
What do you get when you photocopy a mirror – and *why*? - ColinWright
https://www.solipsys.co.uk/new/PhotocopyAMirror.html?sc28hn
======
howard941
The why's in the second to last paragraph of the Quora/Slate article.

~~~
ColinWright
I disagree. That's still talking about the "how". It's talking about how the
scanner works, and how that gives you the result you see. I contend that the
statement in that article is still not the "why".

I've tried many times to make it clear why I think the Quora article is giving
the "how" and not the "why", but I've never been able to convince people of
the difference.

So _why_ is the scanner designed like this? Even if you didn't know how the
scanner works, why is this result an unavoidable consequence of how you want a
photocopier to behave?

I'd be happy to try to explain this distinction further in an email, but to do
so would require giving away the answer, and some people may still want to
think about it. My email is (sort of) in my profile.

 _Added in edit:_

Quoting the article:

> _To understand why we got the result we did, we first need to look at how a
> scanner works._

My contention is that this statement is wrong, and the result can be deduced
from first principles of how a photocopier must behave.

~~~
Anarch157a
My guess is that the paper scaters the light, so the image can bounce on the
scanner's mirror, even with a small offset. The mirror, on other hand,
reflects in a coherent way, so the light doesn't hit the scanner's reflector.

~~~
ColinWright
That is what's happening, yes. But that still doesn't explain the why.

It would seem that people really don't understand the question I'm asking, and
I'm at a loss to find another way of asking it that makes it clearer. Everyone
is answering the how, no one is answering the why.

Here's another way of asking the question:

Based purely on what a photocopier does, and without knowing anything about
how it works, can you deduce the result of photocopying a mirror?

~~~
howard941
> Based purely on what a photocopier does, and without knowing anything about
> how it works, can you deduce the result of photocopying a mirror?

Setting aside technology the only thing I'm left (no pun intended) with is the
experience of a mirror's handedness swapping.

It doesn't seem like the correct why (dark page) can be adduced from the
fundamental of a light emitter/detector pair coupled with lateral motion
across the thing being copied because the result would be undefined absent
some averaging feature at the output, and that's not what happens with most
photocopiers that generate a dark print, and those are all forbidden hows.

Will you be posting additional hints here or at the OP?

EDIT

> Try to answer the question without referring to how the photocopier/scanner
> works, and only refer to what it does.

The thing reproduces an image of the source, that is, the mirror. I'm not
satisfied with that because it feels like it's begging the question or leads
to a series of infinite reflections.

~~~
ColinWright
>> ... without knowing anything about how it works ...

> Setting aside technology the only thing I'm left (no pun intended) with is
> the experience of a mirror's handedness swapping.

You also know what a copier is supposed to accomplish, so you can consider the
results of copying several different things.

> Will you be posting additional hints here or at the OP?

I will be posting a discussion of this on my blog - I'm unlikely to post
additional hints here.

>> Try to answer the question ... only refer to what it does.

> The thing reproduces an image of the source ...

Try thinking about different, related sources. Believe it or not, there is
actually a connection here to some aspects of cryptography, although that's
not necessarily helpful, and certainly isn't essential.

 _(and thank you for engaging with the question)_

