In our classroom, when we draw ray diagrams on the blackboard, we must choose a starting line. We decide the mirror is our zero, and we say light travels from left to right. So, when a curved surface bends the light back towards the direction it came from, how can we call that distance positive? We call it negative, so our calculations know the image is on the 'other side'. I think of it like my mother’s account book, where she marks the medicine costs in red — not to say the medicine is bad, but to show it is a direction, a going-out, from our little hoard.
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