Images in concave mirrors

A concave mirror is a spherical mirror whose reflecting surface curves inward, like the inside of a bowl or the inner bowl of a steel spoon — as opposed to a convex mirror, which bulges outward. Because the surface is curved rather than flat, it does not behave like an ordinary plane mirror: the image it produces changes character as the object's distance changes.

• Object close to the mirror. The image is erect — same orientation as the object, right-side up — and enlarged, meaning bigger than the object itself. This is precisely why concave mirrors are used as shaving mirrors and make-up mirrors: held close to the face, they magnify.
• Object moved farther away. Past a certain distance, the image flips to inverted (upside down). Immediately after the flip it may still look fairly large, but as the object keeps retreating, the inverted image becomes progressively smaller, or diminished.

So there are really two regimes, not a single fixed behaviour: close up, erect and enlarged; far away, inverted and shrinking. There is no intermediate "in-between" image that is neither — the transition from erect to inverted happens at some particular distance, beyond which the image stays inverted and only continues to shrink. A second, separate effect applies to every mirror — plane, concave, or convex — at every distance: lateral inversion, where left and right appear swapped even though top and bottom are not (unless the image has also flipped top-to-bottom, as described above). Raise your right hand in front of a concave mirror held close to your face, and your reflection appears to raise its left hand. Lateral inversion is a constant feature of mirror images; the erect-to-inverted change discussed above is the feature that is special to distance in a concave mirror.

where: "close" and "far" are judged relative to the mirror's own curvature (a more strongly curved bowl-shaped mirror switches over at a shorter distance than a shallow one) — at this stage you are not expected to calculate the switch-over distance, only to recognise and describe the two regimes.

There is no formula to derive at this level, but the pattern is worth reasoning through rather than just memorising, so that "close = enlarged, far = diminished and inverted" feels inevitable rather than arbitrary.

Step 1. A concave mirror's curved surface bends reflected light so that it can either spread the reflected rays out in a way that appears to come from a large point behind the mirror (when the object sits close to the surface), or bring them together and cross in front of the mirror before reaching your eye (when the object sits farther away). You are not expected to trace these rays formally yet — only to accept that the same curved surface can act in these two different ways depending on distance, which is exactly why the same spoon can be a shaving mirror at one moment and show you upside down at another.

Step 2. When the object is close, the light reflecting off the curved bowl behaves gently — the reflection you see appears to originate from a point that is farther behind the mirror's surface and spread wider than the object itself, which is what your eye interprets as "erect and bigger." This is the same reason a small bowl-shaped mirror, like the back of a spoon, magnifies your face when you bring it close.

Step 3. As the object moves farther from the mirror, the curved surface starts to behave more strongly: past a certain distance the reflected light actually crosses over in front of the mirror before spreading out to your eye. A crossed-over image is what appears upside down — this is the physical reason the flip from erect to inverted happens, even though at this stage you describe it only in words, not with a ray diagram.

Step 4. Once the image is inverted, moving the object still farther away makes the crossing happen closer to the mirror and the resulting image cone narrower, which your eye reads as a steadily shrinking inverted image. This is why "farther away" and "more diminished" go together for every distance beyond the flip point.

Step 5. Lateral inversion (left-right swap) is a separate, constant effect present at both close and far distances, caused by how any mirror — flat or curved — reflects the perpendicular direction toward the observer; it does not depend on distance the way the erect/inverted and enlarged/diminished behaviours do, which is why it is worth treating as its own rule rather than folding it into the "close vs far" story.

Question. A student holds a concave mirror very close to her face and observes her reflection. She then slowly moves the mirror farther and farther away, without changing anything else. Describe, in order, how her image is expected to change, and explain briefly why.

Step 1. Start with the object (her face) very close to the mirror. By the close-object rule, the image is erect and enlarged — she sees a bigger, right-side-up version of her own face, just as in a shaving mirror.

Step 2. As she begins moving the mirror away, the object is no longer "close," so the mirror's behaviour starts to shift away from the erect-and-enlarged regime described in Step 1.

Step 3. At some particular distance, the image flips: it becomes inverted rather than erect. Immediately after this flip, the inverted image can still look reasonably large, since she has only just crossed the switch-over distance.

Step 4. As she continues moving the mirror even farther away, the inverted image keeps shrinking — it becomes more and more diminished the greater the distance, following the far-object rule.

Step 5. Throughout every stage, her image also shows lateral inversion — if she raises her right hand, the reflection appears to raise its left hand — because this effect is present at every distance, independent of the erect/inverted, enlarged/diminished changes.

Final answer: the sequence is erect and enlarged (close) → inverted, initially still fairly large (just past the switch-over) → inverted and increasingly diminished (far away), with lateral inversion present throughout.

Competitive stretch

If a quiz asks you to compare two students at a concave mirror — one very close, seeing an erect enlarged image, and one far away, seeing an inverted diminished image — do not conclude they are looking at "different kinds" of mirrors. Both descriptions belong to the same concave mirror; they simply differ because the two students stand at different distances from it. The mirror's identity does not change — only the object distance does.

• Assuming a concave mirror always magnifies — it only enlarges the image when the object is close; at larger distances the image shrinks instead.
• Forgetting that the image can flip from erect to inverted purely because the object moved farther away — this switch is a defining feature of concave mirrors at this level, not something that needs a ray diagram to justify.
• Mixing up "inverted" (image upside down, happens only at larger distances) with "laterally inverted" (left-right swapped, happens at every distance, in every kind of mirror).
• Trying to invoke the pole, centre of curvature, focus, or the mirror formula to explain the behaviour — at Class 8 level the description is built entirely from observation, saved for Class 10's quantitative treatment.

Ask the tutor: Want to practice predicting erect/inverted and enlarged/diminished for a series of close-to-far object positions with a concave mirror?