an object 6cm is placed18cm away from a concave mirror . the image formed is 12cm before the mirror . find f,R, H1,m.
my question is how v=-12. pls tell me.
Dear Student!
The sign conventions for a lens or mirror is same as the conventions in coordinate axes which you must have studied in Mathematics. Center of a lens (or pole of the mirror) is considered as origin, and axis of the lens is considered to be x-axis. All the distances in the left of the center is negative (like negative x-axis) and distances to the right are taken as positive (as positive x-axis). Similarly, a line perpendicular to the axis of lens passing through origin is considered as y-axis. Any distance measured towards positive y-axis will be positive and vice-verca.
Hence in your case, v = -12 cm, u = -18 cm and Ho = +6 cm (object is always erect). I believe you can solve rest of the problem!
Hope it is clear to you!
Best wishes!!