# Diffraction work differently on water waves vs light waves?

Karagoz
When the water waves go through one wide slit (slit wider than wave length), there's no fringes, the water waves spread all over the sides, like in this pic:

http://electron6.phys.utk.edu/light/images1-3/misc3b.jpg

or in this pic: http://philschatz.com/physics-book/resources/Figure_28_02_06a.jpg

But when light waves goes through a wide slit, it's split into multiple sides, there are fringes between them, it's not spread equally all over the place, like in this simulation (if you take width of the slit to 3000-5000 nm):

http://www.walter-fendt.de/html5/phen/singleslit_en.htm

Why there are no gaps when water waves go through a wide slit, but there are gaps when light waves go through a wide slit?

Andy Resnick
Why there are no gaps when water waves go through a wide slit, but there are gaps when light waves go through a wide slit?

Surface waves and light waves have many superficial similarities (and methods of analyses), but they are not equivalent and have important differences- different dispersion properties, for example. One particularly spectacular example of this equivalence is the focusing of water waves by a 'lens' in a large test basin:

https://www.cambridge.org/core/jour...d-experiment/0E9C470B0A35698C77975F06C225AC4E

Don't take the cartoon analogies too seriously.

sophiecentaur
Gold Member
2020 Award
When the water waves go through one wide slit (slit wider than wave length), there's no fringes, the water waves spread all over the sides, like in this pic:

http://electron6.phys.utk.edu/light/images1-3/misc3b.jpg

or in this pic: http://philschatz.com/physics-book/resources/Figure_28_02_06a.jpg

But when light waves goes through a wide slit, it's split into multiple sides, there are fringes between them, it's not spread equally all over the place, like in this simulation (if you take width of the slit to 3000-5000 nm):

http://www.walter-fendt.de/html5/phen/singleslit_en.htm

Why there are no gaps when water waves go through a wide slit, but there are gaps when light waves go through a wide slit?

Exactly the same Maths applies to water waves and light waves, with respect to simple diffraction. The elementary links you refer to are elementary and they don't commit to any 'structure' off axis. If you just look at the visible wavelets on the water surface, it is hard to distinguish anything apart from in the main direction.
The ripple tank demo is pretty much a waste of time for serious analysis. The range of amplitudes that you can actually distinguish is very small and you cannot (at least, I cannot, after a lot of playing around) swear that you are seeing anything but the main 'forward' pattern. Otoh, the side lobes (fringes) of a slit pattern with light is much easier to spot (our eyes are very good with light!!)
If you put in the appropriate values into that calculator (strictly to scale) you will get the same answer for light or water (or sound with water or air, for that matter).

tech99
Gold Member
So you mean it looks like this (if I don't see wrong, I see some:
http://figures.boundless-cdn.com/16694/full/iffraction-4-20wavelengths.png

Or this (at the left side): http://electron6.phys.utk.edu/light/images1-3/single1.jpg

Or this: https://phys.libretexts.org/@api/de...?revision=1&size=bestfit&width=226&height=162

Some fringes appear when water waves pass through singe wide slit?
The first picture shows water waves going through a wide slit, where we see a main beam which starts off parallel and then diverges. There are also side lobes visible. This is correct and is the same as with light and radio waves.

Andy Resnick
Some fringes appear when water waves pass through singe wide slit?

Yes, and there's a simple reason why. That reason is common for all types of waves, and understanding that reason is essential to fully understand any diffraction and interference phenomena.

Here's a hint: describe some properties of the incident wave(s). How can you create waves that look similar?

sophiecentaur
Gold Member
2020 Award
So you mean it looks like this (if I don't see wrong, I see some:
http://figures.boundless-cdn.com/16694/full/iffraction-4-20wavelengths.png

Or this (at the left side): http://electron6.phys.utk.edu/light/images1-3/single1.jpg

Or this: https://phys.libretexts.org/@api/de...?revision=1&size=bestfit&width=226&height=162

Some fringes appear when water waves pass through singe wide slit?
I am not convinced that the top figure is not a simulation. (double negative but you know what I mean). The other figures are better and must have taken quite an effort. The sorts of ripple tank / oscillator/ projector that schools tend to have available do not often produce such good images. Also you really need to be able to strobe the image so that the waves are frozen in time for the viewer.
But the existence of the 'null' rays should help to prove that Diffraction Works the same Everywhere.