NCERT Solutions for Class 8 Science Chapter 10 Light Mirrors and Lenses (PDF) – 2027

Yes. A plane mirror always gives an image of the same size as the object, but a spherical mirror — a mirror whose reflecting surface is curved like a part of a hollow sphere — can give enlarged or diminished images:

• 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.

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.

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.

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?

Yes, clearly. A polished steel spoon reflects light in the same regular way a mirror does, so an image of your face is formed in it.

Yes. In a plane mirror the image of the face is erect and of the same size. In the inner (hollow) side of the spoon the face appears upside down (inverted) and smaller. (In the outer, bulging side it is erect but smaller — see the next steps.)

Yes. In the inner (hollow) side the image stays inverted and becomes smaller as the spoon is moved farther away from the face.

With the bulging outer side facing you the behaviour is far simpler. The image is always erect and always smaller than your face, at every distance. As you move the spoon away the image shrinks a little more, but it never flips over.

Looking along the surface, edge-on, the shape shows up at once:

• If the shiny surface dips down in the middle — the edges stand higher than the centre — it is a concave mirror.
• If the shiny surface rises in the middle — the centre stands higher than the edges — it is a convex mirror.

We can tell them apart by the shape of the reflecting surface and by the images they form:

With the toy only 3–4 cm away:

So the answer to the middle question is no — neither image is the same size as the object, and that is already a clear difference from a plane mirror. Both images are erect at this small distance, and both show lateral inversion.

Concave mirror. The erect image first grows larger. At one particular distance it blurs; past that distance the image reappears inverted, at first enlarged and then getting steadily smaller the farther the toy goes.

Convex mirror. Nothing dramatic happens. The image stays erect throughout and only becomes a little smaller as the toy is moved back.

Conclusion 1. Spherical mirrors do not behave like plane mirrors. A plane mirror always gives an erect image of exactly the same size; in a spherical mirror the size of the image changes as the object distance changes.

Conclusion 2. A concave mirror can give an image that is enlarged, of the same size, or diminished, and erect or inverted — it all depends on how far the object is from the mirror.

Conclusion 3. A convex mirror is far more predictable: its image is always erect and always diminished, whatever the object distance.

Conclusion 4. Lateral inversion is seen in all three types of mirror, so it cannot be used to tell them apart.

Each use is chosen for exactly what that mirror does to a beam of light.

Yes — it is the small round mirror on a long handle that the dentist slips inside the mouth. It is a concave mirror.

Held very close to a tooth, a concave mirror gives an image that is erect and enlarged, so a tiny cavity or crack shows up big enough to examine. The long handle also lets the dentist look at the back of a tooth, which cannot be seen directly at all.

Yes. In 'Beyond Earth' a telescope was introduced as the instrument that lets us see faint and distant objects in the sky — planets, moons and stars — far better than the unaided eye can.

What this chapter adds is how most modern telescopes do it. They are reflecting telescopes: the light-collecting element is a large concave mirror, not a lens.

Yes. In Grade 7 a comb was used as a slit-maker: all its openings were covered with black paper except one in the middle, and a torch shone through that single slit to give one thin beam of light travelling along a sheet of white paper. A plane mirror was stood upright on the paper in the path of that beam.

The beam struck the mirror and came away along a new, definite direction — it did not scatter. Tilting the mirror or the torch changed the direction of the beam coming away, but there was always exactly one reflected beam.

Yes. Everything a mirror does follows from just two laws of reflection, and they apply to every ray, at every point, on every kind of mirror.

1. The angle of incidence is equal to the angle of reflection, both measured from the normal at the point of incidence.
2. The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.

Yes. When the incident beam falls on the mirror at a different angle, the reflected beam also shifts to a new direction. If the incident beam is brought closer to the normal, the reflected beam also comes closer to the normal; if it is moved away from the normal, the reflected beam also moves away from it.

Whatever angle you choose, the reflected beam always comes off on the other side of the normal, tilted away from the normal by exactly as much as the incident beam is tilted towards it.

A typical set of readings from a carefully drawn sheet looks like this. Your own numbers will be close to these but not identical, because you choose your own angles.

