Timeline for Why don't objects get brighter when I reflect their light back at them?
Current License: CC BY-SA 4.0
11 events
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| May 12, 2023 at 9:28 | comment | added | Cloudyman | Re: the second-level bounce, it should be possible to make a separate part of the wall lighter than pitch black with the second-level bounce, but not brighter than the output of the first-level bounce , and redirecting the first-level bounce back at it won't be able to make that spot brighter (based on these optic&thermodynamic theorems) | |
| May 12, 2023 at 9:23 | comment | added | Cloudyman | I posted my own answer --- it's impossible, it would violate the brightness theorem and conservation of etendue! (With the caveat that we're dealing with the diffuse reflections not the specular reflections.) | |
| Apr 22, 2023 at 16:35 | comment | added | Filip Milovanović | You should be able to observe a second level bounce if you are in a completely dark room, and you take a narrow-beam LED flashlight (or maybe a bright laser pointer), have it hit a wall, and then use a mirror or a white piece of paper to bounce the light that has been reflected off of the wall back at the wall (i.e. the light source can be behind the mirror, so no direct light hits the mirror itself). Might be challenging to film though - if I succeed, I'll add it to the answer. 2/2 | |
| Apr 22, 2023 at 16:35 | comment | added | Filip Milovanović | @Cloudyman - the xkcd argument says that it is not possible to use a magnifying glass to make the spot in focus hotter than the surface of the source, not that light can't bounce back and forth multiple times. I guess you're right about my examples not showing exactly your scenario, but my point was, if it bounces once, no reason why it can't bounce again - light is light, doesn't matter if it's coming from the light source, or if it's reflected. 1/2 | |
| Apr 19, 2023 at 7:06 | comment | added | Cloudyman | This answer is well done but I’m asking specifically of reflecting an object’s light back at it, not reflecting additional light from the light source onto the object (which is what the contrast-enhanced image and the mirror and papers at the end show). There is a thermodynamic argument that it’s not possible (see: what-if.xkcd.com/145), and I was wondering if that is valid and if so, why. The answer seems to be that it is possible just the effect isn’t very large? | |
| Apr 18, 2023 at 7:48 | comment | added | João Mendes | The image processing on the OP's photos is quite on point. Nicely done. | |
| Apr 17, 2023 at 8:51 | comment | added | justhalf | Best answer so far +1 | |
| Apr 16, 2023 at 23:07 | history | edited | Filip Milovanović | CC BY-SA 4.0 |
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| Apr 16, 2023 at 22:53 | history | edited | Filip Milovanović | CC BY-SA 4.0 |
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| Apr 16, 2023 at 22:43 | history | edited | Filip Milovanović | CC BY-SA 4.0 |
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| Apr 16, 2023 at 22:34 | history | answered | Filip Milovanović | CC BY-SA 4.0 |