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Say you are in the middle of a desert where there is lots of sunshine, and you are trying to signal for help with a signal mirror.

Either in the distance there is a person, or there is an airplane overhead that you are trying to reach.

How should one aim the signal mirror so that the light that is reflecting from it reaches the objective?

52

How I've practiced is I hold my left hand palm out, and I create a "V" between my ring and middle fingers with the plane or person between my fingers. I then hold the mirror by my face and shine the mirror between my fingers. You should see the light on your hand to confirm you are shining in the right direction.

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    This is the recommended method because it's so simple and intuitive to employ. If the light from the mirror (and that can be anything shiny, CD/DVD, cigarette lighter, credit card, pie tin, etc.) is on your fingers, and the object you're trying to signal is in between them, you can be pretty much guaranteed the object is being flashed by your signal. My method is only slightly different, in that I use the "V" formed by my left index and middle fingers, instead of middle & ring fingers, but I don't think that makes the slightest difference other than personal preference. – delliottg Apr 27 '17 at 14:28
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    Like this? i.stack.imgur.com/4utIA.jpg – uhoh Apr 28 '17 at 13:46
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    I was taught this method, but I was also told to wiggle the mirror back-and-forth slightly so the beam moves around in the target area and appears to flash. – acbabis Apr 28 '17 at 21:53
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A modern (Korean War era and later, at least) signal mirror has a hole in the center, and the hole is surrounded by a grid of retroreflectors. You aim the mirror spot at something close by, then look through the hole. The grid makes a very bright spot where the mirror's reflection is going. You then simply tilt and rotate the mirror to put that spot on your target.

This is a good photo of what it looks like.

This is a good photo of what it looks like.

As the comment in the image says, that hot spot is not the sun itself. It's showing you where the reflected light from the mirror is going. Just put that spot on your target and there you are.

Note that if sun-to-mirror-to-target bounce makes a fairly shallow angle, say more than about 135 degrees, the retroreflecting grid doesn't work and you have to use the methods given in other answers here. Which are also, of course, useful if you don't have a real signal mirror. So the other good answers in this thread should not be ignored.

Image ganked from this flickr page. As it says there, several companies do make "signal mirrors" that appear to have a grid around the hole, but don't actually have retroreflectors and don't make the bright spot. (I can confirm this; I bought one and was disappointed.) If you buy a US signal mirror on eBay and it says "Mark III", it's the right thing.

  • I did not know these existed. I have to go get one. I'll note that my answer is intended for mirrors whose aiming hole contains nothing but air. Everyone who +1'd my answer should +1 this one as well. – cobaltduck Apr 28 '17 at 12:23
  • Well, thanks! I +1'd yours, because as I noted, a) an improvised mirrors may be all you have, and b) the retroreflecting aimer doesn't work at shallow angles (say if the sun is near the horizon and your target is near the opposite horizon). Also +1'd for great diagrams. (You know, if you added an arrow with a 90 deg. bend in it, extending up and left from the lower orange dot, bouncing off of the back of the mirror, and returning to the user's eye, that might address the Q raised by Aaron. Just a suggestion.) – Jamie Hanrahan Apr 28 '17 at 18:54
  • @cobaltduck Now I am looking for an explanation of just how the "retroreflecting coating" on that grid works. I know how retroreflectors work but I don't quite see the application here. (p.s.: Don't you just hate it when you read about something you didn't know about and your reaction is "I must buy one!!!"? At least this is a not-expensive case!) – Jamie Hanrahan Apr 28 '17 at 19:01
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    I have a $10 mirror with this mesh from REI like this one. It's really helpful! – bitsmack Apr 28 '17 at 21:21
  • bitsmack, glad to know about that inexpensive option. Conversely, I bought one of the "Starflash"-branded mirrors that REI also sells for $10. It does not seem to have the true retroflective aiming aid. Be careful! – Jamie Hanrahan Apr 28 '17 at 22:14
19

This answer applies only to signal mirrors which are two-sided and have an aiming hole in the middle, which should be the case with any good signal mirror. My answer will partially duplicate a prior, but will add some rough pictures and also give a description of the geometry behind how it works.

