# Dependance of radiation properties on wavelength

1. Jun 20, 2012

### vin300

Do the absorptivity, reflectivity and transmissivity of a surface depend on the wavelength of the incident light?

2. Jun 20, 2012

### sophiecentaur

Yes- absolutely. It's determined by the atomic / electronic arrangement, the molecular structure of the material on both sides and also the coarse structure of the surface.

3. Jun 20, 2012

### vin300

I was asking about the dependence on wavelength of light being reflected, transmitted or absorbed.

4. Jun 20, 2012

### sophiecentaur

All those factors will affect the way that em waves of different frequencies will behave when they hit the surface.
There are many examples in which different frequencies are affected differently. Imagine a metal plate, painted black. It will reflect microwaves but absorb infra red and light. When you see the irridescent colour of a peacock feather, it is because the surface has reflected certain wavelengths of light and absorbed others. Coloured Paint (pigment) will reflect some wavelengths and absorb others. etc. etc.

What is the actual context of your question? It could be taken in several different ways.

5. Jun 20, 2012

### vin300

That is what motivated me to ask this question. In texts, the absorptivity is often given as the ratio of energy absorbed to the incident energy and interpreted as a property of the surface. They don't say energy of what wavelength.

6. Jun 20, 2012

### sophiecentaur

So you're talking in terms of the way objects interact with thermal radiation, perhaps. The 'colour' of an object, will affect the absorptivity at certain wavelengths (by definition). However, Kirchoff Law of radiation says that the emissivity and absorptivity of a surface are the same at any given wavelength. This is necessary, I think, to avoid the possibility of a system in which heat is constantly pumped 'for free' from one object to another, without needing a temperature difference if you chose the colours of the two objects appropriately.
A highly reflective surface (or transmissive medium) will not be a good radiator or absorber. A highly absorbtive (it would be opaque) substance will also be a good radiator.

If you had two objects in an insulated, massless laboratory (in a perfectly silvered sphere), they would both reach the same equilibrium temperature, whatever their colours (one black, one shiny or one green, one red). Of course, if you have a space craft with a shiny side pointing at the Sun and a black side facing away, it will end up a lot cooler than if it's orientated the other way round - because its surroundings are not in equilibrium (heat is always flowing from the Sun out into space).
The above is a bit garbled but I think it may have some relevant ideas. Was it of help or just more adding confusion?