Refraction versus diffration

BennyA's picture
BennyA started the topic in Friday, 9 Sep 2016 at 10:03pm

So you guys do an amazing job in detailing and defining how and when we should expect great waves. In fact, some of your articles regarding reef shape, contour and depth give us all a fantastic overview of how and why waves break the way they do. You have also provided a great description of swell interval, which the rest of the world would define as frequency.

As surfers we are represented by a phenomenally broad demographic in which some of us are doctors and some of us are making your morning coffee. Regardless, we possess a knowledge of the ocean and weather systems that non-surfers do not.

One thing we do s surfers, particularly badly, is using the right terms to describe our own environment. Swellnet consistently uses the term refraction when the correct term is actually diffraction (yes I'm aware I'm a dirty nerd). Let me explain.

We use the term refraction to explain why a swell bends around a corner into our favourite point, or why we get peaks instead of closeouts. However, refraction is actually a term that refers to the bending of a wave as it transitions through a change in medium. For example, a light wave through air as it enters water. The change in medium alters the speed at which the wave can travel. This is not the case for ocean waves. The medium is always the ocean. Depth (f water), period (frequency), amplitude (wave height) and contour (of the ocean bottom) can all affect the shape of the wave you ride.

Diffraction however, is a completely different animal. Diffraction is what we as surfers refer to as refraction, the bending of waves through the same medium. And frequency (or period) is so important as low frequency (long period) wavelengths bend around corners so much better than high frequencies. A great example is sound waves versus light waves. Because of the very high frequency. light waves cannot bend around corners (that's why you can't see around a wall), whereas sound waves, which are comprised of a much lower frequency, bend around walls quite well (and you can hear them). It is exactly the same for ocean waves which diffract.

I love this site and barely comment but the terminology we use should be addressed.

thermalben's picture
thermalben's picture
thermalben commented Wednesday, 5 Oct 2016 at 9:35am

hey Benny

Sorry this post was missed (our forums aren't very good at alerting new topics, something we're endeavouring to fix).

Really appreciate your insights. To be honest my use of the term 'refraction' stems from what's been ingrained over many years, through uni studies and other external reading. Interestingly, there's plenty of literature that references refraction with regards to the interaction between swells and bathymetry.

My understanding is (and has been) that refraction is the slowing and bending of waves in shallow water, whilst diffraction is the propagation of a wave around an obstacle.

However, on top of this - and you've brought about a great discussion topic too, BTW - I've sometimes had to draw a line between getting overly nerdy (something I tend to gravitate towards) and providing a clear explanation for the layperson (in a limited window of opportunity, every Mon/Wed/Fri afternoon!).

Sometimes this will result in the use of words that may not be technically correct, but is more easily understood to the reader.

A good example of this is a term I coined at the start of my forecasting - some 12-15 years ago. I used to use the term 'radial spread' to describe the way a swell would spread away from the source fetch (this was mainly used for S swells across NSW, originating from W/SW fetched exiting eastern Bass Strait, as we should be experiencing this afternoon).

The correct term is 'angular spreading' but for some reason I decided it was easier for me to convey this information using phrases like 'as the swell spreads radially away from its source,' etc. So technically it wasn't correct but at the time I was of the opinion it would be better understood.

Anyway, back to your point - can you point me to some reference material?

happyasS's picture
happyasS's picture
happyasS commented Wednesday, 5 Oct 2016 at 11:44am

Benny is correct. Swells bending around a point is diffraction. Refraction is a term reserved for change in wave direction due to a change in medium density. Earthquakes refract as they pass through different layers of rock. Water waves aren't really refracting as the energy is being transported through the same medium largely parallel to the sea bed. In shallow water there will be some reflection as the wave energy hits the sea bed with force and bounces back up.

freeride76's picture
freeride76's picture
freeride76 commented Wednesday, 5 Oct 2016 at 1:05pm

Isn't the change in direction caused by the wave energy hitting a different medium, ie the ocean bottom?
The longer period energy feels the bottom in deeper water but it is the different medium, ie hard bottom vs open ocean that is causing the change in direction. It's not bending around a corner as in the example given.

Therefore the term refraction is correct?

Craig's picture
Craig's picture
Craig commented Wednesday, 5 Oct 2016 at 12:58pm

This is correct as well, and the waves follow a similar refraction pattern to light changing mediums.

I think it could be argued correctly both ways, but refraction has been used in literature for as long as I can remember.

blindboy's picture
blindboy's picture
blindboy commented Wednesday, 5 Oct 2016 at 2:00pm

A couple of points. First it is quite correct to talk about swell interval or period instead of frequency as, given a constant wave speed, they are inversely proportional. Second if you are talking about say a south swell bending into a north facing point then it is diffraction. In other situations refraction can be important. For example if you have an offshore canyon at the right depth then the part of the wave travelling over the canyon will travel faster and create a peak.

