Refraction of waves

I remember being on a looooong walk on some Danish dike when my sister was small and really didn’t want to walk any more, telling her about how phase velocity of shallow water waves depended on water depth and how you could observe that when waves are refracted towards the coast (assuming the sea floor has the right slope). And whenever I see this happening now I have to think of that freezing cold and windy day a long time ago.

Wave fronts turning towards the shore

Watch how the angle of the wave fronts changes as they come closer to the shore:


Eddy generation and optics.

Eddies. Dips in the surface and shadows on the ground.

I always get really fascinated by watching how eddies are generated by obstacles in a fluid. But it is especially exciting when you don’t only see the eddies because you see how they deform the surface, but when the water is clear enough so you can see the “shadows” on the ground!


Of course, the dark spots you see aren’t shadows, strictly speaking. As light enters the water from the air, it is being refracted. And since the eddies’ surface imprints are dips in the surface, light is being refracted away from the perpendicular, leading to a less-well lit area – the dark spots.

But isn’t it fascinating to watch how eddies form when the water passes the stick and stones in the water when there is absolutely nothing going on upstream?

Sun dogs

More refraction of light.

Recently I found myself on the ferry from Kiel to Gothenburg, watching the sun rise.


Next to the sun, I noticed a piece of a rainbow.


Now as we all know, rainbows are supposed to only be visible when we are facing away from the sun. Clearly not the case here.


By the way, I’m talking about the rainbow-y thingy to the right of the sun, the spec close to the sun is probably something on the lens of my camera, or some other artefact of some sort.


So I read up on the rainbow-y thingy, and apparently it is called a sun dog.


There are supposed to be two of those, on either side of the sun.

IMG_9494Do you know those medieval pictures of three suns, with the outer two facing the inner one? Apparently those are supposed to be sun dogs! I never knew.


Anyway, I stood, fascinatedly watching the rainbow-y thingy.


Occasionally distracted by cool ships.

IMG_9522The higher the sun rose, the more colorful the rainbow-y thingy became. While it had been colorful for the naked eye (ok, I’m wearing glasses, but you know what I mean. No filters or polarization or anything), it started to show up on pictures, too.


In the beginning I tried finding the second sun dog on the left of the sun, but there was nothing. But the one on the right got prettier and prettier!


Eventually we arrived in the port of Gothenburg and I got distracted by container terminals and other exciting things that you will surely hear about very soon.

IMG_9551But for now I’ll leave you with this amazing view of the little islands right before you enter Gothenburg. Ready for a Scandinavian holiday? :-)


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!

Screen shot 2014-07-11 at 6.16.13 PM
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?

Screen shot 2014-07-13 at 3.34.19 PM
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!

Sun halo

A rainbow that isn’t one, technically speaking.

Browsing through the photos on my phone, I came across the one below that I took two years ago in Bergen. I remember taking the picture with Nadine on our way home, and wanting to look up what phenomenon caused this ring around the sun, but I never did – until now.

photo 3
Halo in Bergen

So, according to Wikipedia and various other internet sources, what we see in that picture is a 22° halo. In contrasts to rainbows which you see when you are facing away from the sun, this kind of halo forms as a circle around the sun. Also in contrast to rainbows, in this case sun rays aren’t refracted at raindrops, but at ice crystals. Since ice crystals typically have a hexagonal shape, this causes the radius of the halo to be on average 22°. On average, since refraction still depends on the wavelength of the refracted light – hence the halo is red-ish towards the middle and blue-ish towards the outer rim. This is also different in a rainbow, where the outer rim is red and the inner rim is blue.

Why is that? Stay tuned for the next posts, I’m still trying to figure out a good explanation. For those of you who saw my post on teaching videos a week ago, you might have noticed that I was working on something related to refraction and reflection and light already then… ;-)