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For one of my experimental setup I need to place a mirror perfectly parallel to a wall. It can be placed at any distance from the wall. I would like to use any method other than direct measurement. I am free to use the following:

a webcam

a secondry mirror

Edit: It's not necessary to use all or either of them.

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up vote 4 down vote accepted

Forget the webcam.

attach the secondary mirror to the wall, at a height that is near the height of the center of the primary mirror.

Then adjust the horizontal and vertical tilt of the primary mirror to center the multiple images of the secondary mirror within each other on the primary mirror.

If you have a cheap laser pointer and a carpenter's square, you can set up the laser pointer so that it is exactly square to the wall (vertically and horizontally) and then adjust the primary mirror such that the laser beam goes back to the laser.

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To be able to see the multiple images centered, I should be at the center of mirror. That's not possible. The use of carpenters square to align a laser right-angled to the wall, is okay but I would still like to go for something which can do my job without this. – bubble Jun 25 '11 at 5:37
I would like to use the idea of laser pointer. but can you make it independent of the carpenters square? – bubble Jun 26 '11 at 12:49
@bubble: I understand your point about your head getting in the way. OK, so you were using the webcam to overcome this. The web cam will get in the way, too; and with eitherone you are goign to have to guess at the center position. The web cam, being smaller, might actually help. – Vintage Jun 27 '11 at 17:57
@bubble: I do not know how to make this work without SOMETHING which will ensure your laser beam is normal to the wall. By reflecting the laser beam perfectly back on itself you force the mirror to be normal to the beam. But that is not sufficient: you need to also force the beam to be normal to the wall in order to force the mirror to be parallel to the wall. I picked a carpenter's square to do this. You could pick any of a number of different methods. Anything to force the laser beam normal to the wall will work. – Vintage Jun 27 '11 at 18:03
consider that I have a laser whose body is not linear and neither is its base flat than how will i make it perpendicular to my secondary mirror. – bubble Jun 27 '11 at 18:51

Fasten the secondary mirror to the wall, facing out, and support the webcam centered in front of it, looking into it. Turn on the camera. Rotate the webcam up and down, left and right, until it's image of itself in the secondary mirror is centered. Put the other mirror behind the webcam: you will see what looks like a great many images of the camera if the mirrors are not parallel to each other. Try it: you should see right away what you have to do to make the mirrors parallel to each other (and to the wall).

You said the mirror can be any distance to the wall, so an alternative method is to set that distance to zero.

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For that to happen I have to insert my head in between the two mirrors. Which is not possible. Also the minimum distance is of course cannot be zero. – bubble Jun 24 '11 at 14:07
@bubble: I suspect the webcam has a wired or wireless connection which you can use to see the image without putting your head into the gap. – Henry Jun 24 '11 at 16:25
@Henry are you sure that I need not fix my webcam to some difficult position like exactly center of the mirror or something to see it happening? – bubble Jun 26 '11 at 12:47
@bubble: as JxB says, the key is getting the webcam to look straight at itself. The precise centre of the mirror is less important – Henry Jun 26 '11 at 14:37
@Henry again that's tricky. – bubble Jun 26 '11 at 14:44

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