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Author Topic: How can transparent glass act like a mirror?  (Read 2886 times)

evan_au

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How can transparent glass act like a mirror?
« on: 21/02/2013 10:32:27 »
From Scientist: We get reflection (as I get it) when there are enough free electrons on surface then how a transparent glass becomes a reflecting surface when there is dark in one side and light in another side? (e.g cars windows, or house's windows at night when seen from inside the house)

...split off from different colours from single atom at same time...??

evan_au

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Re: How can transparent glass act like a mirror?
« Reply #1 on: 22/02/2013 10:30:31 »
The speed of light in glass is lower than the speed of light in a vacuum.
The index of refraction of glass is about 1.5, so the speed of light in glass is about c/1.5=2x108m/s

Whenever the impedance of a medium changes, the propagation speed changes, and some energy will pass through, and some will be reflected. The ratio is determined by the change in index of refraction - for window glass, about 5% is reflected when it hits the window, and another 5% when it hits the other side of the window, or 10% total.  See: http://en.wikipedia.org/wiki/Refractive_index#Reflectivity

So if the light intensity on the other side of the window is (say) <1% of the illumination on this side of the window, then the window will reflect mainly light from this side of the window, ie it acts as a mirror.

lightarrow

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Re: How can transparent glass act like a mirror?
« Reply #2 on: 22/02/2013 18:20:33 »
From Scientist: We get reflection (as I get it) when there are enough free electrons on surface then how a transparent glass becomes a reflecting surface when there is dark in one side and light in another side? (e.g cars windows, or house's windows at night when seen from inside the house)
With free electrons you have a kind of reflection (metallic reflection) but free electrons are not necessary: a polarizable medium (a dielectric) can do it as well, and it's still a collective mechanism, because it involves electron displacements of many molecules at the same time.

By the way, the more efficient mirrors (at a specific narrow range of wavelengths, used e.g. in lasers) are made exactly with dielectric reflection:
http://en.wikipedia.org/wiki/Dielectric_mirror
« Last Edit: 22/02/2013 18:25:46 by lightarrow »

Spacetectonics

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Re: How can transparent glass act like a mirror?
« Reply #3 on: 24/02/2013 18:58:44 »
evan-au

when I was a kid I was thinking of mixing glass(sand)with something which goes dark when connects to a battery!!
jes.. never got those dreams done!!:))

evan_au

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Re: How can transparent glass act like a mirror?
« Reply #4 on: 25/02/2013 09:14:56 »
The "one-way mirrors" often seen in detective and spy movies use a thin, partially-reflective film of aluminium deposited on the glass.

This makes it harder to see from the bright room into the dark room, and means that you don't have to have the dark room quite so strictly black (which would include dressing the viewers in black, and wearing dark balaclavas...)

http://en.wikipedia.org/wiki/One_way_mirror

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Re: How can transparent glass act like a mirror?
« Reply #4 on: 25/02/2013 09:14:56 »