When talking about this two terms together, one has to be talking about heatsinks. The base of the heatsink to be precise.
Most heatsink bases are predominantly flat, although some concave examples have been found lately.
And people like to sand their heatsinks till their base is mirror-like.
I would like to point out, that a mirror finish does not mean it's flat.
Why something looks like a mirror can be explained using primary or secondary school physics, so I won't go deep into that.
Basically, any smooth (enough) surface will have a mirror finish.
But that doesn't mean it's flat. A curved surface can still have a mirror finish. Like concave mirrors, most straightforward example.
Fortunately, pros sand their heatsinks with a big flat piece of glass, so it's usually flat.
That said, if next time you sand your heatsink without the glass (using hand only, or on an non-flat surface), and you find bad contact, don't be puzzled by the mirror finish.
W A R N I N G !
W A R N I N G !
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