Question
Describe all possible cases for image formation by concave and convex mirrors depending on object position. What are the nature, position, and size of the image in each case?
(CBSE 10 and 12 boards ask 2-3 ray diagram questions; NEET tests image characteristics)
Solution — Step by Step
Concave mirror — 6 positions of the object
Case 1: Object at infinity → Image at F, real, inverted, highly diminished (point image).
Case 2: Object beyond C → Image between F and C, real, inverted, diminished.
Case 3: Object at C → Image at C, real, inverted, same size.
Case 4: Object between F and C → Image beyond C, real, inverted, magnified.
Case 5: Object at F → Image at infinity, real, inverted, highly magnified.
Case 6: Object between F and P → Image behind mirror, virtual, erect, magnified.
Convex mirror — always the same type
No matter where the object is placed:
- Image is always behind the mirror (between P and F)
- Image is always virtual, erect, and diminished
- As the object moves closer, the image gets larger but never exceeds the object size
This predictability is why convex mirrors are used as rear-view mirrors in vehicles.
The mirror formula connects all cases
Sign convention (New Cartesian): distances measured from pole, along principal axis. Towards the incident light = negative. Concave mirror: is negative. Convex mirror: is positive. Object distance is always negative.
Magnification: . Positive = erect image, negative = inverted image.
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Why This Works
A concave mirror converges light, so it can form real images (when rays actually meet) or virtual images (when rays appear to diverge from behind the mirror). The special case at F creates parallel reflected rays — hence the image "at infinity."
A convex mirror diverges light, so reflected rays always diverge. They can never actually meet in front of the mirror — the image is always virtual, formed where the diverging rays appear to originate from (behind the mirror).
Alternative Method
💡 Expert Tip
The "crossover" trick for concave mirrors: as the object moves from infinity to the pole, the image moves from F to infinity (real) and then jumps behind the mirror (virtual). The crossover point is at F — this is where the behaviour changes. For any MCQ, first determine: "Is the object inside or outside F?" Inside F = virtual. Outside F = real.
Common Mistake
⚠️ Common Mistake
Students confuse the sign convention and write as positive for a concave mirror. In the New Cartesian convention used in CBSE and competitive exams, concave mirror has negative focal length (focus is in front of the mirror, in the negative direction). Convex mirror has positive focal length. Getting this sign wrong will give you the wrong image position every time.