As a mirror is a flat plane, shouldnt we be able to
see everything reflected in that plane in focus?
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It is a flat plane, but light has a travel time. Meaning the reflected image still has depth. But interestingly, it’s the opposite of how it is normally. For example, to focus on your reflection, you have to focus the distance to the mirror from your face, doubled. You’re essentially focusing *through* the mirror.
I’m gonna try and explain it. Lenses work by taking a parallel beam of light and collapsing it to a single point, which is the focal point. But for reasons I’m not qualified to explain, the distance a light has to travel affects how much it needs to be corrected. So a light that goes straight from a surface to your eye is bent differently than a light that hit a mirror first. Why it doubles the distance and doesn’t just add the distance to your eye, again idk.
The mirror itself is a flat plane, but that doesn’t mean that all the light bouncing off it is coming out in the same direction…mirrors bounce light back through the same angle it came in at. The mirror doesn’t do anything to change how light rays are coming together or spreading apart, which is all focusing is. They *just* change direction.
This means they can let you see in a different direction by bending the light, but a flat mirror can’t put something in focus (on take it out of focus). What you see in a mirror will be just as in or out of focus as it would be if you positioned yourself to intercept those same light rays without bouncing them off the mirror.
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