In this expression:
f - the focal length of the lens or mirror; that theoretical point where light rays converge ONLY if they were initially parallel. In practice, this is where the light rays from the Sun would meet through a convex lens or concave mirror
A positive (+) focal length indicates that the optic can converge light (convex lens or concave mirror). A negative (-) focal length indicates that the optic can NOT converge light (concave lens or convex mirror).
do - the "object distance," or distance between object and lens or mirror
di - the "image distance," or distance between the formed image and lens or mirror
If you do the calculation for di and it is negative, the image is virtual. Otherwise, the image is real.
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