Reflections on reflections

When we think about reflections in water, we usually think of calm lakes and trees on the shore opposite to us. Or clouds. Or at least that’s what I think of: Everything is so far away, that it seems to be reflected at an axis that is a horizontal line far away from us.

Then the other day I walked along Kiel Fjord and it hit me that I had never actually consciously observed reflection of things that are located close to my position, and especially things who are not pretty much equidistant to me, but where one end is a lot closer than another one. Consider the picture below: Do you notice something that looks kinda odd to you (while at the same time looking super familiar)?


If you are wondering what I mean, I marked it in red in the picture below: The rope and its reflection! It’s embarrassing to say that (as someone who has been sailing A LOT since the age of 7) this was the first time I really noticed, but it struck me how the maximum of the parable of the reflected rope isn’t right below the minimum of the parable of the rope, but seems shifted to the left. Of course this is exactly how it should be if we think about the optics, but I was really shocked that I had never noticed before and never thought about it before! I bet if I had had to draw the reflection I would have done it wrong and probably not even noticed…


Here is another picture to show you what I mean. This is what it looks like:


Below I’ve drawn in the original objects in blue, the axis of reflection in red and then the reflection in green:


So far, so good, everything looking the way it’s supposed to look. Right? Then look at the picture below:

Sorry if this seems obvious to you, but I’m fascinated with this right now :-)

But it leads to another interesting thought: Asking people to draw stuff in order to both check their understanding and also make them reflect on their understanding. I recently had the opportunity to observe a class of master students draw the SST of the mean state of the Pacific Ocean (which was an exercise that I had suggested in connection with a class on El Nino. I thought it would be neat to have them draw the mean state and then later the anomalies of El Nino and La Nina to activate prior knowledge) and it was surprising how difficult that was even though I’m sure they would all have claimed to know what the mean state looks like. Having to draw stuff really confronts us with how sure we are of things we just assumed we knew…

And then I’m pretty sure that once we’ve drawn something that we have constructed ourselves from what we knew (rather than just copied a drawing from the blackboard or a book, although I think that also helps a lot), we are a lot less likely to forget it again.

Anyway, this is a type of exercise I will use — and recommend — a lot more in the future!

Another wave hunt expedition: Learning to discover ocean physics wherever you go

One of my favourite topics right now: Learning to “see” ocean physics wherever you go. For example here: A visit to my goddaughter in Schleswig, and this time we are practicing all she and her mom read about in MY BOOK (and if you have good ideas for a title for that book, please let me know!). So today I’m showing you pictures of phenomena similar to those in my book, but discovered on this recent visit.

For example diffraction when waves pass this pier:


In the image below, I’m showing what I mean: Waves coming in from the right have straight crests (red). As they pass the pier, they get diffracted and bent around (green).

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In this spot, this phenomenon can be seen on most days. I wrote about it before, but I have more pictures from previous visits, where the same thing happens in the opposite direction, too: Waves propagating in from the left and being bent around the pier to the right.

Or we can see other wave crests, meeting a rock that breaks the water’s surface.


Those waves (shown in red in the image below) get reflected from the rock, and circular waves radiate away from the rock (green).

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A similar thing can also be observed from a flag moored out in the water:


This time, incoming waves are green and the circular waves radiating off the flag are red.

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And we also got to see awesome criss-crossing again, albeit in a different spot:


Here we have the red wave crests coming in, and the green reflections.

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If we look at it from a little more distance, we can also see another phenomenon: The wave crests are refracted towards the shallower shore:


Again the red crests are the original, incoming ones, and the green ones are the reflection:

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And then finally, let’s look at duckies again. And on waves being created by wind:


Below you see the direction of the wind (white): One side of this little channel is shaded from the wind, so hardly any ripples there. But then on the other side, we clearly see ripples and small waves. And we see the wake the ducky made!

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And one last picture: Which direction does this little channel flow in?


Yep. From the left to the right!

If you enjoy discovering this kind of stuff on your walks, or know someone who enjoys it, or want someone to learn to enjoy it, you might want to consider checking out my book. In my book, I show many pictures like those above, but I actually explain what is shown in the pictures rather than assuming (like I do on this blog) that my readers are oceanographers anyway… :-)

Diffraction and reflection of waves

Last night, we saw really nice wave phenomena on the Schlei in Schleswig.

