# Lets say you had a box made of mirrors... **URL:** \ **Category:** Factual Questions\ **Created:** [June 3, 2011, 1:12am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155 "2011-06-03T01:12:22Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![samjones](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@samjones](https://boards.straightdope.com/u/samjones)\ **Post date:** [June 3, 2011, 1:12am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/1 "2011-06-03T01:12:22Z")
and you shut the lid. It would get dark inside, right? Why doesn’t the light trapped inside the box continually reflect off the mirrors and keep the inside of the box illuminated? ---
**Author:** ![treis](https://avatars.discourse-cdn.com/v4/letter/t/bc79bd/32.png) [@treis](https://boards.straightdope.com/u/treis)\ **Post date:** [June 3, 2011, 1:16am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/2 "2011-06-03T01:16:09Z")
Mirrors aren’t perfect reflectors. Some of the light is absorbed and increases the temperature of the mirror. After some time (probably very rapid), all of the energy of the light has been absorbed by the mirrors, and the inside is dark. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [June 3, 2011, 1:19am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/3 "2011-06-03T01:19:27Z")
i’ve been in a box of mirrors and it does have light for just a bit longer. nothing is perfect though. ---
**Author:** ![Shmendrik](https://avatars.discourse-cdn.com/v4/letter/s/ee7513/32.png) [@Shmendrik](https://boards.straightdope.com/u/Shmendrik)\ **Post date:** [June 3, 2011, 1:24am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/4 "2011-06-03T01:24:10Z")
> [@johnpost](#): > > i’ve been in a box of mirrors and it does have light for just a bit longer. nothing is perfect though. Sure, because you were absorbing all the light. Try it next time in a tin foil suit and hat and then get back to us. ---
**Author:** ![Ruminator](https://avatars.discourse-cdn.com/v4/letter/r/b9bd4f/32.png) [@Ruminator](https://boards.straightdope.com/u/Ruminator)\ **Post date:** [June 3, 2011, 1:32am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/5 "2011-06-03T01:32:52Z")
this type of question sounds like a vaguely related topic of [Olber’s Paradox](http://en.wikipedia.org/wiki/Olber's_paradox). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 3, 2011, 1:33am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/6 "2011-06-03T01:33:24Z")
There’s no way that you were able to perceive the light persisting for any amount of time after the box was closed. If you thought you did, you were fooling yourself. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 3, 2011, 1:33am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/7 "2011-06-03T01:33:32Z")
The characteristic number of bounces for the light to decay in a box with sides tht have a reflectivity of **R** is **R/(1 - R )**. You can show this through infinite series, but I’m not going to go into it right now. Conveniently, if **R** is very close to **1** , this means that the number of bounces is really close to **1/R**. So a box made of mirrors that reflect 99% of the light will require about 100 bounces before it’s mostly absorbed. So say that your box is 100 mm on a side, very nearly 4". If you let the light bounce back and forth, you’ll find that it makes those bounces in under 4 nanoseconds (0.000000004 seconds). If your walls are 99.999% reflecting, you can shift the decimal point three places over and make it 4 microseconds. And I’d like to point out that 99.999% redflectivity is really, really hard to do. ---
**Author:** ![Washoe](https://avatars.discourse-cdn.com/v4/letter/w/ecae2f/32.png) [@Washoe](https://boards.straightdope.com/u/Washoe)\ **Post date:** [June 3, 2011, 2:01am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/8 "2011-06-03T02:01:10Z")
> [@Chronos](#): > > There’s no way that you were able to perceive the light persisting for any amount of time after the box was closed. If you thought you did, you were fooling yourself. Apparently the cat survived the experience and related to him that the light did indeed extinguish itself after the lid was shut. ---
**Author:** ![Arrendajo](https://avatars.discourse-cdn.com/v4/letter/a/e0b2c6/32.png) [@Arrendajo](https://boards.straightdope.com/u/Arrendajo)\ **Post date:** [June 3, 2011, 2:18am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/9 "2011-06-03T02:18:53Z")
Is the box on a treadmill? ---
**Author:** ![Onomatopoeia](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/onomatopoeia/32/4145_2.png) [@Onomatopoeia](https://boards.straightdope.com/u/Onomatopoeia)\ **Post date:** [June 3, 2011, 2:30am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/10 "2011-06-03T02:30:05Z")
> [@Arrendajo](#): > > Is the box on a treadmill? HAH! I love this place 😃 ---
**Author:** ![Onomatopoeia](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/onomatopoeia/32/4145_2.png) [@Onomatopoeia](https://boards.straightdope.com/u/Onomatopoeia)\ **Post date:** [June 3, 2011, 2:31am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/11 "2011-06-03T02:31:17Z")
> [@CalMeacham](#): > > The characteristic number of bounces for the light to decay in a box with sides tht have a reflectivity of **R** is **R/(1 - R )**. You can show this through infinite series, but I’m not going to go into it right now. Conveniently, if **R** is very close to **1** , this means that the number of bounces is really close to **1/R**. So a box made of mirrors that reflect 99% of the light will require about 100 bounces before it’s mostly absorbed. > So say that your box is 100 mm on a side, very nearly 4". If you let the light bounce back and forth, you’ll find that it makes those bounces in under 4 nanoseconds (0.000000004 seconds). If your walls are 99.999% reflecting, you can shift the decimal point three places over and make it 4 microseconds. And I’d like to point out that 99.999% redflectivity is really, really hard to do. This sounds like a very cool project for, er…someone. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [June 3, 2011, 8:00am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/12 "2011-06-03T08:00:36Z")
