Can we make mirrors which can give enlarged or diminished images?
Answer
Yes. We only have to bend the reflecting surface. A plane mirror sends every ray back without changing how much the beam is spreading, so the image is always exactly the size of the object. A spherical mirror — a mirror shaped like a part of a hollow sphere — is different, because its surface faces a slightly different direction at every point.
A concave mirror (surface hollowed inwards) pulls the reflected rays together. Held close to your face it gives an enlarged, erect image; taken farther away the image turns inverted and then shrinks.
A convex mirror (surface bulging outwards) spreads the reflected rays apart, so its image is always erect and diminished.
Seen edge-on, the difference is obvious: the shiny side of a concave mirror is hollowed inwards; the shiny side of a convex mirror bulges outwards. The shaded part is the non-reflecting back.
Why it happens: reflection at every point still obeys the same law — the angle of reflection equals the angle of incidence. But on a curved surface the normal tilts a little more as you move along the mirror, so rays that arrived parallel leave in different directions. Concave surfaces tilt the normals towards each other (rays converge); convex surfaces tilt them apart (rays diverge).
Check it yourself: a steel spoon is both mirrors in one. Its hollow inner side is concave; its bulging outer side is convex.
Q2.
On side-view mirrors of vehicles, there is a warning that says “Objects in mirror are closer than they appear”. Why is this warning written there?
Answer
Because a side-view mirror is a convex mirror, and a convex mirror always makes the image smaller than the object.
We judge how far away something is largely from how big it looks. A motorcycle whose image is half the expected size reads to the eye as being twice as far away. So the bus behind you really is nearer than the small image suggests, and the printed warning is there to stop a driver from misjudging that gap while changing lanes.
Why a convex mirror is used at all: because it curves outwards, it gathers light from a much wider strip of the road behind and squeezes it into the same small mirror. A plane mirror of the same size would show a correct-sized image but a far narrower view, leaving a bigger blind spot. The manufacturer accepts the distorted sense of distance in exchange for the wider field of view — and prints the warning to cover it.
Did you know? The Hindi edition states the same warning as “वस्तुओं के बीच वास्तविक दूरी दर्पण में दिखाई देने वाली दूरी से कम होती है।” — the actual distance is less than the distance the mirror suggests. Same idea, said the other way round.
Q3.
Why is there a curved line on some reading glasses?
Answer
That line is the boundary of a second lens built into the lower part of the same spectacle lens. Such a lens is called a bifocal — literally two focusing regions in one piece of glass.
The large upper region has a gentle curvature and is used for looking at distant things.
The small lower region, marked off by the curved line, is more strongly curved. Being a stronger converging (convex) surface, it magnifies near print, so the wearer can read by simply dropping the eyes.
Why two curvatures are needed: how much a lens bends light depends on how sharply its surfaces are curved. One curvature cannot suit both a book at 30 cm and a signboard 30 m away, so the two are ground into one lens and the join shows up as a visible curved line.
Tip: the chapter itself does not answer this opener — it is meant to make you look closely at real spectacles. Look at an elder's reading glasses in a slanting light and the line becomes easy to see.
Q4.
Share your questions
Answer
Write down the questions the pictures on this page actually raise for you, and keep them at the front of your notebook — you should be able to answer each one by the end of the chapter.
Sample questions:
Why does my face look upside down in the inner side of a spoon but the right way up in the outer side?
Is there a distance at which the image in a concave mirror flips over? What is special about that distance?
Why can a concave mirror set paper on fire but a plane mirror of the same size cannot?
A mirror shows an image in it, a lens shows an object through it. Why the difference?
Convex mirrors are used at sharp bends on roads. Would a concave mirror work there instead?
Tip: a good question names something you actually observed and asks 'why' or 'what if'. 'What happens if I use two mirrors facing each other?' is a better question than 'What is a mirror?'.