Do you see the waves being diffracted by the pier in the picture below?


Waves are coming in from the right (see the three lines on the right in the picture below) and at the head of the pier they get bent around (all other lines).

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Now look at the line on the very left. What happens where that wave hits the pier?



Awesome criss-crossing of wave crests!


An annotated picture of what happens below: The red lines show the incoming original wave crests, and the green lines show the wave crests of the wave that got reflected by the shore.

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If all those lines are a tad confusing, thankfully a ducky in a fairly wave-less spot made a single wake which also got reflected on the sea wall:


Same picture as above, this time with the original wake marked in red, and the reflected wake marked in green:

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Nice evening, isn’t it?


And since everybody else is asleep, I put together some short video clips into a movie for you:

What I learned from the movie-making? I need to take longer footage and practice my editing-skills! :-)

More wave phenomena on a lake, and a bit about wind

Waves on Aasee in Münster. By Mirjam S. Glessmer

Last week I showed you the results of my “wave hunt expedition” on Aasee in Münster. Today, I am following up with the same lake on the day after and the day after that. Even more wave phenomena to observe!

First, on my second day in Münster on my way to the conference:

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Clearly it had been windy for a while with more or less constant winds: You see Langmuir circulation cells.

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So imagine my surprise when, on day 3, I wake up to this view:


Absolutely no waves at all, and no wind! Reason enough for a pre-breakfast stroll.


As I was walking the wind picked up, as you can see in the increased surface roughness in the middle of the lake.


But many parts of the lake were still completely calm. For example that weird building, which I sat at for the next half hour or so.


Sitting there, I watched the “sea state” turn to slightly more wavy (see above — aren’t those pretty reflection patterns? :-))


And I love how you have those tiny wave trains. So pretty!

At some point it got too windy for my liking, and I wandered on. And noticed a spot that I had missed on my last walk: A drain going into the lake, making more pretty patterns!


Eventually I had walked all the way around the lake again into the lee of the land, which would have been really boring if it had not been for some duckies:


Oh, and of course more pretty reflections.


Hope you have a great day, too! :-)

Wave hunt expedition. You don’t need to live close to the coast to observe all kinds of wave phenomena!

Waves on Aasee in Münster. By Mirjam S. Glessmer

A 1.5 hour walk around a lake — and 242 photos of said lake — later I can tell you one thing: You definitely don’t need to live close to the coast in order to observe wave phenomena!

The idea to go on a “wave hunt expedition” is actually not mine (although it definitely sounds like something I could have come up with!), it’s Robinson’s idea. Robinson had students go on wave hunt expeditions as part of their examination, and present their results in a poster. I was so impressed with that, that I had to do it myself. Obviously. So the second best thing about work travel (right after the best thing, again, obviously!) is that I find myself in a strange place with time on my hand to wander around and explore. Not that Münster might not have been a nice city to explore, but the lake…

Anyway. I only want to show you 53 out of the 242 pictures. I was going to annotate all of them so you actually see what I mean. And I started annotating. But since I am giving a workshop tomorrow (which is all prepared and ready, but I do need my beauty sleep!) I only drew the key features in the pictures, and you will have to come up with the correct keywords all by yourself (have your pick: refraction! diffraction! fetch! interference! :-)) So click through the gallery below and see first the original photo and then one that I drew in. Do you spot the same stuff that I saw, or what else do you see? Let me know!

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If you think it would be useful to see all those pictures with proper annotations and descriptions at some point please let me know. I might still be excited enough to actually do it, who knows…

P.S.: I actually really enjoy work travel for the work parts, too. For example, I went to a great workshop in Dortmund earlier this year to learn about a quality framework for quantitative research, and that workshop was amazing. And a week ago, I went to Stuttgart for a meeting with all the fellows of the Stifterverband für die Deutsche Wissenschaft, which was also great. And now I am giving this workshop in Münster, that I am actually really excited about because I managed to condense pretty much all I know about “active learning in large groups” into a 2.5 hour workshop. Just so you don’t get the wrong idea about my priorities. Obviously water comes first, but then work is a very close second ;-)

The difference between secondary rainbows and double rainbows

More reflection or more rain?