> [@CalMeacham](#): > > The characteristic number of bounces for the light to decay in a box with sides tht have a reflectivity of **R** is **R/(1 - R )**. You can show this through infinite series, but I’m not going to go into it right now. Conveniently, if **R** is very close to **1** , this means that the number of bounces is really close to **1/R**. So a box made of mirrors that reflect 99% of the light will require about 100 bounces before it’s mostly absorbed. > So say that your box is 100 mm on a side, very nearly 4". If you let the light bounce back and forth, you’ll find that it makes those bounces in under 4 nanoseconds (0.000000004 seconds). If your walls are 99.999% reflecting, you can shift the decimal point three places over and make it 4 microseconds. And I’d like to point out that 99.999% redflectivity is really, really hard to do. Does that also mean the box never gets completely dark, just dimmer and dimmer, since it drops to 1% each time? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [June 3, 2011, 8:12am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/13 "2011-06-03T08:12:27Z")
> [@AaronX](#): > > Does that also mean the box never gets completely dark, just dimmer and dimmer, since it drops to 1% each time? Light is quantized, so you can’t split it up infinitely. Eventually, you reach zero. ---
**Author:** ![Pitchmeister](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pitchmeister/32/3459_2.png) [@Pitchmeister](https://boards.straightdope.com/u/Pitchmeister)\ **Post date:** [June 3, 2011, 8:13am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/14 "2011-06-03T08:13:07Z")
> [@AaronX](#): > > Does that also mean the box never gets completely dark, just dimmer and dimmer, since it drops to 1% each time? Well, there’s a finite amount of photons, once you’re down to that level (maybe after another 4 microseconds) it indeed is completely dark. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [June 3, 2011, 8:16am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/15 "2011-06-03T08:16:59Z")
> [@AaronX](#): > > Does that also mean the box never gets completely dark, just dimmer and dimmer, since it drops to 1% each time? Or does it fade to black box? (I couldn’t resist putting the question this way, but isn’t it true?) ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [June 3, 2011, 11:50am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/16 "2011-06-03T11:50:18Z")
> [@AaronX](#): > > Does that also mean the box never gets completely dark, just dimmer and dimmer, since it drops to 1% each time? Just as no surface is a perfect reflector, no surface is a perfect absorber, either. This means that no matter what the interior of the box is coated with, there will be a non-zero fade-to-black time after the lid is closed, with the length of time depending on the reflectivity of the material. The other issue is that [All matter with a temperature greater than absolute zero emits thermal radiation,](http://en.wikipedia.org/wiki/Thermal_radiation) which means the interior of the box never goes completely dark (i.e. there will never be zero photons bouncing around in there). Something at or near room temperature emits _mostly_ infrared photons, but the emissions are a spectrum: there _will_ be some photons emitted by the box’s interior surfaces which are in the visible spectrum (albeit not many). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 3, 2011, 5:29pm UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/17 "2011-06-03T17:29:49Z")
Slight quibble to what **Machine Elf** said: There will _sometimes_ be some photons in there in the visible range. Most of the time, the number of visible-light photons will be zero. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [June 3, 2011, 5:40pm UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/18 "2011-06-03T17:40:24Z")
As a hypothetical, what if you assumed a perfect mirror that reflected 100% of the light 100% of the time? (As an aside, I think SF writer Gene Wolf used the conceit of a box of perfect mirrors in one of his books. In his story, it formed an interstellar - or interdimensional teleportation device; unfortunately, I doubt he had much of a scientific basis for his theory). ---
**Author:** ![Ignatz](https://avatars.discourse-cdn.com/v4/letter/i/3be4f8/32.png) [@Ignatz](https://boards.straightdope.com/u/Ignatz)\ **Post date:** [June 4, 2011, 1:30am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/19 "2011-06-04T01:30:58Z")
They sell them at Tarjay, in the form of cube tables. [http://www.target.com/Clear-Mirrored-Cube-Table-15-75x15-75x15-75/dp/B004GJ49VC](http://www.target.com/Clear-Mirrored-Cube-Table-15-75x15-75x15-75/dp/B004GJ49VC) ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [June 4, 2011, 4:10am UTC](https://boards.straightdope.com/t/lets-say-you-had-a-box-made-of-mirrors/584155/20 "2011-06-04T04:10:11Z")
The OP is describing something close to an integrating sphere used to measure the strength of light sources. Such spheres use a white internal coating (often paint) instead of mirrors, but do show over unity gain…that is to say that due to reflections, a detector at one point in the sphere will receive more photons than just the ones making a direct path from the source. > **[Integrating sphere](https://en.wikipedia.org/wiki/Integrating_sphere)** > > An integrating sphere (also known as an Ulbricht sphere) is an optical component consisting of a hollow spherical cavity with its interior covered with a diffuse white reflective coating, with small holes for entrance and exit ports. Its relevant property is a uniform scattering or diffusing effect. Light rays incident on any point on the inner surface are, by multiple scattering reflections, distributed equally to all other points. The effects of the original direction of light are minimized. ...