Re: Absorption/Reflection

["Philippe Boucher" <pb31416@xxxxxxxxx>]

I have a question, mainly theorical...
Is there any way to "shift" wavelength?

I've found a way to rotate polarisation of light, but I was wondering if
that (shifting wavelength) was possible.  I know when light travels in
material with refaction factor, the speed of light decrease accordingly.

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----- Original Message -----
From: "Andy Finney" <andy@xxxxxxxxxxxxxxxx>
To: <infrared@xxxxx>
Sent: Tuesday, June 27, 2000 1:15 PM
Subject: Absorption/Reflection


> Dirk Wright wrote:
>
> I was wondering about infrared reflectance and absorption of various
> materials. I shot some EIR recently, and materials like brick, gravel,
> and mulch recorded as not reflecting (ie, not red). If a material does
> not reflect IR, can it be assumed that it absorbs light at that
> frequency? Does this further mean that these materials will soak up
> heat, or rather get hotter, than materials that reflect IR, given
> identical solar illumination? This can have implications for housing
> and landscaping design beyond the normal artistic effects.
>
> And my take on this is ....
>
> Surely it's relatively simple. If a substance does not reflect
> radiation (whether visible or not) then the energy of the radiation
> has to go somewhere. Usually it heats up the object but in some
> (rare) cases it will generate an electrical potential difference or
> some other equally curious effect.
>
> Clark says that objects with temperatures of between 250 Celsius and
> 500 Celsius can be photographed by their own illumination using
> infrared film. Above about 600 Celcius the objects are hot enough to
> glow visibly. A chapter in Clark is devoted to the characteristics of
> materials in infrared light including some charts. From that I
> appropriate the following bits and pieces:
>
> For example: Water has a pronounced dip in infrared absorption at
> around 800nM, but it is still three times as absorbent as at 700nM.
> This dip gets more pronounced as the water heats up mainly because
> the absorption around the dip increases.
>
> Substances such as copper and gold show around 90% reflectivity at
> 800nM whereas at 400 they are only 30% reflective. (This makes sense
> if you consider what colour gold and copper appear to be in visible
> light.) Carbon on the other hand (as graphite) has only a very slight
> rise - almost a straight line - from 400 to 1400nM. Aluminium is
> almost constant at about 85% from below 400nM to above 1400nM.
> Chromium has a shelf of sensitivity which rises from around 40% at
> 400nM to 55% at 600nM. It then stays virtually constant up to 1400
> and perhaps beyond.
>
> For building materials the reflection generally stays constant (+/- a
> few percentage points) between 500nM and 840nM but often climbs at
> 1780nM. Exceptions to this include clay tiles, which steadily climb
> in reflectivity as the wavelength gets longer.
>
> Snow has an undulating reflective curve: 35% at 300-400, 40% at
> 400-800, 15% at 800-2600 ... only climbing again above 7000nM.
>
> Regards
>
> Andy
> www.invisiblelight.co.uk
> *
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