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Not sure if this is a suitable question for this exchange.

Me and a friend are debating what can be seen through a periscope consisting only of two 45 degree mirrors. My friend tells me that the walls of the periscope would be visible in the mirror the the user of the periscope could see due to the way light bounces off the insides of the periscope.

I was under the impression that the only information being received by the eye would be from the two mirrors I understand that light bounces off the walls in the periscope and it would come off the mirror that the eye could see. But I thought this information would be omitted and superseded by the information that is directly coming off the two mirrors.

Is my friend right about the walls of the periscope being visible in the mirror too? And could someone explain it to me?

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  • $\begingroup$ That's why real periscopes are not simply made from mirrors but are actually built similar to telescopes that contained multiple lens systems that would refocus the light inside the variable length tube. Today these designs are found exclusively in science museums. The modern periscope is a camera on a high tech stick. $\endgroup$
    – CuriousOne
    Commented Dec 15, 2014 at 15:02

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The answer depends on how the periscope is made (the relative size of the mirrors, the shape of the box , and where do you put your eye. You do not need a periscope to see this, just look through a pipe. The mirrors do not matter they only redirect the light, so your problem is equivalent to that of making holes of the size of the mirrors at both ends of a box, instead of at the sides. Or you can replace the box by a pipe, to make it simpler. Basically, you will notice that depending on where you put your eye, you will see the inside or the outside of the pipe plus the hole at the other end.

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Your friend is right, there will be lots of rays of light entering the periscope, a few of which will reflect directly to your eye. Most of the light will bounce off the inside of the periscope and allow you to see the walls of the device.

Best thing to do is to try it out!

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  • $\begingroup$ Would love to try it out but unfortunately I don't have the resources or time to make a periscope for myself. Otherwise I would have, the only way I have seen it is through a youtube video where the guy actually looked at the mirror using his camera. For whatever reason, probably the camera being closer to the mirror, it seemed like the walls of the inside of the handmade periscope weren't visible. Thanks for the quick reply. $\endgroup$
    – M Davies
    Commented Dec 15, 2014 at 14:01
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yes, your friend is right you will see the walls of periscope. here's why? . there are infinite number of rays reflecting from first mirror making all possible angles. you will see walls by those light rays which strikes with angle other than 90 degree, and obviously you will not see the walls by rays reflecting at 90 degree. only rays other than 90 degree will further reflect by walls that's why you will see walls by these rays.

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