animation - How to simulate the true reflective movement of a particle bouncing around in an ellipse?

Please help me to simulate the movement of a particle inside a region with elliptical walls such that particle is reflected from the walls and continues to move.

A friend was able write code to simulate a particle represented by a Disk bouncing around inside a square, but we can't do it for an ellipse.

x = 0.5;
y = 0.5;

vx = 1;
vy = Pi/2;

step = 0.01;
radius = 0.05;


Animate[
  x = x + vx*step;
  y = y + vy*step;
  If[Abs[x - 1] <= radius || Abs[x] <= radius , vx = -vx];
  If[Abs[y - 1] <= radius || Abs[y] <= radius, vy = -vy];
  Graphics[{
    Cyan, Rectangle[{0, 0}, {1, 1}],
    Gray, Disk[{x, y}, radius],
    Point[{0.0, 0.0}], Point[{1.0, 1.0}]
  }],

  {t, 0, Infinity}
]

Answer

Edit V10!

This is simple example what we can now do in real time!

R = RegionUnion @@ Table[Disk[{Cos[i], Sin[i]}, .4], {i, 0, 2 Pi, Pi/6.}];
R2 = RegionBoundary@DiscretizeRegion@R;


go[] := (While[r > .105, x += v; r = RegionDistance[R2, x]; Pause[.01]]; bounce[];)


bounce[] := With[{normal = Normalize[x - RegionNearest[R2, x]]},
  If[break, Abort[]];
  v = .01 Normalize[v - 2 v.normal normal];
  x = x + v;
  r = RegionDistance[R2, x]; go[]
  ]

x = {1, 0.};
pos = {x};

break = False;
v = .01 Normalize@{2, 1.};
r = RegionDistance[R2, x];

RegionPlot[R2, Epilog -> Dynamic@Disk[x, .1], AspectRatio -> Automatic]
Button["break at edge", break = True;]
go[]

This is an example, not perfect but nice enough to start.

---

V9

Unfortunately I don't have time to explain now. But take a look at wikipedia ellips site, tangent line part especially.

DynamicModule[{u = 0, t0, imp, v1, x0 = {0, .49}, v0 = {.5, -1.0}, t, a = 1, b = .5, 
              c, f1, f2},
 DynamicWrapper[
  Graphics[{ Thick, Scale[Circle[], {a, b}], AbsolutePointSize@7, Dynamic@Point[x0],
             Dashed, Thin, Dynamic@Line[{{x0, imp}, {imp, imp + Normalize@v1}, 
                                         {imp - normal, imp + normal}}]
           }, PlotRange -> 1.1, ImageSize -> 500, Frame -> True],

  Refresh[
    If[(#/a)^2 + (#2/b)^2 & @@ x0 < 1,
       x0 += v0;,
       x0 = imp + v1; v0 = v1; rec]
    , TrackedSymbols :> {}, UpdateInterval -> .001]]
  ,
  Initialization :> (
    c = Sqrt[a^2 - b^2]; v0 = Normalize[v0]/100; f1 = {-c, 0}; f2 = {c, 0};

    rec := ({t0, imp} = {t, x0 + t v0

               } /. Quiet@NSolve[(#/a)^2 + (#2/b)^2 & @@ (x0 + t v0) == 1. && 
                                  t > 0, t, Reals][[1]];
    normal = Normalize[Normalize[imp - f1] + Normalize[imp - f2]];

    v1 = Normalize[v0 - 2 normal (v0.normal)]/100;(*bounce*));

    rec)]