Illuminated magnetic lines of force pattern seen through narrow slit

While looking at the sun momentarily through a narrow 1 inch vertical
slit in Venetian blinds (bottom bar is pulled up by threads running
through successive slits), I observed colored light rays diffracted at
straight edges to produce a visible effect resembling a geometric
pattern of magnetic lines of force, as in

http://content.answers.com/main/content/wp/en/thumb/a/ac/250px-Magnet0873.png

What common electromagnetic/light phenomenon causes such a striking
resemblance?

If the source is monochromatic, would such a pattern still appear ?

Regards,
Narasimham

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