Concept:The Cartesian sign convention fixes the signs of distances based on the direction of incident light from the origin.
Explanation:The pole of a spherical mirror, or the optical centre of a lens, is taken as the origin of the coordinate system.
The principal axis of the mirror or lens is taken as the X-axis.
Distances measured in the direction of incident light, that is to the right of the origin, are taken as positive.
Distances measured opposite to the direction of incident light, that is to the left of the origin, are taken as negative.
A real object is always placed on the left side of the mirror or lens.
Therefore, the object distance
u is measured from the origin towards the left.
Hence, the value of
u is always negative for a real object.
Option A and option B are incorrect because distances measured to the left are never taken as positive.
Option D is incorrect because the object must be at a finite or infinite distance from the mirror to form an image.
Answer:So, the object distance
(u) is taken as negative when the object is real.
Correct option: C. Negative when the object is real.