Why take image distance positive in concave mirror.
Since, object is always placed in front of the mirror hence the sign of object is taken as negative.
Since, the centre of curvature and focus lie in front of the concave mirror, so signs of radius of curvature and focal length are taken as negative in the case of concave mirror.
When image is formed in front of the mirror, the distance of image is taken as – (negative) and when image is formed behind the mirror, the distance of image is taken as + (positive).
Height of image is taken as positive in the case of erect image and taken as negative in the case of inverted image.