Question

When an object is between the focal point and a concave mirror, so is negative. What other values must be negative for your equations to still work? (Hint: The distance between the focal point and the lens (f) is always positive for a concave mirror.)

Answer 1

Given

The mirror is concave.

The object is placed between the focal point and a concave mirror. From the sign convention, the object distance is taken as negative.

As the focal point is on the left side of the concave mirror, the focal length is also taken as negative from the sign convention.

Thus, the focal length must be negative so that the mirror formula can work.