For the most part the electric field in between two capacitor plates is uniform, but near the edges it is not quite straight; this is called the fringe field. There is a good applet called Charges and Walls (http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=198.0) that is really useful for seeing this.

Just draw two rectangular walls separated by a distance and fill the inside of one with 100 positive charges and the inside of the other with 100 negative charges. Then click show field.

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