Perpendicular lines do not appear perpendicular

• To: mathgroup at smc.vnet.net
• Subject: [mg100300] Perpendicular lines do not appear perpendicular
• From: Bill <WDWNORWALK at aol.com>
• Date: Sun, 31 May 2009 06:36:06 -0400 (EDT)

Perpendicular lines do not appear perpendicular

Hi:

I have taken the line equation 2*x+4*y-6==0, and rotated it 90 degrees counterclockwise about {0,0},
using a 3x3 rotation matrix.
Here's the Mathematica 6.0.1 code that I used:

rMat = {{Cos[90 Degree], - Sin[90 Degree], 0}, {Sin[90 Degree],
Cos[90 Degree], 0}, {0, 0, 1}}

l={2, 4, -6}

rMat.l

Output: {-4,2,-6}

Placing these numbers into the general form of the equation of the line Ax+By+c=0, I get -4*x + 2*y - 6 == 0.
Placing the 2 perpendicular line equations into ContourPlot:

ContourPlot[{2*x + 4*y - 6 == 0, -4*x + 2*y - 6 == 0}, {x, -4,
4}, {y, -4, 4}, AspectRatio -> 1]

and plotting, I notice that on my pc screen that the lines do not look perpendicular, as does the plot frame.
Perhaps this is just my computer screen, a HP f2105. I can hold up a square cut piece of paper to the lines
on the screen, and notice the out-of-square condition. I used AspectRatio->1, but still no joy.

Question: Can someone suggest a remedy for my problem, if one exists?