Chapter 10

Light: Mirrors and Lenses

At a science centre, Meena looked into a row of curved mirrors — her face appeared unusually large in one, while in another she saw a tiny version of herself, and her brother's reflection looked upside down. Unlike the plane mirror she'd used before, which always shows an erect, same-size image, these were spherical mirrors — curved inward or outward, and behaving quite differently.

What Are Spherical Mirrors?

Looking into the curved inner surface of a shiny metal spoon shows an inverted image, while the outer, bulging surface shows an erect but smaller image — a preview of how curved mirrors behave. A spherical mirror is a common type of curved mirror, shaped like part of a hollow glass sphere, with a reflecting surface that curves either inward or outward.

A spherical mirror whose reflecting surface curves inward is a concave mirror; one that curves outward is a convex mirror. Viewing a mirror from the side — checking whether its surface bulges toward or away from you — is enough to tell which type it is.

What Are the Characteristics of Images Formed by Spherical Mirrors?

Placing an object close to a concave mirror produces an erect, enlarged image; moving the object farther away flips it to inverted, first still enlarged, then shrinking. A convex mirror behaves more simply: its image is always erect and diminished, only shrinking slightly further as the object moves away. Unlike a plane mirror — always erect and same-sized — both concave and convex mirrors change the image's size as the object's distance changes, and a concave mirror additionally inverts the image once the object is far enough away. Lateral inversion — where left and right appear swapped — shows up in all three mirror types: plane, concave, and convex.

These behaviours aren't just classroom curiosities. Torch reflectors and vehicle headlights use concave mirrors, and a dentist's mirror is concave too, giving an enlarged view of teeth when held close. Reflecting telescopes use a large concave mirror as their main mirror.

Vehicle side-view mirrors are convex: they always show an erect, smaller image of traffic behind, and their outward curve gives a wider view of the road than a plane mirror could. That's also why they warn "objects in mirror are closer than they appear" — the diminished image makes following traffic look farther away than it really is. Convex mirrors are also installed at blind road intersections and sharp bends so drivers from both directions can see each other, and in large stores to monitor wide areas against theft.

What Are the Laws of Reflection?

Shining a thin beam of light onto a plane mirror and tracking it as it reflects reveals a precise relationship. The ray that falls on the mirror is the incident ray, and the one that bounces back is the reflected ray. Drawing a line perpendicular to the mirror's surface at the point where the beam strikes it gives the normal. The angle between the normal and the incident ray is the angle of incidence; the angle between the normal and the reflected ray is the angle of reflection. Measuring these angles across many trials shows they're always equal — the first law of reflection. (When a beam strikes exactly along the normal, both angles are zero.)

Reflecting a beam off a mirror lying flat on a sheet of paper that extends over a table's edge, then bending that extended part, makes the reflected beam vanish — it reappears once the paper is flattened again. This shows the incident ray, the normal, and the reflected ray all lie in the same flat plane — the second law of reflection; bending the paper breaks that shared plane.

Both laws of reflection hold for every kind of mirror — plane and spherical alike. But shining multiple parallel beams onto spherical mirrors reveals something extra: a plane mirror keeps reflected beams parallel, a concave mirror makes them converge (move closer together), and a convex mirror makes them diverge (spread apart) — even though each individual ray still obeys the ordinary laws of reflection, the mirror's curve reshapes the beam as a whole.

A concave mirror's converging beams can concentrate real energy: aiming one at the Sun and reflecting the light onto paper produces a small, intensely bright, hot spot that can make the paper smoke and even burn — because sunlight reflected from across the whole mirror gets focused onto one tiny point.

What Is a Lens?

A drop of water placed on an oiled glass strip naturally forms a curved, dome-like surface — and looking down through it at text underneath makes the letters appear larger, just like a magnifying glass does. A lens is a piece of transparent material, usually glass or plastic, with curved surfaces — and like mirrors, lenses come in convex and concave forms. Unlike mirrors, though, light passes through a lens rather than reflecting off it, so we see objects through a lens, not in one.

A lens that's thicker in the middle than at its edges is a convex lens; one thicker at the edges than in the middle is a concave lens. Looking at a nearby object through a convex lens shows it erect and enlarged; moving the object farther away flips the image to inverted, first still enlarged and then shrinking — much like a concave mirror. A concave lens, by contrast, always shows the object erect and diminished, changing size only slightly as distance changes.

Shining multiple parallel light beams through a thin flat glass plate leaves them passing straight through unchanged. A convex lens converges those beams together — earning it the alternate name converging lens — while a concave lens spreads them apart, earning the name diverging lens. Just like a concave mirror, a convex lens can concentrate sunlight enough to burn paper, since it converges the light to a point too.

Lenses are everywhere: eyeglasses, cameras, telescopes, and microscopes all rely on them. Even the human eye contains a convex lens — a remarkable one that changes its own shape, letting us focus on both a nearby book and something far away.