# Palpable darkness... well, almost. **URL:** \ **Category:** Factual Questions\ **Created:** [February 18, 2003, 11:57am UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858 "2003-02-18T11:57:25Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 18, 2003, 11:57am UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/1 "2003-02-18T11:57:25Z")
The other day I took my kids to Marwell Zoo; we went into the bat house, where it was warm and very dark indeed - the habitat was very dimly illuminated by yellowish lights, the area where the public walk was almost completely dark. Immediately, I lost track of my two children and, as the place was quite busy, feared a little for their safety. “Not to worry”, thought I , and whipped out my keychain, upon which I have a very bright little white LED torch; aware of the fact that use of this light could cause problems for the bats, I shielded it carefully with my hand as I switched it on. A strange thing happened; it cast hardly any light at all upon the area; I eventually tracked down the kids by voice, touch and blind luck, but even then, shining the light onto the back of their coats hardly illuminated them at all. Thinking that perhaps the batteries were just going flat, I foolishly looked into the beam, which was an experience as dazzling and painful as ever. The light usually (both before and since) casts a moderately bright illumination on anything - I use it to find my way up the stairs each night in total darkness and I use it for inspecting various things in tight corners where the light is bad, but in the bat house, it simply didn’t seem to illuminate anything - What could possibly have caused this phenomenon? ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [February 18, 2003, 12:39pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/2 "2003-02-18T12:39:04Z")
Walking in out of the sunlight? I’m guessing it was during daylight, right? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 18, 2003, 12:44pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/3 "2003-02-18T12:44:03Z")
It was daytime but overcast, even so, this is not unlike the transition that occurs when I turn off the main lights at home and ascend the stairs in darkness. ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [February 18, 2003, 12:59pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/4 "2003-02-18T12:59:57Z")
Excessive humidity and dust absorbing the light maybe? Try it out on a humid summer night. As an aside, I find that warm water is “thick” to swim in, so what do I know. 🙂 ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 18, 2003, 1:05pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/5 "2003-02-18T13:05:15Z")
Even when overcast, daylight is one or two orders of magnitude brighter than indoor house lights. Your eyes hadn’t recovered from the bright light (i.e. your pupils hadn’t opened up all the way). ---
**Author:** ![Bongmaster](https://avatars.discourse-cdn.com/v4/letter/b/6a8cbe/32.png) [@Bongmaster](https://boards.straightdope.com/u/Bongmaster)\ **Post date:** [February 18, 2003, 5:03pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/6 "2003-02-18T17:03:51Z")
I’m totally guessing here, but could it have been the color of the other lights in the bat room? ---
**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\ **Post date:** [February 18, 2003, 5:08pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/7 "2003-02-18T17:08:41Z")
What was the colour of the construction inside the batroom? If everything was black, very little light from your LED would be reflected to your eyes. ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [February 18, 2003, 5:10pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/8 "2003-02-18T17:10:35Z")
scr4 has, I believe, the most plausible explanation. You said “Immediately, I lost track…” which tends to support his hypothesis. I’d also suggest that the area, which can be large enough to accomodate several people, may be a bit larger, and probably painted considerably darker than your house stairwell, lending to the sense that even with your torch (heh heh, torch. We say flashlight here. Torch. heh.) you weren’t going to illuminate the area very well. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [February 18, 2003, 6:35pm UTC](https://boards.straightdope.com/t/palpable-darkness-well-almost/155858/9 "2003-02-18T18:35:42Z")
The yellow lights in the bat house may have been causing your pupils to contract, as if from a much brighter light source. That’d make your LED’s illumination appear dimmer. > [@](#): > > \>What this means with regard to the red vs. green discussion is that a red > \>display will do a better job stimulating the color/resolution detecting > \>systems of the eye (cones). Although the eye, overall, is less sensitive to > \>red (because the rods are not stimulated by red light); it is BETTER to use > \>red light on our computer screens because it will NOT stimulate the rods. > \>Remember, stimulating the rods has a negative effect on dark adaptation. > \>Green light will be easier to read (and allow us to turn down our > \>displays); but you will diminish your capability to observe dim > astronomical objects [here](http://observers.org/tac.mailing.list/2002/Dec/0314.html) The visual purple that scr4 alluded to likely also played a part.