What happens when infrared light passes through a glass prism?

[Scorpii]

Hi all, got a bit of an experimental/theoretical confusion here.

A group of us are doing an investigation into limiting the heating of solar panels while still allowing the majority of visible light to reach the panel cells.

One theory we're intending to test out is using prisms to change the direction of the light around the visible range of the spectrum (directing it onto the solar panel) and hoping the infrared does not also get too absorbed by the prism and remains with its direction mostly unchanged.

However, despite a fair bit of reading on the topic, there seems to be little information available about what happens if you pass infrared light through a glass prism. Does it just get absorbed? Or does it pass through?

Another issue is that we require a rather large prism to be constructed (around 20cm dimensions), as they are prohibitively expensive, so the plan was to build one out of acrylic and fill it with water. Now obviously these two materials have different optical properties to glass, and different refractive indices, but would it still work as a prism? As long as it works to change the direction of visible light to some extend but not do the same to infrared is the idea.

Thanks for any help!

 

[Bobbywhy]

Scorpii, Welcome to Physics Forums!

Glass, water, and acrylic plastic all have different infrared absorption coefficients. There is data available on individual specific materials, but when you fill an acrylic prism with water I doubt that sort of detailed absorption information is published. You will have to rely on your own measurements. Here is some suggested reading to get you started:

http://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation )
http://en.wikipedia.org/wiki/Absorption_spectroscopy
http://en.wikipedia.org/wiki/Absorption_(optics )
http://www.habmigern2003.info/future_trends/infrared_analyser/ndir/IR-Absorption-GB.html

Here is an excellent tutorial and an example of a graph of absorption vs wavelength:
http://www.lsbu.ac.uk/water/vibrat.html

 

[Dr_Morbius]

Why not just put an infrared filter in front of the solar panel? Or how about just mounting the solar panel on a heat sink so that it will stay at a relatively cool temperature? There's a lot of ways of dealing with the problem of solar cell overheating.

 

[Scorpii]

Thanks for the info Bobbywhy, I will have a read through those sources.

And Dr_Morbius, we are experimenting with a number of different ways, including different types of infrared filtering, the prism idea is just one of them.

The application for all this is a proposed very close approach solar probe, passing within the range 0.01-0.1AU of the Sun. Solar panels are currently only usable to ~0.2AU, even then tilted at 75 degrees (and that's with the cooling system running).

We are looking at filtering light before bouncing it off an angled mirror, so reducing the incident light on a solar panel that is stowed behind the heat shield for the duration of the perihelion of the probe's orbit.

The prism idea is just an alternative one of the group working on this thought of, but we need to quickly construct one just as a proof of concept before researching properly into materials with suitable refractive indices and absorption properties that could be used in a real scenario. It may turn out there are no feasible ones, but before doing the research we need to show that it's worth the time researching it!

 

[Drakkith]

Why not coat the panels in a reflective substance to reflect a large amount of light away from the panel so it doesn't get absorbed and converted to heat? Or a material that reflects infrared light only or something?

 

[Dr_Morbius]

Prisms are large and bulky; they're the last thing you need on a space probe where weight and volume are at a premium.

 

[Scorpii]

Yep Drakkith we are looking at various IR reflective coatings to effectively filter the light.

Filtering the light then bouncing it off a highly angled mirror to the solar panels behind the heatshield look like the most promising option at the moment. The issue with coating the panels directly is that - due to them being behind the heat shield - you need a way of getting the light to them. Using the mirror idea would mean the light would land at and angle other than 90 degrees onto the solar panels, and might end up bouncing back off the IR reflective material to the rear of the heat shield or other parts of the probe. By filtering the light with - for example - a dielectric mirror we can bounce light back at the incident angle before passing the heat shield (in theory!).

And Dr_Morbius, we are well aware that space and weight are prime concerns on a space probe, but we are looking at this as an alternative to the proposed near sun power units.

If you'd have to use prisms weighing a few kilos vs a radioisotope generator for the close approach weighing even more, then perhaps prisms are the better option, as you can utilise the solar panels already present (which is the point of this investigation).

In any case, it's only an idea we're exploring. Perhaps we will conclude that prisms aren't a very good option, but if we find that dispersing the light is very beneficial then we can look into other ways to do that. But for an early stage lab experiment we just want to experimentally find what the effect of dispersing the light is :)