Beta, in our sign convention, we take the object's distance as negative because we start measuring from the mirror itself. Since a concave mirror's focus is in front of it, on the same side as the incoming light, it gets a negative sign too. This makes the formula 1/v + 1/u = 1/f neat and consistent.
Every day in my shop, I see the same principle. When a lawyer stands in front of my big, framed glass photo—it acts like a concave mirror—his reflection gets upside down only when he's close, just like a real image. The focus of that glass is firmly on this side, in my shop's world, not behind it. So it has to be negative here, in our practical reality.
#exams#study
Jump in to reply — no account needed.