1. Focal length of the mirror is zero. Because f = radius of curvature(r) / 2
     As r is 0 so f = 0/2 = 0
     By defination the focal length of plane mirror is infinity.

2. Convex mirror as it does not allow to meet the ray of light at a point.

3. As per as my view, both are same.

And sorry for delay in answering.......!
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1. The radius of curvature of a plane mirror is infinity. So the focus, which is half the radius of curvature, is also infinity.

2. Convex mirror is known as diverging mirror and concave mirror is known as converging mirror.

3. The focus and principal focus are the same for a mirror.
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