# Science Questions

## How many times can an image be reflected between two facing mirrors?

Sun, 12th Oct 2008

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### Question

How many times can an image be reflected between two facing mirrors?

Dave - Fundamentally, basically what happens when something hits the mirror light gets reflected. Depending on how good your mirror is a certain proportion of that light will get absorbed by the mirror. Most of it will get reflected. Normal mirrors around the house will reflect about 70% of the light. Really high-quality mirrors they use in optics labs in universities and research places will reflect 99.9% of the light. Basically it just depends how faint you can still see that light. With a normal mirror you probably won't be able to see it after ten or twenty times because it will get so dim. With really high-quality mirrors, maybe several hundred, several thousand times.

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colin_murray@mts.net asked the Naked Scientists: Hello, When you look at two mirrors reflecting each other it seems like the image reflects for infinity (until my big head gets in the way). Is there a limit to how may images are reflected? Or is the image really reflected forever? Is there an official name for this effect? There is a similar situation where in a book I'm reading there is a picture of someone reading the same book in a chair. Then in that book there is ostensibly the same picture of the same person reading the same book. Could this repeat for infinity or is there a theoretical limit? Thanks. Love the show. I'm listening to the podcasts trying to get caught up to the current show. Colin Murray Canada What do you think? colin_murray@mts.net , Thu, 11th Sep 2008

No mirror is perfect so some light gets scattered at each reflection. Eventually the image will get too dim (even if there were no distortion problems). In simple theory the image would get dimmer and dimmer but never wholly disappear, but in reality there are quantum limits to this. Physically seeing the image would be impossible long before these limits were reached though. graham.d, Thu, 11th Sep 2008

For a finite mirror, diffraction will also mean that it doesn't carry on "forever".

There's an obscure form of spectroscopy that relies on bouncing a beam back and to hundreds of times between two mirrors.

http://en.wikipedia.org/wiki/Cavity_ring-down_spectroscopy Bored chemist, Thu, 11th Sep 2008

When I worked on the HELL scanner/recorders one of my task was to service the SPECTRA PHYSICS gas discharge lasers.
These worked by bouncing a light beam back and forth thru a cloud of ionised gas over a path of about 20 cm until it acquired enough energy to 'burst out'.
I do not know how many transits it made but great care had to be taken to get the mirrors quite clean. syhprum, Sat, 13th Sep 2008

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