which intensifies the wave. is that what causes it to "fill" in the missing area behind the obstacle?

Answer 16

I don't like how that example was worded. The water is level without any waves present. When a wave is created, the height of the crest will be equal to the depth of the trough. The average height will still be the level of the water.
The use of "fill in" seems to imply that something is missing until the wave gets there. No correct. When the wave goes through the opening, it will spread out in almost 180 degrees - as if a rock had been dropped into the water right at the opening. If we were thinking about light going thru the opening, it would not spread out. That is because the opening is really huge compared to the wavelength of light.
I think is this case the author is picturing that the waves spreading out 180 degrees is what is "filling in" means.

Answer 17

ok ok i understand thank you!

Answer 18

You're welcome.