Ha, aren’t you enjoying talking about optics again?

Sometimes you see two rainbows that both have red on the outside and blue on the inside. And according to my post on secondary rainbows, that should not be the case. Yet is has been observed. Why?

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Rainbow and secondary rainbow, seen at Heathrow Airport. Picture by my friend F.

As you remember, secondary rainbows form outside the primary rainbows because the light is reflected twice inside the raindrop rather than only once as in the case of a primary rainbow. But that second rainbow with red on the outer rim and blue on the inner is formed differently.

Until now we’ve assumed that all the rainbows appear on the same rain front. This is not the case for the rainbow we are talking about here – it is formed on a second rain front behind the first one. So the path of light within rain drops of both rainbows on both fronts is similar, with light being only reflected once for each rainbow.

When you google double rainbows, you sometimes find pictures of two rainbows, both with red on the outer rim, nicely separated from each other. And when you see those pictures, you can be pretty sure that they’ve been photoshopped. Double rainbows of the kind we are talking about here overlap, and usually you see one full rainbow with all its colors, and then a slightly smaller rainbow with only green, blue and purple peeking out:

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If you look closely, there is a green-and-purple band on the inside of the complete rainbow. Double rainbow!

Sun dogs

Recently spotted: sun dogs, a special form of halo! Or rather sun dog (singular), since there was only one to be seen and not a second one at equal distance from the sun but on its opposite side.

Sun dog spotted somewhere between Mölln and Hamburg

These pictures are exactly as my camera took them without any filters or color enhancement or anything. Isn’t it weird that we appeared to be the only car stopping every couple of minutes to watch while everybody just continued driving?

Sun dog spotted somewhere between Mölln and Hamburg

Waves radiating from an object

In the last post, I showed you flow separation on a pylon in Elbe river. Remember?

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Flow separation at a pylon in Elbe river

Today, we are back at the same pylon, only that this time the tidal current is a lot less strong, but there is a lot of wind, so our focus is on wind-generated waves.

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Waves running towards the pylon and radiating radially away from the obstacle.

It might be admittedly a bit hard to see, but if you watch closely and use your imagination, you might be able to see the waves propagating towards the pylon and then being reflected and radiating radially outward from where they hit the pylon. Pretty fascinating!

Can you see the locally generated waves to the left of the pylon? All those tiny waves where the wind is funneled around the pylon?


What are the ingredients of a rainbow?

Still collecting materials for our instructional short movies.

A while back I talked about how my colleague and I were experimenting with short instructional screen casts, and I shared some first attempts at movies on how rainbows form. We are still working on a story board for an improved version, but I was lucky enough to see a very pretty rainbow in a fountain the other day.

The picture below is a good demonstration of how rainbows form where there are water droplets in the air (provided there is enough sunlight, too, and we are watching from the right position) – we still see a bit of the rainbow to the right of the fountain, even though the wind direction has changed and the fountain is now blown to the left, visible because of the mist and the lower part of rainbow.

Fascinated as I was I had to film clips of this, too, which are combined in the movie below. There you see the rainbow appearing and disappearing, depending on where the fountain is moved by the wind, i.e. whether it is moved to the part of the sky where all the angles are right for us to see a rainbow, or not.

It was a magical moment – enjoy! :-)

Why do we only see rainbows in the mornings and evenings, but never at noon?

Another movie on rainbows

My dearest readers, I hope you are still as fascinated by rainbows as I am? Today I’m giving you another movie explaining something rainbow-related, namely why we do not see rainbows when the sun is too high up in the sky. The video is stylistically similar to the ones I did before, and while practice really helps and I am getting pretty fast in making this kind of videos now, I am ready to try something new. But using doceri is something that I could imagine doing operationally if I was to use this kind of movies in my courses. It is really a nice tool!

So here is my movie. As always, let me know how you like it and what I could do better!

P.S.: Whenever I say or write 82, what I mean is 84! But according to my colleague it is actually beneficial to learning if movies aren’t perfect, because hesitation or small mistakes create irritations in the learner, which then make him think about what you were saying. And as the learner is now engaging more actively, the learning process is more successful. So there you go! :-)