The process

Stand facing roughly half-way between the sun and the object you want to signal. Hold the mirror up and observe your own reflection in the back of the mirror. Move the mirror around until you see the shadow of the mirror on your reflection, and in particular note the little dot of light on your cheek due to the aiming hole.

enter image description here

Now, angle the mirror so you can see the target object through the hole, and also align the light dot on your cheek with the hole. Wiggle the mirror slightly so that the target sees a flashing.

Why it works

With mirrors, the idea is that the angle of incidence is equal to the angle of reflection. For us, we need the angle between the mirror and the sun (θ1) and the angle between the mirror and the target (θ2) to be equal. We do this by making the corresponding angles on the backside of the mirror equal (θ2A and θ1A), by aligning the reflected dot and the target object via the hole in the mirror.

enter image description here

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    Thanks for adding this answer - the pictures are much more helpful than my purely verbal description! – Toby Speight Apr 27 '17 at 15:31
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    There is still something lacking in your description which prevents me from fully grasping your concept. "Align the light dot on your cheek with the hole." What does that mean? If you mean to align the light with the shadow's hole, then the light is the shadow's hole as you've described it. If you mean to align the light with the actual physical hole on the held object, what does that even mean? Also, in both of your pictures you have a blue spot on the face-shadow-side, which is obviously the point created at your face by a line from target through mirror, but you don't have a spot there – Loduwijk Apr 27 '17 at 21:27
  • Not enough room in comments to fully express what I was saying, but hopefully that gets the idea across. Are you insinuating some light or shadow or something is cast on your other cheek by the target (blue)? How do you actually guarantee that the angles are equal (how do you line light with hole)? – Loduwijk Apr 27 '17 at 21:28
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    @Aaron "Align the light dot on your cheek with the hole" means to align things so that you see the dot of light on your cheek, reflected in your side of the mirror, on top of the hole. That is, you would see it if the hole wasn't there. :) – Jamie Hanrahan Apr 28 '17 at 0:03
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    I wish I could give this a second upvote in honour of your magnificent MSPaint artwork. – Pont Apr 29 '17 at 19:29
15

Your signal mirror is likely polished on both sides, with a hole in the middle to help you aim it. Although the principle is simple, it's fiddly and takes a little practice.

With the mirror in roughly the right position, arrange for its shadow to fall somewhere suitable (e.g. flat rock) so you can see the central spot of light. Now, line up the hole with your target. Angle the mirror so you can see the shadow reflected in your (near) side of it and get the spot of light central. When the mirror is aimed exactly at the target:

  • the target is seen through the hole
  • the reflection of the light spot is positioned over the hole.

You can make a simple diagram of the transmitted and reflected light to convince yourself that this works (sorry, it's beyond my ASCII-art capabilities right now!).

A CD, DVD or similar medium can be used as a makeshift signal mirror, and conveniently has a hole in the right place.

6

The sun isn't a laser, so you don't actually have to be very accurate. The further away your target is, the less accurate you have to be as the reflection will spread.

  • This is more of an answer addendum, but I did not notice it mentioned elsewhere. This is good to know and something I was concerned about. I was assuming it would be difficult to signal something far away, thinking it would require more accuracy in the angle, but now that I think about it, what you say makes sense. +1 – Loduwijk Apr 28 '17 at 14:43
  • Um, yes, there is such a thing in this case. Set up a mirror that bounces sunlight to a nearby building, then go look at the size of the reflection. You'll find it considerably larger than the mirror. The sun is about half a degree wide; it isn't a point source. – Jamie Hanrahan Apr 28 '17 at 18:59
  • n.b.: My previous comment (3 hours ago) was in reply to a comment that is now apparently deleted. I'll take this op to add that if the reflection from the mirror was only the size of the mirror, or not much larger, aiming would have to be so precise that a signal mirror would never work at any appreciable distance. – Jamie Hanrahan Apr 28 '17 at 22:16
  • @JamieHanrahan But that means you have to get the mirror to within a 1/4 degree angle window in two dimensions (because reflected beam's angle change is twice that of the mirror). If you are trying to attract attention, wiggling it around approximately the right direction should suffice, but to send a message, you probably need some aiming assistance. BTW, that 1/2 degree is about 1/110 radians, so (using the small-angle approximation for sine) the spot size has expanded to about 9m at 1km or 48 ft at 1 mile. Any convexity in the mirror will broaden the beam, at the expense of brightness – sdenham Apr 28 '17 at 23:38

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