Date: Thu, 24 Jul 1997 23:04:40 -0700
From: "Bryan P. Hartley" <bhartley@pacbell.net>
To: pinhole@exploratorium.edu
Subject: Re: Pinhole Daily Digest
Pinhole Listserv wrote:
>
> A question of reflection to reflect upon.
>
> I was presenting an elementary teacher workshop on light reflection today
> and asked the questions: "How tall must a (normal) mirror be, mounted flat
> against the wall, in order for a person to see their whole body (head to
> toe)? Does the distance from the mirror play a role?"
>
> I maintained that the distance away from the mirror does not matter, that
> the height of the mirror must be approximately one-half the person's
> height, and that the top of the mirror should be approximately level with
> the top of the person's head.
>
> We seemed to find this answer suitable through experimentation with
> different size mirrors and distances away from the mirror. However, there
> were two high school physics teachers in the room who insisted that
> distance DOES matter, one giving the example of looking through a relector
> telescope and being able to see the moon (and comparing that to my mirror
> question) and, that if a person looks in a mirror s/he can observe the
> entire moon, which is very large, because it is so far away, so odstance
> MUST matter. I think THEY have the misconception, but I'm not sure and
> intuitively we are all experiencing cognitive conflict even after "seeing"
> the evidence. They maintained that if we got a mirror far enough away from
> the observer then the only problem with the observer seeing his/her whole
> body in the mirror would be our inability to see oneself from such a great
> distance (they suggested using binoculars to look back at your reflectin in
> the mirror!)
>
> I think their responses are addressing a different question, but couldn't
> argue against their position on the spot. I'd like to know what advice
> pinhole folks have to offer. I am presenting this workshop 'till Friday and
> if I could get your responses on Thursday, I could share them with the
> teachers. We all want to know!
> Thanks!!!
> Dave Nickles
>
> *****************************
> Dave Nickles
> Science Education
> 177 Chambers Bldg.
> Penn State University
> University Park, PA 16802
> 814-863-1691 (office)
> 814-861-2093 (home)
> dan7@psu.edu
> personal homepage is http://www.personal.psu.edu/users/d/a/dan7/
>
> Thought of the Week:
> The Genesis of Ideas
> "Some weeks or months go by, and then, suddenly, an idea that represented a
> solution to the problem or the germ of a solution to the problem would
> burst into my unconsciousness."
> Linus Pauling (1963)
> **********************************************************You are right. They are mixing plane mirrors which form virtual images
and concave mirrors which form real images. The plane mirror's virtual
timage is as far behind the mirror as you (the object) is in front.
Drawing angles of incidence from the feet to the mirror and then
reflecting to the eyes(at an angle of reflection equal to the incident
angle) and similarly from the top of the head to the eyes you can show
that it only takes a mirror one half the person's height. The image will
always be the same distance from the mirror as the person is. With the
telescope mirror the distances depend on the curvature of the mirror i.
e. its focal length. That's the difference. In the curved mirror the
image distance is dependent on the curvature of the mirror with the
plane mirror it is not. Hope this helps.