Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 07/12/2015 10:50:02
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Ed Wilson asked the Naked Scientists:
Most definitions of albedo talk in terms of real-world planetary surfaces, so a value of 1 describes a "perfectly white" surface. Some definitions say the perfectly reflective surface is a mirror. How do the two definitions relate to each other? Does a white surface, if it gets more and more perfectly white, become a mirror?
When considering albedo with reference to global warming, wavelengths other than the visible spectrum are relevant. Does a surface have the same albedo value for all wavelengths? I doubt it.
What do you think?
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a value of 1 describes a "perfectly white" surface. Some definitions say the perfectly reflective surface is a mirror. How do the two definitions relate to each other?
The essence of both definitions is that they reflect 100% of the light that falls on them, without absorbing any of it.
One difference is that:
- a white surface gives a diffuse reflection, ie incident light is reflected in all directions, like snow.
- A mirror gives a specular reflection, obeying the "angle of incidence=angle of reflection" rule;
- from certain angles, an illuminated mirror may appear black, while an illuminated sheet of paper will appear bright when viewed at all angles.
Does a white surface, if it gets more and more perfectly white, become a mirror?
I would say that
- a perfectly white surface, as it gets more and more smooth, provides a more specular reflection (like magazine paper vs newspaper)
- A mirror surface (like aluminum foil or aluminized mylar), as it gets more and more crumpled, provides a more white reflection.
- ...although you could probably still tell the difference by looking at the reflected polarization of a polarized light source.
Does a surface have the same albedo value for all wavelengths?
No. All chemicals (like paper) have an infra-red spectrum, corresponding to energy levels of the molecules.
Metallic mirror surfaces have fairly uniform reflectance over a wide range of frequencies (although the color of gold & copper shows that they are not entirely uniform over the visible spectrum).
But if you go too high in frequency (eg X-Rays and Gamma rays), they tend to go straight through most materials, rather than being reflected.
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One anomalous case of albedo is with optical whiteners, as used in some clothes detergents.
These use the fact that sunlight and some indoors lighting have a component of invisible ultraviolet (UV) light.
Using fluorescence, the invisible UV produces visible light, so the object appears to emit more than 100% of the visible light falling on it. This results in an "impossible" albedo at visible wavelengths, although it is compensated by a very low albedo at UV wavelengths.
Clothes treated this way glow brightly in locations lit by UV lamps.
Some clothes detergents were advertised as being "whiter than white"; when challenged as being an unscientific advertising slogan, they were able to show that it is based on good science (and good chemistry).