Refraction of waves towards the beach

As you see from my parents’ attire and the lack of leaves on the trees, these pictures are not brand new. But still interesting: Do you see how the waves are bent towards the coast as they run into shallower water?

And can you believe I still have to look up which one is refraction and which one is diffraction to make sure, despite having a PhD in oceanography? They really should consider retracting that. I am pretty sure I “was instructed” (avoiding to say “learned”, since I clearly didn’t) about this in a lab during first year physics. On the other hand, I know where to look it up. Does that count?

And maybe it’ll help if I write it down once more:

Diffraction is what happens to waves behind slits or obstacles.

Refraction is what happens when waves run into shallower/deeper water (or into a different medium if we are talking about light waves) due to dispersion.

Does anyone know a nice “Eselsbrücke”/mnemonic/way to remember?

Waves change their direction because of changes in water depth

When waves run up a beach, they change their direction because for shallow water waves, phase speed depends on water depth (see this post for a nice little visualization for why the whole wave crest swings in towards the beach).

Unfortunately, it is really difficult to take good pictures of this phenomenon, but I tried:


See below for an annotated version that shows the wave crests:

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It is easier to see in a movie, so here you go with a movie from that exact same spot:

Observing hydrodynamic phenomena on a creek

Wave phenomena on the Pinnau in Mölln. By Mirjam S. Glessmer

Looking at a creek on a Sunday stroll, and seeing lots and lots of concepts from hydrodynamics class.

For example below, you see waves radiating from each of the ducks. And you see interference of waves from all those ducks.


What happens if the ducks bring their waves closer?


At some point, all those waves from the ducks are going to hit the weir in the picture below.


And there, they are going to somehow react to the flow field caused by the changes in topography.


And you can spot so many different phenomena: Standing waves, hydraulic jumps, and lots more!


Watch the movie below to see the whole thing even better!

Btw, you might remember this spot, I have talked about standing waves from right there before. Interestingly, the wave pattern in the other post looks really different, probably due to different water levels or changes in topography (maybe someone threw in rocks or they did some construction work on the weir?). But it is still just as fascinating as last time :-)

And for those of you who like to see a “making of”:

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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! :-)

Rainbows III

Updated movie following Arne’s advice.

When I asked for feedback on the rainbow movies the other day, Arne had a pretty good idea for how one of the explanations could be made more intuitive. I have other people’s comments still in the queue and I’m working on them, this is still very much in the trial & error phase… And unfortunately it’s in german, which I didn’t realize until I had uploaded it.

But please do keep the comments coming, I will include them eventually!

Secondary rainbows

Sometimes you get lucky and see a double rainbow. But how does the second rainbow form?

On my first 17th of May in Bergen, Ellen invited me to her home for a traditional dinner, which was exceptional. And as a bonus we got to see a double rainbow over Store Lungårdsvann!

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Double rainbow in Bergen on May 17th 2011

The outer rainbow is the so-called secondary rainbow, and as you can see the colors in the secondary rainbow are reversed, with red being on the inside and blue being on the outside.

Having watched my explanations in the textbook-style movie or in the short movie collection, is the sketch below enough information for you to make sense of how a secondary rainbow forms?

If the sketch isn’t clear – what additional information would you need to make sense of the sketch?

Here comes the movie in case you’d like to watch it:

I have yet a newer version of the rainbow movies as well as the one pictured above ready for you, but I thought I’d give you a bit of a break from rainbows and talk about something else for a while. But we’ll be back to rainbows soon, promise!

Rainbows and refraction II

Taking the same graphics as in this post, but presenting them differently.

In the previous post, I presented a screen cast explaining, in a very text-booky way, how rainbows form. Today, I am using the same graphics, but I have broken the movie into six individual snippets.

I’m starting out from the schematic that concluded last post’s movie and ask  five questions that you could ask yourself to check whether you understand the schematic:

Ideally I want to link the other five of the movies into the one above, but I haven’t figured out how to do that yet, so here you go for the answers:

What do you think of this way of presenting the material? Do you like it better than the textbook-y movie? I’m curious to hear your opinions!

For both this and the other way of displaying the material, I am toying with the idea of adding quizzes throughout the movies, in a programmed learning kind of way. But considering all the pros and cons, I haven’t made a final decision on it yet. What do you think?

Rainbows and refraction

Why is a rainbow always red on top and blue at the bottom?

We always talk about prisms and refraction and stuff, but be honest – would you be able to explain the order of colors in a rainbow without pausing and thinking first?

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Rainbow. Picture taken in Laufenselden in 1996

As I said the other day, I am currently experimenting with screen casts. This is my very first attempt – I didn’t write a script so it is pretty chaotic, I have a cold (which you can hear from my voice), my handwriting sucks, the movie is, at 4 minutes, about twice as long as I wanted it to be – it is not perfect and I will certainly modify it before using it in teaching. But I would be very interested in your feedback so I can improve it!

As you noticed, this is a very textbook-y screen cast. I’ll present an alternative model for the same topic in my next post.

P.S.: As you might have noticed from the watermarks in the video, I have continued experimenting with screen cast programs and am currently using Doceri. And I am very happy with it!