# The Naked Scientists Forum

### Author Topic: How many times is an image reflected between two facing mirrors?  (Read 3347 times)

#### colin_murray@mts.net

• Guest
##### How many times is an image reflected between two facing mirrors?
« on: 11/09/2008 10:36:31 »
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

What do you think?

#### graham.d

• Neilep Level Member
• Posts: 2208
##### How many time is an image reflected between two facing mirrors?
« Reply #1 on: 11/09/2008 11:58:05 »
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.

#### Bored chemist

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##### How many time is an image reflected between two facing mirrors?
« Reply #2 on: 11/09/2008 19:13:33 »
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

#### syhprum

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##### How many time is an image reflected between two facing mirrors?
« Reply #3 on: 13/09/2008 11:29:29 »
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.

#### The Naked Scientists Forum

##### How many time is an image reflected between two facing mirrors?
« Reply #3 on: 13/09/2008 11:29:29 »