Why would a lake reflect a mountain?

The mountain with and without a name. Source: https://upload.wikimedia.org/wikipedia/commons/9/90/Mt._Rainer-Reflection_Lake.JPG
Diffuse reflection. Source: https://upload.wikimedia.org/wikipedia/commons/9/90/Mt._Rainer-Reflection_Lake.JPG

There are many texts out there which discuss the reflection of light from surfaces. A good example would be found here. Now, as well as these articles are written, I do have a question.

How come that water can reflect light? Isn’t it supposed to be transparent?

Mirrors work in such a way that electrons in the so-called conducting band of a metal redistribute on the incidence of light to counter the electromagnetic field of the photon. In this way, the electrons effectively reflect the photon outwards again.

It is a bit like a trampoline, except, of course, it isn’t.

The water of a mountain lake is not a metallic solid. So, how does it work?

I will try to explain. Many photons hit a lake and interact with it. Quantum mathematics happens. If the surface of the lake is smooth enough, i.e. smooth at the scale of the wavelength of the light, there will only be constructive interference to this interaction if you locate yourself at an angle to the surface which is equal to angle of the incident photons. To translate that, it means, that you will see a finely defined picture. When the surface is not quite as smooth, you see a more diffuse picture. You can do experiments on it and observe. The image above attests to it. You can do the maths to go along with that experiment.

Without mathematics it boils down to this: Water is transparent, unless, of course, it isn’t. Nature is like that sometimes.


12108755_1507250599599792_3692866167745201820_nAlexander is a physicist, teacher and science communicator who is currently working at the Norwegian Centre for Space-related Education at Andøya Space Center in Norway. Even though, in his case, work and play do overlap, the content on this webpage is entirely private. You can follow Alexander on Twitter, Facebook and Google +

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