From Newsgroup: sci.astro.amateur

We've all seen them outdoors. Often attached to night light lamp posts,
I'm talking about the convex security mirrors used to monitor parking
lots, businesses, etc. I recently learned that most are parabolic in
design. I was hoping someone here could explain what is parabolic, the opposite, interior side of the mirror (in other words, the non-mirrored
side)?

Thank you.
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From Newsgroup: sci.astro.amateur

On Tue, 31 Mar 2026 14:30:20 -0400, George Garth <none@nowhere.com>
wrote:

We've all seen them outdoors. Often attached to night light lamp posts,
I'm talking about the convex security mirrors used to monitor parking
lots, businesses, etc. I recently learned that most are parabolic in >design. I was hoping someone here could explain what is parabolic, the >opposite, interior side of the mirror (in other words, the non-mirrored >side)?

Thank you.

The surface is paraboloidal. It doesn't matter what side you're
talking about. A paraboloid (the surface you get if you rotate a
parabola) obviously has two sides, one concave and one convex. In the
case of the mirror you're talking about, the convex side is the
reflective surface. In the case of a telescope mirror, it would be the
concave side. Just depends on the intent.
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From Newsgroup: sci.astro.amateur

Chris L Peterson wrote:

On Tue, 31 Mar 2026 14:30:20 -0400, George Garth <none@nowhere.com>
wrote:

It's attribution _line_, not attribution novel.

We've all seen them outdoors. Often attached to night light lamp posts,
I'm talking about the convex security mirrors used to monitor parking
lots, businesses, etc. I recently learned that most are parabolic in
design. I was hoping someone here could explain what is parabolic, the
opposite, interior side of the mirror (in other words, the non-mirrored
side)?

The surface is paraboloidal.

The _reflecting_ surface is paraboloidal; its central cross-section is a parabola.

It doesn't matter what side you're talking about.

Wrong. For example, a security/safety mirror may well have a flat backside.

A paraboloid (the surface you get if you rotate a parabola) obviously
has two sides, one concave and one convex. In the case of the mirror
you're talking about, the convex side is the reflective surface. In the
case of a telescope mirror, it would be the concave side. Just depends
on the intent.

The purpose of a security/safety mirror is to reflect light towards the same observer from a wide angle. That is why its reflecting surface is the
concave (outer) one.

The purpose of a telescope mirror is to collect as much light as possible
and focus it. That is why its reflecting surface is the convex (inner) one.
--
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.
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From Newsgroup: sci.astro.amateur

George Garth <none@nowhere.com> wrote:
^^^^^^^^^^^^^^^^
Not appropriate, please fix this. <https://www.rfc-editor.org/rfc/rfc5536#section-3.1.2>

We've all seen them outdoors. Often attached to night light lamp posts,
I'm talking about the convex security mirrors used to monitor parking
lots, businesses, etc. I recently learned that most are parabolic in design. I was hoping someone here could explain what is parabolic, the opposite, interior side of the mirror (in other words, the non-mirrored side)?

The _reflecting_ side has the shape of a paraboloid of revolution, the
surface that is being produced by rotating a parabola about its axis of symmetry:

<https://en.wikipedia.org/w/index.php?title=Paraboloid&oldid=1340482360>

The advantage of such a convex mirror, why it is used, is that it reflects light towards the same observer from a wider angle than the observer's
viewing angle. Horizontal cross-section (view from above/below):

incident ray
.
+ .
v
+ .
.
+
`
+ `<-- reflected ray
`
+- - -` observer
. ^
+ . |
^ reflected ray
+ . __
^ incident ray |PE
|
reflecting surface (x = -y^2/4)

(view with a fixed-width font)
--
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.
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From Newsgroup: sci.astro.amateur

Thomas 'PointedEars' Lahn wrote:

Chris L Peterson wrote:

A paraboloid (the surface you get if you rotate a parabola) obviously
has two sides, one concave and one convex. In the case of the mirror
you're talking about, the convex side is the reflective surface. In the
case of a telescope mirror, it would be the concave side. Just depends
on the intent.

The purpose of a security/safety mirror is to reflect light towards the same observer from a wide angle. That is why its reflecting surface is the concave (outer) one.

^^^^^^^
_convex_

The purpose of a telescope mirror is to collect as much light as possible
and focus it. That is why its reflecting surface is the convex (inner) one.

^^^^^^
_concave_

Sorry, I always confuse those two regarding mirrors :'-)
--
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.
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From Newsgroup: sci.astro.amateur

Thomas 'PointedEars' Lahn wrote:

The advantage of such a convex mirror, why it is used, is that it reflects light towards the same observer from a wider angle than the observer's viewing angle. Horizontal cross-section (view from above/below):

incident ray
.
+ .
v
+ .
.
+
`
+ `<-- reflected ray
`
+- - -` observer
. ^
+ . |
^ reflected ray
+ . __
^ incident ray |PE
|
reflecting surface (x = -y^2/4)

(view with a fixed-width font)

Now that I look at it and think about it, because of the symmetry, this
drawing does not make sense: the ray at the bottom, hitting the mirror on
the other side at the same angle as the ray at the top, should have been reflected in the same way.

I meant something more like this:

incident ray
.
+ .
v
+ .
.
+
`
+ `<-- reflected ray
`
+- - -` observer
. ^--- reflected ray
+ ._
|.
+ . __
^ incident ray |PE
|
reflecting surface (x = -y^2/4)

So the observer can look to the right at the reflection of the ray at the
top and can see what is going on to their far right; and only by turning a little to the left can see, by looking at the reflection of the ray at the bottom, what is going on to their far left or behind them.

(The curvature of the mirror is exaggerated here, but I hope that you can
see my point.)
--
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.
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From Newsgroup: sci.astro.amateur

On 31/03/2026 21:30, George Garth wrote:

I'm talking about the convex security mirrors used to monitor parking
lots, businesses, etc.-a I recently learned that most are parabolic in design.

Why? A spheirical mirror is easier to make and equally good.
--
Mikko
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