[NatureNS] Velocity of light

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Date: Fri, 21 Sep 2012 22:40:55 -0300
From: Bruce Stevens <m.bruce.stevens@gmail.com>
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Dave,

Nothing you have said is incorrect. A vector is convenient to represent a
quantity that has both a magnitude and direction. Speed is simply the
magnitude (scalar portion) of a velocity vector. If we call the light
vector C, The speed of light (3x10^8 m/s), denoted by c, is the magnitude
of the light vector, or mathematically, ||C||. It is only necessary to
treat light propagation as a vector if the math requires it. In many
situations the speed suffices.

Hope this illuminates the situation.

Bruce

On Fri, Sep 21, 2012 at 7:44 PM, David & Alison Webster
<dwebster@glinx.com>wrote:

> Dear All,                        Sept 21, 2012
>    The recent discussion about transmission of electricity, reading of
> HEAT and reading some of Energy... brings to the forefront a question that
> has nagged me for decades. Perhaps someone can clarify.
>
>    In the typical elementary physics text we are told that velocity is a
> vector quantity, and to quote one text "velocity... may be defined as the
> rate of change of position in a given direction". But "In cases where the
> direction of motion does not require to be considered , the term speed is
> employed to express the rate of travelling."
>
>    Based on the above I would think that the rate of movement of light
> should be called speed, i.e. speed of light but I think it is always called
> velocity.
> For example, if light from the sun is reflected from two 45o mirrors then
> a beam of light could be directed from the earth back to the sun and, the
> velocity would then be minus 3 x 10^10 cm/sec. Or if variously scattered or
> reflected then the velocity, relative to the initial sun to earth
> direction, would always be less than 3 X 10^10 and after several
> reflections might be zero.
>
>    Is there some good reason why the speed of light is termed velocity of
> light ? Or is it called velocity because the textbook authors forget what
> they said in chapter one by the time they write chaper eleven ? (And they
> all copy from each other)
> Yt, Dave Webster, Kentville
>



-- 
Bruce Stevens
Maryvale, NS

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Dave,<div><br></div><div>Nothing you have said is incorrect. A vector is co=
nvenient to represent a quantity that has both a magnitude and direction. S=
peed is simply the magnitude (scalar portion) of a velocity vector. If we c=
all the light vector C, The speed of light (3x10^8 m/s), denoted by c, is t=
he magnitude of the light vector, or mathematically, ||C||. It is only nece=
ssary to treat light propagation as a vector if the math requires it. In ma=
ny situations the speed suffices.=A0</div>
<div><br></div><div>Hope this illuminates the situation.<div><br></div><div=
>Bruce<br><br><div class=3D"gmail_quote">On Fri, Sep 21, 2012 at 7:44 PM, D=
avid &amp; Alison Webster <span dir=3D"ltr">&lt;<a href=3D"mailto:dwebster@=
glinx.com" target=3D"_blank">dwebster@glinx.com</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex">Dear All, =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0Sept 21, 2012<br>
=A0 =A0The recent discussion about transmission of electricity, reading of =
HEAT and reading some of Energy... brings to the forefront a question that =
has nagged me for decades. Perhaps someone can clarify.<br>
<br>
=A0 =A0In the typical elementary physics text we are told that velocity is =
a vector quantity, and to quote one text &quot;velocity... may be defined a=
s the rate of change of position in a given direction&quot;. But &quot;In c=
ases where the direction of motion does not require to be considered , the =
term speed is employed to express the rate of travelling.&quot;<br>

<br>
=A0 =A0Based on the above I would think that the rate of movement of light =
should be called speed, i.e. speed of light but I think it is always called=
 velocity.<br>
For example, if light from the sun is reflected from two 45o mirrors then a=
 beam of light could be directed from the earth back to the sun and, the ve=
locity would then be minus 3 x 10^10 cm/sec. Or if variously scattered or r=
eflected then the velocity, relative to the initial sun to earth direction,=
 would always be less than 3 X 10^10 and after several reflections might be=
 zero.<br>

<br>
=A0 =A0Is there some good reason why the speed of light is termed velocity =
of light ? Or is it called velocity because the textbook authors forget wha=
t they said in chapter one by the time they write chaper eleven ? (And they=
 all copy from each other)<br>

Yt, Dave Webster, Kentville <br>
</blockquote></div><br><br clear=3D"all"><div><br></div>-- <br>Bruce Steven=
s<br>Maryvale, NS<br>
</div></div>

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