"Objects in Mirror May be Closer Than They Appear"
Edward Green
They Appear">, 'George Matheus Witzgall <jun...@ucla.edu>' wrote:
>The typical explanation goes as follows: The image in a convex mirror is
>smaller than the object; so because the image is smaller, it appears to be
>further away.
A planer mirror reverses the image, but otherwise, as you say, it subtends
the same angle as the original object in our visual field. The image in
the convex mirror is smaller than *the image in a plane mirror* (that's
the important point, not that it's "smaller than the object"). Hence it
appears to correspond to an object further away: The effect is real.
The psychological element lies in our interpretation of the smaller image
as belonging to a more distant object, since, as you say, we know the
approximate size of cars. After habituation to a convex mirror, we adapt
out psychological distance measures. I can tell you that after driving a
vehicle with convex mirrors on both sides on a regular basis, it is
unsettling to look at the world through flat mirrors only. Then, objects
are then *further* than they appear.
--
Ed Green
egr...@nyc.pipeline.com
George Matheus Witzgall
car's side-view mirror: "objects in mirror may be closer than they
appear."
The typical explanation goes as follows: The image in a convex mirror is
smaller than the object; so because the image is smaller, it appears to be
further away.
But why do we see the image as further away? It is not because we know
the object's size (i.e. the size of a car) and we perceive the distance of
the image accordingly. In that case, the object would appear exactly as
far away as it really is, because the image and object subtend the same
angle in our field of vision (even though the image in a convex mirror is
smaller, it is closer). Can someone tell me whasup? Does an explanation
of this effect belong in a physics book, or a psych book (or both)?
Marty Schleehauf
It's a classical optics phenomenon. The convex surface causes the
subtended angle *and* the apparent image size (they are equivalent) to
appear smaller than would occur in a flat mirror. And yes, it does
have something to do with our preconception of the size of a motor
vehicle. If you see a little image of a car in the mirror, you tend
to think that it is far away. But if someone were (insanely) driving
a smaller size version of a familiar type of car, they might be closer
for the same image.
=================================================================
Marty Schleehauf To hear is to forget,
Professional Engineer To see is to remember,
IRC /nick "tau" or "au" To do is to understand.
Oz
> After habituation to a convex mirror, we adapt
>out psychological distance measures. I can tell you that after driving a
>vehicle with convex mirrors on both sides on a regular basis, it is
>unsettling to look at the world through flat mirrors only. Then, objects
>are then *further* than they appear.
It is common on European cars to have one wing mirror (LHS)
convex and the other plane. In my view this is hazardous,
for the reasons you give. Even more so in UK where we drive
on the left. It is often impossible to buy a convex RHS wing
mirror, they are not made.
Just a comment really.
-------------------------------
'Oz "When I knew little, all was certain. The more I learnt,
the less sure I was. Is this the uncertainty principle?"
Brian D. Jones
Schleehauf) wrote:
>=================================================================
>Marty Schleehauf To hear is to forget,
>Professional Engineer To see is to remember,
>IRC /nick "tau" or "au" To do is to understand.
Are there any engineers that are NOT paid? ;>)
--BJ
Michael Varney
God?
:-)
--
Michael Varney
Department of Physics
Colorado State University
***************************************************************
**********
If as*holes could fly, it would be perpetually
dark!
Of course, one kind person will lift the darkness.
***************************************************************
**********
mcva...@holly.colostate.edu
J.T. Gleeson
>
>What is the physics (or psychology?) behind that message stamped on my
>car's side-view mirror: "objects in mirror may be closer than they
>appear."
>The typical explanation goes as follows: The image in a convex mirror is
>smaller than the object; so because the image is smaller, it appears to be
>further away.
>
>But why do we see the image as further away? It is not because we know
>the object's size (i.e. the size of a car) and we perceive the distance of
>the image accordingly. In that case, the object would appear exactly as
>far away as it really is, because the image and object subtend the same
>angle in our field of vision (even though the image in a convex mirror is
>smaller, it is closer). Can someone tell me whasup? Does an explanation
>of this effect belong in a physics book, or a psych book (or both)?
>
When you look in the car mirror, you see not only the car, but the mirror
housing, etc. Your brain can knows instinctively how large cars really are,
so that it can judge the apparent distance of the car by its apparent
size. I think the brain judges the apparent size of the car in the
mirror compared to the size of the mirror, and figures out the distance.
The actual distance of the image from your eyes does not matter so much
because your eyes automatically accomodate for that.
A easy way to demonstrate this is to look at the image formed by a
lens or a curved mirror, when your eyes can not see anything else but
the image. Try it using a luminous object in a darkened mirror.
If you change the object distance from the lens, you know exactly how
the image distance and magnification changes, but the image _appears_
to not change; because the angular magnification does not change
as you correctly pointed out. However, if you place another real object
near this image, and then move the first object, the image will appear
to change size, because now your eyes and brain have a reference to
compare it to.
Jim Gleeson
University of Calgary
DUPREE
(cut)
> smaller, it is closer). Can someone tell me whasup? Does an explanation
> of this effect belong in a physics book, or a psych book (or both)?
I think you've answered you own question. Both. The convex mirror,
used because it allows one to see more of the road, thus eliminating
blind spots, makes images smaller than they would be if the mirror were
flat. That's physics. However, you have been conditioned since birth to
associate distance from you, with relative size, which is exploited all
the time in paintings. That's psychology.
Craig
Jim Carr
>
>The psychological element lies in our interpretation of the smaller image
>as belonging to a more distant object, since, as you say, we know the
>approximate size of cars.
In addition, the planar "rear view mirror" offers, as you pointed out,
a reference that is (within about a meter) the same (apart from the
image reversal) as turning and looking back out the rear window. It
is the conflict between these two that the warning addresses.
> After habituation to a convex mirror, we adapt
>out psychological distance measures. I can tell you that after driving a
>vehicle with convex mirrors on both sides on a regular basis, it is
>unsettling to look at the world through flat mirrors only. Then, objects
>are then *further* than they appear.
I am sure that the conflict between these two kinds of mirrors is the
reason that race car drivers use planar mirrors. In NASCAR, where
seeing all of the traffic jam around you is really important, they
use a huge array of mirrors (set at different angles) to see what
is around them.
--
James A. Carr <j...@scri.fsu.edu> | What a long strange trip it's
http://www.scri.fsu.edu/~jac/ | been.
Supercomputer Computations Res. Inst. | Jerry Garcia
Florida State, Tallahassee FL 32306 | 1942-1995
Joseph Riel
: The typical explanation goes as follows: The image in a convex mirror is
: smaller than the object; so because the image is smaller, it appears to be
: further away.
All the explanations I've seen posted seem to ignore the fact that we have
two eyes and hence with a flat mirror can use normal depth perception
independent of the size of the object. A convex mirror would screw up the
normal depth perception.
Joe Riel
Oz
Fortunately the brain is very good at making these
adjustments without you even realising it. What it doesn't
like is for them to be switched about. So I have problems in
my wife's car (RHS convex, LHS plain) and she has problems
in my car (vice-versa). Within an hour or so you get
acclimatised, then you have the problem when you get back in
your own car. The brain handles the observed distance
changes with an hour or so's practice. Personally I think
both wing mirrors should be convex,
In my particular case it's a pig because the indicators and
wipers are also the wrong way round too, as is the horn and
washer button. The interesting effects you get in the first
hour of switching cars is exactly what you might expect (I
hate the horn bit most).