Laurie McGinness

happyasS's picture
happyasS's picture
happyasS commented Wednesday, 5 Oct 2016 at 2:12pm

The change in direction that freeride is referring to is still diffraction as the medium hasn't changed. The wave hasn't transfered into the seabed, it's merely hitting the seabed and being affected by friction. The bending around a headland is due to part of the wave hitting the seabed and slowing down while the rest of the wave out in deeper water is travelling at normal speed. The wave particles are tied together as a group and so the slower particles pull the faster particles around with them. Hence the bending occurring. Longer wavelength waves bend around the point better because they engage more seabed so the effect is stronger.

blindboy's picture
blindboy's picture
blindboy commented Wednesday, 5 Oct 2016 at 3:33pm

Sorry happy but you are wrong. A wave wrapping around a headland is diffraction. The wave spreads out into areas that were originally blocked by the headland. You can demonstrate this in a ripple tank in which the depth is constant. Set up a train of waves travelling towards a block and they will bend around it even though the depth does not change.

Laurie McGinness

wally's picture
wally's picture
wally commented Wednesday, 5 Oct 2016 at 4:04pm

Yeah, I agree with bb. I think happy is getting too caught up with the refraction definition that specifically applies to optics.

In optics, refraction occurs because waves change speed in different media - faster in less dense medium and slower in denser medium.
With ocean waves, varying water depth has the same speed effect as different media in optics.
So the refraction definition is broadened to include :- faster in less dense medium (or deeper water) and slower in denser medium (or shallow water).

Diffraction is waves curving around an obstacle, where the bending in not due to a changed medium or water depth.

Around headlands, depending on swell direction, you will get refraction and probably some diffraction as well.

happyasS's picture
happyasS's picture
happyasS commented Wednesday, 5 Oct 2016 at 4:06pm

i agree bb. i am arguing its diffraction - said that in my first post. although we appear to have different understanding on how the wave diffraction around a headland occurs. i do know about the ripple tank effect with an obstacle you are describing. can you explain what is the mechanism why a longer wavelength wave diffracts better than a shorter wave? i was sure that headland seabed effects where taking place like i described, but is it something else, or a combination of seabed + ripple tank effects? cheers

freeride76's picture
freeride76's picture
freeride76 commented Wednesday, 5 Oct 2016 at 6:50pm

I think we are arguing about interpretations of different phenomena.

The phenomena we observe of swells changing apparent direction as they feel the effects of the seafloor whether in deep water or shallow around headlands is best known to us as refraction.

The phenomena of swells curving around islands and offshore reefs that the Polynesians used for navigation is best understood and widely known as diffraction.

OldSouth's picture
OldSouth's picture
OldSouth commented Wednesday, 11 Oct 2017 at 10:59am

Here is an interesting old thread which I can contribute to in a positive way. I studied physics with a passion when I was at school and then at uni( Mech. Eng. ). I always wondered why surfers call the change in direction af ocean waves around a headland or reef etc as refraction , because there is no density change. Wave propagation follows Huygens Principle which treats all particals along a wave front as an independent source of spherical waves with each cycle ,(with most energy being destructively cancelled to leave just front line.) When we have a wave front moving into a medium of different density (where it will change speed) at an angle, the first wavelets to strike the interface change their speed first followed by the next along the front and the next and the next. The whole wave will change direction or bend, refraction, simple right. This happens in light waves and also waves in solids liquids and gasses. Now just imagine this, I remember this experiment, a perfect flat bottomed ripple tank with a barrier sticking up from the bottom which part of the wave can move past. As a wave meets the obstruction the unaffected line of wavefront just moves past as it was with NO DIRECTION CHANGE. But some energy ,or wave leaks around the obstruction and tries to fill in. This is our diffraction and it is totally due to Huygens description of the spherical wavelets from the particals at the edge of the obstruction loosing some of the destructive interference on the obstructed side. You get a circular or spherical wave coming around the obstruction but no bending. So where does this leave us, it seems it's not refraction or diffraction with our ocean waves around headlands etc. BUT quite simply refraction is really not just associated with medium density but just anything that can impart a change in speed as the wave travels. When a good GROUNDSWELL (which is feeling bottom) comes to the contoured change in depth ,which the land presents we all know it will change speed and direction accordingly and this folks is definitely refraction. So diffraction has very little relevance to our ocean waves and it's all refraction. Huygens Principle is the best way to understand wave propagation and there is loads of good material to read on it. If I must I will put up some references.