Spherical mirrors
Flip a shiny steel spoon in your hand and something odd happens — the inside bowl shrinks your face into a tiny, upside-down blob, while the outside back swells it into a wide, cheerful grin. Nothing about the spoon's metal changed; only the curve of the surface facing you changed. That single idea — that a curved reflecting surface bends light differently from a flat one, and bends it differently again depending on which way it curves — is the whole story of spherical mirrors. This article stays exactly where CBSE Class 8 leaves it: naming the two kinds of curved mirror, telling them apart by sight, and confirming that even a curved surface still obeys the same law of reflection as an ordinary flat mirror. No pole, no centre of curvature, no focal length, no formula — those arrive in Class 10. Here the goal is simple: given any curved shiny surface, know instantly whether it is concave or convex, and why a flat mirror cannot do what either of them does.
A spherical mirror is a curved reflecting surface, concave if it curves inward like a bowl and convex if it curves outward like a ball, and both still obey the ordinary law of reflection at every point of the curve.
The core idea
What makes a mirror "spherical." Picture a large hollow glass or metal sphere. Now imagine cutting a small curved patch out of its surface and silvering one side of that patch. The patch is no longer flat — it carries the gentle curve of the sphere it came from. That curved, silvered patch is a spherical mirror. Real mirrors are ground and polished to this curve rather than literally sliced from a ball, but the shape is identical to a piece of a hollow sphere.
Two mirrors, one shape, opposite curves. Depending on which side of the curved patch is silvered, the same basic shape gives two very different mirrors:
- A concave mirror has its reflecting surface curving inward, like the inside of a bowl or the inside surface of a spoon. The silvering sits on the outer, bulging side, so your eye looks into the hollow, cave-like face — hence "con-cave."
- A convex mirror has its reflecting surface curving outward, like the outside of a ball or the back of a spoon. The silvering sits on the inner side, so your eye looks at the bulging outer face.
A single spoon conveniently carries both: its inner, bowl-shaped face is a concave mirror, and its outer, rounded back is a convex mirror, both cut from the same sheet of steel and differing only in which way the surface bends toward you.
How this differs from a plane mirror. A plane mirror has a perfectly flat reflecting surface, so it always returns an image the same size as the object, right-side up, simply swapped left-for-right. A spherical mirror's curve changes how the reflected light spreads out or bunches together, so the size and orientation of the image can differ from the object — exactly how much, and under what conditions, is worked out with ray diagrams in Class 10; at this stage it is enough to know that curvature is why concave and convex mirrors look different from a flat one.
Why it works
There is no formula at this stage, but there is a reason both mirror types still count as ordinary mirrors: the laws of reflection — the incident ray, the reflected ray and the normal all lie in one plane, and the angle of incidence equals the angle of reflection — hold at every single point on a curved surface, exactly as they do on a flat one. A flat mirror has the same normal direction everywhere, so parallel rays stay parallel after reflecting and the image sits the same size as the object. A curved mirror's surface tilts a little differently at every point, so its normal direction changes from point to point across the surface. Each point still obeys angle of incidence equals angle of reflection individually, but because the normals point in slightly different directions, rays that were parallel before hitting a concave or convex surface no longer stay parallel afterward — this is why curved mirrors can enlarge, shrink or otherwise change an image while a plane mirror cannot. Concave and convex mirrors bend the reflected light in opposite senses because their surfaces curve in opposite senses, but both are still, at heart, ordinary reflectors following the same single law.
Worked example
Question. You are handed two unlabelled curved steel mirrors, A and B. Looking at mirror A, its centre bulges out toward you. Looking at mirror B, its centre curves away from you like the inside of a bowl. A friend then hands you a serving spoon and asks which of its two faces behaves like mirror A and which behaves like mirror B.
Step 1. Recall the two definitions: a concave mirror's reflecting surface curves inward, away from the viewer, like a bowl; a convex mirror's reflecting surface curves outward, toward the viewer, like a ball or a spoon's back.
Step 2. Classify mirror A. Its centre bulges out toward the viewer, matching the outward-curve definition, so mirror A is a convex mirror.
Step 3. Classify mirror B. Its centre curves away from the viewer like the inside of a bowl, matching the inward-curve definition, so mirror B is a concave mirror.
Step 4. Match the spoon's two faces to A and B. The spoon's inner, bowl-shaped face curves away from you exactly like mirror B, so the inner face is concave. The spoon's outer, rounded back bulges toward you exactly like mirror A, so the outer face is convex.
Final answer: mirror A and the spoon's outer back are both convex; mirror B and the spoon's inner bowl are both concave — one spoon, two mirror types, distinguished purely by which way each face curves.
Common traps
- Swapping the names — remember "concave" contains "cave," a hollow you look into, while "convex" bulges out at you.
- Thinking a spherical mirror is physically sliced out of a solid glass sphere with a knife; real mirrors are ground and polished to that curve, not cut from a ball.
- Treating a spherical mirror as just a fancy plane mirror because both show a reflection — a plane mirror is flat and always gives a same-size image, while a curved mirror's image size and orientation can change.
- Forgetting that "concave or convex" depends on which side is silvered — the very same curved sheet of glass could, in principle, be silvered on either face.
Ask the tutor: Curious how concave and convex mirrors change the size of your reflection as you move closer or farther away — ask the tutor to walk through it with everyday examples like torches and car mirrors.
How can I quickly tell concave from convex without touching the mirror?
Look at how your reflection changes as you move, or simply notice which way the surface curves relative to you. If the middle of the mirror looks like it dips away from you, like looking into a shallow bowl, it is concave. If the middle bulges toward you, like the curved back of a spoon or the outside of a shiny ball, it is convex. With a spoon you get both answers on one object just by flipping it over.
Why doesn't a spherical mirror give the same size image as a plane mirror?
Because a plane mirror's surface points in exactly the same direction everywhere, so every ray reflects in a perfectly uniform way and the image stays the object's own size. A spherical mirror's surface tilts a little differently from point to point since it is curved, so reflected rays that started out parallel no longer stay parallel — that is the seed of why concave and convex mirrors can shrink, enlarge or otherwise distort an image. The Class 8 syllabus only asks you to know that this difference exists; working out exactly how much bigger or smaller the image gets is Class 10 material, using the mirror formula and ray diagrams.
Is the "law of reflection" different for curved mirrors compared to flat ones?
No, it is the same law — angle of incidence equals angle of reflection, with the incident ray, reflected ray and normal all in one plane. What changes on a curved mirror is simply the direction of the normal itself at each point, because the surface is bending. So every individual reflection still obeys the ordinary law; it is only when you compare reflections from different points on the curved surface that you see the curve's effect, since each point aims its reflected ray in a slightly different direction.