## Friday, 2 February 2007

### Friday's Physical Law - Reflections

In the first two weeks of this series I started out with simple motion as my fundamentals and then built from there, and I do intend to continue in that vein but every now and then I am going to feel like something different and this week is one of those times.

Today I am going to discussing some thing that is part of my PhD research and is also something that merits a comment when talking about Climate change. That is reflection.

To begin with we must have a wave travelling along that interacts with an object (or the boundary between two media - ie water and air). Now when this wave (and for the most part I will be dealing with electromagnetic (EM) waves such as light, radio waves etc., although this applies to all types of waves) interacts with an object any of three things can happen:

1. Reflection
2. Transmission
3. Absorption.

Which of these occurs depends on the properties of the object and the wave. And in most cases two or even three of these things happen together.

Reflection is where the light strikes the object and bounces off. Transmission is where the light passes through the object. Absorption is where the light is ... well ... absorbed.

If we start of with a wave intensity of 1 then:

• R + T + A = 1

where R, T, and A are the amounts of the wave reflected, transmitted and absorbed respectively.

The obvious examples are a mirror for reflection, a window for transmission, and a black object for absorption.

And you will notice that I mentioned a colour specifically for the absorption, in fact it is due to reflection and absorption that we see colour. Light shines on an object and if red light is reflected and the green and blue light absorbed then we see the object as being red.

Now what does all this have to do with my research, well, I study the electrical interactions of the upper atmosphere (more about this another time) and the main tool we use is by using the ionosphere and ground as a mirror (albeit and imperfect ones) and sending radio waves long distances between these two "surfaces" and so the reflective properties of particularly the ionosphere are very important. And by looking at the changes in the signals we receive we can determine the changes in the reflections along the way and hence the local (or global) changes that has caused them.

And of course I also mentioned that reflections can affect climate change, if you have snow, ice or tundra then the sunlight reflects of this quite well, as any one who has gone skiing can attest to. But as these melt less of the sunlight is reflected and hence more is absorbed (since the Earth is not very transparent). This leads to more heat being trapped in the Earth-atmosphere system leading to more warming (even if only locally) which will lead to more melting which will lead to more absorption and so on. In other words it is a positive feedback loop for the warming which is not a good thing, especially since we can quite easily measure the amount of ground covered by snow, ice and tundra and it is decreasing.