In every row the two angles are equal to within a degree, and the small differences change sign from row to row. That is the signature of measurement error, not of a real difference — so the conclusion is

Both angles are zero.

The beam therefore travels straight back along the path it came by, retracing itself.

Yes. In a single uniform medium light travels in straight lines, and that is exactly why we are allowed to draw it as straight lines with arrows — as rays.

The evidence is all around us:

• A shadow has the sharp outline of the object, and a bigger object makes a bigger shadow — which is what straight-line travel predicts.
• Light from a torch passing through a slit shows up as a straight streak on the paper, never a curved one.
• You cannot see round a corner, and a beam through a bent pipe does not come out at the far end.

No. The moment the overhanging part of the chart paper is folded down along the edge of the table, the bright streak on it disappears. The reflected beam still lies in the same plane as the incident beam and the normal, and the bent part is no longer in that plane.

The reflected beam reappears on the extended portion, exactly where it was before.

This appearing-and-disappearing is the whole point of the activity. It shows that the reflected beam lies in one particular plane — the plane that also contains the incident beam and the normal — and nowhere else.

Yes — the laws of reflection are valid for all kinds of mirrors: plane, concave and convex. They are laws about a single ray meeting a surface, and a spherical mirror is made of exactly such surface points. Each ray obeys i = r at the point where it strikes; because the surface is curved, the reflected rays of a parallel beam come together (concave mirror) or spread out (convex mirror).

Yes, the observations match the figure exactly:

• Plane mirror (Fig. 10.11b) — the reflected beams are still parallel to one another.
• Concave mirror (Fig. 10.11c) — the reflected beams come closer together, that is, they converge, and meet at a point.
• Convex mirror (Fig. 10.11d) — the reflected beams spread apart, that is, they diverge.

Yes, and that is exactly what happens. A concave mirror gathers light falling on its whole surface and delivers it to a very small patch.

Move the paper slowly towards and away from the mirror. The patch of light first shrinks, becomes a small, very bright, sharply-edged spot, and then starts to widen again. Stop at the position where the spot is smallest and brightest.

Yes. Held steady, the paper at the bright spot first darkens to brown, then gives off a thin curl of smoke, and can finally catch fire. This is because the concave mirror concentrates sunlight on that small spot, which produces enough heat to ignite the paper.

Yes — in Chapter 4, Electricity: Magnetic and Heating Effects. There we learnt that the heating effect of electric current is used in a specially designed high-temperature furnace, an enclosed space built to generate heat, in which scrap steel is melted and recycled into usable steel.

A solar furnace reaches the same very high temperature by a completely different route. Instead of pushing a current through a conductor, a large concave mirror (or a field of mirrors) concentrates sunlight onto a small target.

They look changed in size, and if the surface is curved enough they can even look upside down — much as they do in a curved mirror, except that now we are looking through the material rather than in it.

No, they would not.

Through a flat window pane every object keeps its size and shape — that is why we can use windows at all. The moment the transparent surface is curved, the letters or objects behind it look larger or smaller than they really are. Activity 10.8 shows this with nothing more than a drop of water.

The surface of the drop is curved outward — it bulges upward like a tiny dome. It is neither flat nor hollowed inwards.

Yes — the letters under the drop look clearly enlarged, bigger than the very same letters lying just outside the drop. The water drop, curved outwards, acts like a simple lens (a convex lens).

The object can change in size and, with a convex lens, even in orientation.

Yes. A convex lens converges a parallel beam of light and a concave lens diverges it — which is why they are also called a converging lens and a diverging lens. Activity 10.10 shows both.

With the object a small distance behind the convex lens, it appears erect and enlarged — clearly bigger than it looks with the naked eye, and the right way up.

The distance decides everything:

1. Small distance — erect and enlarged.
2. At one particular distance — the view blurs and no clear image can be seen.
3. Beyond that distance — the object appears inverted, at first still enlarged.
4. Farther still — it stays inverted and now steadily diminishes in size.

With a concave lens the answer is the same at every distance: the object appears erect and diminished. As you move it farther from the lens it just keeps getting smaller. It never flips over and it is never enlarged.

Conclusion. The image formed by a convex lens can be enlarged, diminished or of the same size, and erect or inverted, depending on the distance of the object from the lens. The image formed by a concave lens is always erect and diminished.

