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Author Topic: How many mirrors would it take for me to be able to look at the sun?  (Read 1648 times)

Offline bizerl

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When I see the sun reflected in a mirror, it's bright, but not as bright as looking straight at the sun.

How many times would the sun need to be reflected off a standard bathroom mirror, before I could safely look at it?


 

Offline Phractality

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You need to reduce the brightness by about 10,000 to 1.

The visible spectrum is roughly 400 to 800 nm.

Between 600 and 800 nm, Silver reflects about 97%, and Aluminum about 86%.

If you assume a value of 90% reflectance, for simplicity, you need to reflect the light x times, where .9^x = 10^-4. According to Wolframalpha, that's about 87 reflections.

After reflecting 87 times, your image of the sun will be extremely fuzzy. This is not a good way to view the sun.

You could do it with fewer reflections by using a glossy black surface. The blackest substance on Earth reflects about .1% of incident light. If you can polish it to a mirror finish, you might only need to reflect the light twice.

A better way to view the sun is to project the light from a telescope's eyepiece thru a window onto a viewing screen in a dark room. Or you can buy a solar filter to put over the front of your telescope.
« Last Edit: 22/10/2012 03:23:18 by Phractality »
 

Offline CliffordK

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You could do it with fewer reflections by using a glossy black surface. The blackest substance on Earth reflects about .1% of incident light. If you can polish it to a mirror finish, you might only need to reflect the light twice.
I don't think you could polish a carbon nanotube forest & preserve the optical properties.  However, you might do well with a homogenous forest wtih all nanotubes having the same height, direction, and diameter.

Superblack might have better optical properties, but does not have equal absorption across the spectrum, and thus IR might be too bright.
 

Offline Bored chemist

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The sun is rather small, so it's hard to get a good look at.
If you project an image of it that's bigger than the lens you use then that image is less bright than the sunlight.
Also, if you project the image onto, for example, a bit of paper then that will scatter the light in all directions so only a small fraction of it will reach your eyes.

So projecting the light with a telescope is a very good solution but
Don't ever look through a telescope at the sun.
 

Offline evan_au

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The glass of a bathroom mirror absorbs light surprisingly strongly.

We are used to looking through the thin direction at a mirror, but if you look at the mirror from its side, it looks very dark green to black. This is due to the light being strongly absorbed by passing through the width of the mirror, with no reflections.

Compare this to optical fibers, which lose less than 25% of the light after passing through a kilometer.

But the safest way to observe the sun is to project an image onto paper, without your eye looking in the direction of the sun (even accidentally).
 

Offline syhprum

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The problem with trying to get any good quality pictures of the sun is that you need a room where most of the ambient light can be excluded and the only light that comes in is via the telescope, I was able to get good pictures of the first transit of Venus by this method but could not record the second one due to having moved and having no suitable line of sight.
I think for high quality pictures a Density 5.0 filter is the only answer.
I think Evan-au is rather pessimistic about the loss in high grade fibre optic cables
« Last Edit: 24/10/2012 21:34:15 by syhprum »
 

Offline evan_au

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Astronomical telescopes using a mirror (as invented by Isaac Newton) avoid light absorption through the glass by coating the mirror surface facing the light source, rather than coating the rear surface as is done for bathroom mirrors.

Being exposed to the atmosphere, astronomers have to clean off the reflective metal surface and recoat it at regular intervals.

You also need to ensure that visitors don't leave fingerprints on the mirror....
 

Offline evan_au

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Re: loss in high grade fibre optic cables

For visible-light applications, loss is around 1dB/km.

Communications applications traditionally use infra-red wavelengths around 1500nm, where the loss of silica fiber is a minimum.

Other, more exotic fibers have losses less than 0.1dB/km. If my arithmetic is correct, 0.1dB/km translates into 1db/10km, which is about 25% loss of optical power in 10km.

http://en.wikipedia.org/wiki/File:Zblan_transmit.jpg
 

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