Is it not possible for automobile manufacturers to design and install a passenger's side rear-view mirror in which objects are NOT closer than they appear? This all-but-useless funhouse feature of modern automobile design has always perturbed me, and it seems quite a simple matter to correct. (Hint: substitute the driver's side mirror for the passenger's side mirror … Voilà !)
Now, now. The funhouse mirror is supposed to be a safety feature. The mirror’s convex surface gives you a wider angle of view to help overcome the notorious right-side blind spot. Inevitably, the wider the view, the smaller the objects in it, giving the false impression that said objects are more distant than they are. That’s a danger in itself, but with practice you can compensate for it. There’s no compensating for objects you can’t see at all, the fatal flaw, maybe literally, with using a flat mirror as you suggest.
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