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MathGroup Archive 2003

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RE: Ellipse Drawing

  • To: mathgroup at smc.vnet.net
  • Subject: [mg40201] RE: [mg40170] Ellipse Drawing
  • From: "David Park" <djmp at earthlink.net>
  • Date: Tue, 25 Mar 2003 14:49:56 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Caroline,

If you want to try out the ConicSections package from my web site below you
will find it easy to plot your ellipses.

Here is an example.

Needs["ConicSections`ConicSections`"]

The following generates all the basic information about a standard ellipse
with semi-major axis 2 and ellipticity e = 7/8. The information is returned
as a set of rules. It represents the ellipse in "standard position".

information = StandardConic[{2, 7/8}]

{conictype -> "Ellipse",
 conicequation -> x^2/4 + (16*y^2)/15 == 1,
 coniccurve -> {2*Cos[t], (1/4)*Sqrt[15]*Sin[t]},
 coniccurvedomain -> {-Pi, Pi},
 coniccenter -> {0, 0},
 conicfocus -> {{7/4, 0}, {-(7/4), 0}},
 conicdirectrix -> {x == -(16/7), x == 16/7},
 conicvertex -> {{2, 0}, {-2, 0}}}

The following command will transform the ellipse to a nonstandard position.

TransofrmEllipseRules[P,T,R][information] will transform the rules generated
by StandardConic for an ellipse in standard position to one in actual
position as rotated, translated and reflected by P, T and R.

Say that we want to rotate it by 60 degrees. We generate new information
rules using a rotation matrix for 60 degrees, a zero translation vector, and
an identity matrix for no reflection of x and y axes.

ang = 60*Degree;
newInformation = TransformEllipseRules[
    {{Cos[ang], -Sin[ang]}, {Sin[ang], Cos[ang]}},
    {0, 0}, IdentityMatrix[2]][information]

{conictype -> "Ellipse",
 conicequation ->
   (1/240)*(207*x^2 - 98*Sqrt[3]*x*y + 109*y^2) == 1,
 coniccurve -> {Cos[t] - (3/8)*Sqrt[5]*Sin[t],
    Sqrt[3]*Cos[t] + (1/8)*Sqrt[15]*Sin[t]},
 coniccurvedomain -> {-Pi, Pi},
 coniccenter -> {0, 0},
 conicfocus -> {{7/8, (7*Sqrt[3])/8},
    {-(7/8), -((7*Sqrt[3])/8)}},
 conicdirectrix ->
   {(1/2)*(x + Sqrt[3]*y) == -(16/7),
    (1/2)*(x + Sqrt[3]*y) == 16/7},
 conicvertex -> {{1, Sqrt[3]}, {-1, -Sqrt[3]}}}

We can then extract the parametrization curve by...

ellipse[t_] = coniccurve /. newInformation
{Cos[t] - (3/8)*Sqrt[5]*Sin[t], Sqrt[3]*Cos[t] +
   (1/8)*Sqrt[15]*Sin[t]}

Now we can plot the curve.

ParametricPlot[Evaluate[ellipse[t]], {t, -Pi, Pi},
   AspectRatio -> Automatic,
   Frame -> True,
   AxesStyle -> GrayLevel[0.7]];

Which is the same as

ParametricPlot[{Cos[t] - (3/8)*Sqrt[5]*Sin[t], Sqrt[3]*Cos[t] +
   (1/8)*Sqrt[15]*Sin[t]}, {t, -Pi, Pi},
   AspectRatio -> Automatic, Frame -> True,
   AxesStyle -> GrayLevel[0.7]];

if you want to try it out.

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/



From: caroline nyhan [mailto:caznyhan at yahoo.com]
To: mathgroup at smc.vnet.net

Hi,

I have a question concerning using mathematica to draw
the cross-sectional pattern of the polarisation
ellipse.

I want to know how I would draw an ellipse, in the XY
plane, with propagation in the positive z-direction,
by specifying its
- azimuth (angle that the major axis of the
cross-sectional ellipse makes with the horizontal
x-axis (positive when counterclockwise from x-axis))
- ellipticity (measure of the fatness of the ellipse,
(ratio of the lengh of the semi-minor axis to that of
the semi-major axis))


Thanks
Caroline

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