Yes. Anything that concentrates sunlight into a small enough spot can char and ignite paper, and a convex lens converges light just as a concave mirror does.

No — only the flat plate leaves the beam alone.

• Thin transparent glass plate — the beams pass through as they are and are still parallel on the far side.
• Convex lens — the beams are brought together; they converge and meet at a point.
• Concave lens — the beams are spread apart; they diverge.

Analysis. A convex lens is also called a converging lens and a concave lens a diverging lens. Comparing with Activity 10.6, the convex lens does to transmitted light what the concave mirror does to reflected light, and the concave lens does what the convex mirror does.

Yes. Hold the convex lens so that sunlight passes through it onto the paper, and adjust the distance until the patch of light shrinks to the smallest, brightest spot. Held steady there for a minute or two, the paper browns, smokes and can catch fire — exactly as it did with the concave mirror.

Lenses are used almost everywhere we need to control where light goes.

(ii) 50°

Draw the normal first in every case — at 90° to the mirror surface at the point where the ray strikes it. Then measure i from that normal and set r equal to it on the other side.

Compare each image in the mirror with the actual cap standing beside it.

All three pictures are taken at the same object distance, so any difference in size is caused by the glass alone.

(ii) Angle of incidence is 0°

The light is reflected straight back along its own path.

Left: convex mirror; middle: plane mirror; right: concave mirror.

Compare the squares seen in each mirror with the squares of the real graph sheet standing behind them.

(iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.

Follow the walk from a long way off:

1. Far from the mirror — the image is inverted and small, exactly as in Fig. 10.26 where she is shown upside down in the mirror.
2. Walking closer — the image stays inverted but grows larger.
3. Very close to the mirror — the image turns over and becomes erect and magnified.

A magnifying glass is a convex lens — a lens that is thicker at the middle than at its edges.

What you notice, step by step:

1. Held close to the page, the letters look erect and enlarged; there is a range of a few centimetres over which the print is both big and sharp.
2. As the lens is lifted, the letters get bigger still, up to a point.
3. At one particular height the print blurs completely and cannot be read.
4. Lift it further and the print reappears upside down and now gets smaller the higher you go.

(i) – (a), (ii) – (b), (iii) – (c), (iv) – (d)

(i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Assertion — correct. Side-view mirrors on vehicles are convex mirrors.

Reason — correct. Because a convex mirror curves outwards, it collects light from a much wider strip of the road behind and shows it in the same small mirror.

And the Reason really is the explanation. A driver needs to see as much as possible of the road behind before changing lanes, and the widest view in the smallest mirror is precisely what a convex mirror gives. That is why it is chosen over a plane mirror of the same size.

(ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror.

Compare the arrow marked I with the arrow marked O in each figure. In both figures the image is erect and lies on the far side of the mirror M, so only the size can tell them apart.

Through the empty tumbler the pencil looks normal. Once the tumbler is half filled, the part of the pencil seen through the water looks distinctly broader than the part above the water level, and it may look shifted a little to one side — so the pencil seems to break at the water surface. This is because the water-filled tumbler has a curved surface and acts like a convex lens, which makes the part of the pencil seen through it look broader.

What you will find.

What a good proposal must contain: (1) the need it meets, (2) the physics it uses, (3) a labelled design with dimensions, (4) a materials-and-cost table, (5) how you will test it, and (6) its limitations.

Sample proposal — a box-type solar cooker with a plane reflector for the school kitchen

3. Design. Outer wooden box 60 cm × 60 cm × 20 cm; inner aluminium tray 50 cm × 50 cm painted matt black; 5 cm of thermocol or glass wool between the two as insulation; two glass sheets 4 mm thick, 2 cm apart, as the lid; a plane mirror 60 cm × 60 cm hinged to the lid, propped so that the reflected sunlight falls into the box; four black-painted aluminium cooking pots with lids.

A set of runs worth doing

What you should be able to conclude. The concave mirror and the convex lens behave alike, and the convex mirror and the concave lens behave alike. In each pair the difference lies only in whether the light is reflected or transmitted; what decides the image is whether the rays are made to converge or to diverge.