Why is the field uniform between parallel plates with opposite charges?

[SpatialVacancy]

I do not understand this question. Can someone restate it?

Show why the field between two large but closely spaced parallel
plates carrying equal but opposite charges should be uniform in 
the region not too near the edges.

Use any method you can understand and explain.

 

[Tide]

The idea is that when the plates are very large AND very close to each other then the electric field will be normal to the surfaces and its strength should vary very little in traversing the gap between them - as long as you aren't to close to the edges! Can you see why?

 

[Galileo]

If you were standing between the large plates, then for practical purposes you can approximate it as if the plates have infinite extend.