Physics · Ray Optics And Optical Instruments · NEET
In real problems the object is placed in front of the mirror or lens, on the same side as the incoming light. Incoming light travels left to right, so the object sits to the LEFT of the origin (pole or optical centre). Distances measured to the left are against the positive direction, so u comes out negative. You will see u = -40 cm, u = -20 cm etc. in almost every NEET numerical. A positive u would mean the object is behind the mirror, which is not a normal case.
Negative. A concave mirror has its focus in FRONT of the mirror, on the left side where the light comes from, so f = -R/2 is negative. A convex mirror has its focus behind it (to the right), so its f is positive. Same idea for lenses: converging (convex) lens f is positive, diverging (concave) lens f is negative. Fix these four signs in memory and you rarely go wrong.
You do NOT guess it. Put in the known signs of u and f, then let the formula give you v with its own sign. If v comes out negative for a mirror, the image is real (in front of the mirror, same side as the object). If v is positive for a mirror, the image is virtual (behind). For a lens it is the opposite side rule, but the method is the same: trust the algebra, do not force the sign.
Yes. Heights measured upward from the principal axis are positive, heights measured downward are negative. This matters for magnification: an inverted image has negative height, so magnification m comes out negative, telling you the image is upside down. An erect image has positive height and positive m. So the sign of m is not decoration; it carries the erect or inverted information.
No, the RULES are identical: origin at the pole or optical centre, light travels left to right as positive, distances against light are negative, upward heights positive. What differs is only the physical geometry, so real images form on opposite sides. Learn one convention and apply it to both. This is why NCERT calls it a single New Cartesian Sign Convention for the whole chapter.
An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Origin at the pole or optical centre, incoming light left to right is positive, distances against the light are negative, and upward heights are positive.
One formula, 1/v + 1/u = 1/f, must handle real and virtual images, concave and convex, all at once. Signs let a single equation describe every case, so you never memorise separate formulas.
All of u, v, f and R are measured from the pole for a mirror, or from the optical centre for a thin lens, along the principal axis.
Yes. Negative m means the image is inverted (usually real for a mirror or lens), positive m means erect (usually virtual). The size is the magnitude of m.
Yes. It is the foundation of the entire Ray Optics chapter and every mirror, lens, refraction and prism numerical silently depends on it.
