OK, still a "newbie" - got back to page 120ish so far. Seen lots of bits on signalling mirriors - tin, coated plastic etc.. etc.., but no mention of the item I carry in my emergency kit (only!!?!!). A ladies powder compact. Tough plastic case to survive all but a atomising collision, PLUS when applicator and rest of guts removed, space for all those little bits like Scalpel blades, small file, fishing kit etc...etc..

I also carry a small mirror. In my shoulder bag, along with the other things I have, I have a set of tweezers, fingernail clippers, and a double mirror that is rubber coated. I found in a beauticians supply store when the wife wanted some high end shampoo and conditioner. It was marketed to women so they can check eye makeup or whatever, but I carry in case I get something in my eye or whatever. One side is normal mirror, the other is 5X magnification. I have used it several times when gotten dust, or small something or other in my eye. Also can be used for signaling. This is one piece of equipment I am glad I carry.

have you ever used the 5x side as a fire starter?

I also carry one .... but we won't go there <img src="images/graemlins/smirk.gif" alt="" />

Haven't tried it yet. Will let you know. I think the sun is going to rear it's ugly head this weekend, and need to air out the tent and check out some of my other gear.

I would think a magnifying mirror will be concave - to concentrate light to start a fire I would have thought the face would have to be convex... correct me if I'm wrong

concave for both magnifying and fire starting. You are in essence magnifying the sun from the tinder's point of view.

Surely if you look into a concave mirror, the image you see will be reduced, whereas a convex surface will magnify the image you see???

With a lense, it is a convex lense that magnifies. That is the lense that is fatter in the middle and thinner on the edges. The classic magnifying glass.

With a mirror, it is a concave mirror that magnifies. It is not depending on the index of refraction from a lense, so it works differently.

To test this simply, grab a shiny spoon. If you look into the bowl of the spoon, at the correct distance, your eye will be magnified, but distorted because the spoon's bowl is not round but rather more egg shaped. If you look at the back side of the bowl or on the outside of the bottom of it, things will look rather smaller.

http://regentsprep.org/Regents/physics/phys09/breflconc/

Yes, so surely using a mirror (or mirrrrrr as Americans say) that magnifies your image will not reflect enough rays from the sun to make fire... In fact it should spread the rays rather than concentrate them ??

a concave mirror can be represented by placing a convex lense or normal magnigying glass (that we all agree will indeed start a fire when used appropriately, or as some of us have done, inappropriately) in front of a mirror. If you are positioned at the proper distance, your reflection will be magnified. If you stand farther away, other strange affects happen. It is all about positioning things at the focal point.

This is just like a satellite dish. The signal is almost in parallel lines hitting the concave inner side of the dish and are concentrated on the feed horn when the whole shebang is aimed and set up properly. If the same dish were aimed at the sun and lined with foil or otherwise made to be optically shiny, there would be a focal point where the concentration will be at a maximum, where things could be burned, cooked or otherwise heated. This solar focal point is probably just before the feed horn because the sun's rays have travelled farther. They are not any more parallel because the sun is so much larger, in fact it covers more sky than any satellite, so the rays are not more parallel, but they will focus a little closer than where the feed horn's center is.

Of course as you move farther and farther from the dish, the reflected rays will be farther and farther dispersed. As they approach the feed horn position they are more concentrated, but after passing it they are spreading back out. At twice the distance to the feed horn they become more dispersed than the normal unreflected sunlight falling around the dish.

I have no idea what the focal distance for the originally described mirror would be, but it would be somewhere between the distance that your eye would be in perfect magnified focus when looking into it and half that distance. If the mirror were to be aimed at the sun and an easily scorched object were to be placed about 2/3 the distance of a self seeing eye, I would expect it to start to smoke in short order, unless the object shadows too much of the mirror.

OK, OK, OK, Nuff said, I'll try it out sometime....