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According to the Cartesian sign convention, the object distance (u) is taken as:
Positive when the object is real
Positive for all types of objects
Negative when the object is real
Zero for all mirrors
Negative when the object is real
According to the New Cartesian Sign Convention, all distances are measured from the pole (or optical centre) of the mirror (or lens). Distances measured in the direction opposite to the incident light rays are taken as negative. Since a real object is always placed in front of the mirror, the light travels from the object to the mirror, making the object distance (u) negative for all real objects.
According to the New Cartesian Sign Convention, all distances are measured from the pole (or optical centre) of the mirror (or lens). Distances measured in the direction opposite to the incident light rays are taken as negative. Since a real object is always placed in front of the mirror, the light travels from the object to the mirror, making the object distance (u) negative for all real objects.
Physics / Geometric Optics (NCERT Class 10 Science Chapter 10: Light - Reflection and Refraction)
Object distance (u) is measured from the pole (P) to the object location along the principal axis.
By convention, the object is always placed to the left of the mirror, so light travels from left to right.
Distances measured against the direction of incident light (to the left of the pole) are taken as negative.
Option A is incorrect because real object distance is never taken as positive in the standard Cartesian convention.
Option B is incorrect because real objects have negative distance, whereas virtual objects (in complex optical systems) are treated with positive distance.
Option D is incorrect because object distance equals zero only if the object is placed directly on the pole, which is physically impossible for mirrors.
C is correct — According to the Cartesian sign convention, object distance (u) for a real object is always taken as negative.
Always remember: Object distance u is ALWAYS negative in mirror and lens numericals. Focal length (f) is negative for concave mirrors/lenses and positive for convex mirrors/lenses.