NCERT Class 10 Science Chapter 9 Activities — Light: Reflection and Refraction

NCERT Class 10 Science — Light: Reflection and Refraction, Activities 9.1 to 9.13 on pages 135 to 153.

Also here. Solutions to the end-of-chapter questions for this chapter and the in-text questions are on their own pages.

Thirteen activities, and they divide almost exactly in half: 9.1 to 9.6 are about mirrors, 9.7 to 9.13 are about refraction and lenses. The two halves are built the same way — a rough qualitative look first, then a measurement, then a systematic sweep through every object position.

What makes this chapter different from the chemistry chapters is that most of its activities produce a table rather than a single observation. Activities 9.3 and 9.12 each work through six object positions, and the tables they build are the ones examination questions are set from. They are reproduced in full here.

Key insight. Nearly every result in this chapter is a consequence of one thing: where the object sits relative to F and C (or F₁ and 2F₁). Students who memorise the six rows of Table 9.1 forget them; students who can place the object on a chalk line on a table and predict what happens next do not. That is what Activities 9.3 and 9.12 are for, and why they are worth the table space and the lesson time.

Activity 9.1

Take a large shining spoon and look at your face in its curved surface. Is the image larger or smaller? Move the spoon slowly away and watch how it changes. Then reverse the spoon and repeat.

What you should observe. The two sides of the spoon behave completely differently, and one of them flips.

Why. The inner surface of the spoon is a concave mirror and the outer is a convex one. Held close, your face is between the pole and the focus of the concave side, which gives a virtual enlarged image. Move it past the focus and the image becomes real and inverted — that flip is the moment the object crosses F.

Why it comes first. It gives every student a personal encounter with the whole chapter before any terminology exists. The flip is the thing to make them notice, because explaining it is what Activity 9.3 and Table 9.1 exist to do.

What goes wrong. A small or dull spoon. It needs to be large and genuinely shiny, or the image is too dim to judge, and the flip is missed altogether.

Activity 9.2

Hold a concave mirror facing the Sun and direct the reflected light onto a sheet of paper held close to the mirror. Move the paper back and forth until you get a bright, sharp spot. Hold it there for a few minutes. The book warns: never look at the Sun directly, or into a mirror reflecting sunlight.

What you should observe. A tiny brilliant spot forms at one particular distance, and the paper begins to smoke.

Why this is a measurement, not a trick. The Sun is effectively at infinity, so its rays arrive parallel to the principal axis, and parallel rays converge at the focus. That makes the distance from mirror to spot a direct measurement of f. The chapter adds the relation it needs later: the radius of curvature is twice the focal length, so the principal focus lies midway between the pole and the centre of curvature.

What goes wrong. The genuine hazard here is not the mirror, it is the eye. Looking at the reflected image is as dangerous as looking at the Sun. Point the mirror, then look only at the paper. Do this outdoors with the paper on a non-flammable surface and nothing else nearby.

Activity 9.3

Find the mirror’s approximate focal length as in Activity 9.2. Mark a chalk line on a table and stand the mirror with its pole on the line. Draw two more parallel lines at that same spacing, so they mark F and C. Now place a burning candle at each position in turn — beyond C, at C, between C and F, at F, and between P and F — and each time try to get a sharp image on a screen. Record nature, position and relative size.

What you should observe. Six distinct cases, which the chapter collects into Table 9.1.

Why the chalk lines matter. They turn “roughly beyond C” into an exact position. The whole table depends on where the object is relative to F and C, so measuring f first and marking the lines at spacing f is not preparation — it is the experiment.

A wording worth noting. NCERT’s own table says that with the object at F, the image “would not be formed”. Many other books write “highly enlarged, at infinity”. Both describe the same physics — the reflected rays emerge parallel and never meet — but NCERT’s phrasing is the one printed in this book, so it is what is reproduced above. If a student writes “at infinity, highly enlarged”, they have understood it correctly.

What goes wrong. Looking for a screen image of the last row. Between P and F the image is virtual and cannot be caught on a screen at all — it has to be looked at in the mirror. Students who keep moving the screen about are chasing something that is not there, and that failure is worth naming in advance.

Activity 9.4

Draw neat ray diagrams for each object position in Table 9.1, using any two of the standard rays. Compare your diagrams with Fig. 9.7 and describe the nature, position and size of each image.

Why it is set as an activity and not an explanation. The ray diagram has to reproduce the result Activity 9.3 obtained by experiment. Drawing the diagram for “object at C” and finding the image at C, having already measured it there, is what makes the construction trustworthy rather than a recipe.

Why ray 3 works. A ray through the centre of curvature hits the mirror along the normal, so the angle of incidence is zero and it comes straight back. Saying that is much more useful than “rays through C reverse”.

What goes wrong. Drawing rays that cross where they should not, because the mirror is drawn as an arc and the rays reflected off the arc rather than off the plane through the pole. For small apertures the construction is done as if the mirror were flat at the point of incidence, which is what the chapter’s own figures show.

Activity 9.5

Hold a convex mirror in one hand and a pencil upright in the other. Is the image erect or inverted, diminished or enlarged? Move the pencil away slowly — does the image get smaller or larger? Does it move towards or away from the focus?

What you should observe. No flip, ever, however far the pencil goes.

Why there are only two rows. A convex mirror diverges every incoming ray, so the reflected rays never actually meet — the image is always virtual, always behind the mirror, and always between P and F. There is no object position that changes this, which is exactly why the table is short.

Why cars use them. The chapter gives both reasons: convex mirrors “always give an erect, though diminished, image”, and they have a wider field of view because they curve outwards. A driver gets more road in the glass, at the price of everything looking further away.

What goes wrong. Expecting a flip because Activity 9.1 had one. The flip belonged to the concave side of the spoon. This activity’s whole point is that the convex side never does it.

Activity 9.6

Look at the image of a distant object — a tree, say — in a plane mirror. Can you see a full-length image? Try plane mirrors of different sizes, then a concave mirror, then a convex one.

What you should observe. Only one of the three shows the whole thing.

Why. It follows directly from Activity 9.5: the convex mirror’s image is always diminished, so a large object fits into a small area of glass. The same property that makes things look small makes the field of view wide.

The example the chapter gives. A convex mirror set in a wall of Agra Fort facing the Taj Mahal shows the whole building; you have to stand in the right place on the adjoining terrace to see it distinctly.

What goes wrong. Trying it with a nearby object. It needs a genuinely distant one, because the comparison is about how much of the scene fits, not about image size.

Activity 9.7

Put a coin at the bottom of a bucket of water. With your eye to one side above the water, try to pick it up in one go. Try again, and get your friends to try.

What you should observe. Your hand misses, consistently, and in the same direction each time.

Why it works as a hook. The mistake is involuntary — knowing about refraction does not stop you missing the coin, which makes the point that this is about light and not about judgement. A class that all miss together is convinced in a way a diagram cannot manage.

What goes wrong. Looking straight down. Refraction shifts the apparent position only when you view obliquely, so the eye must be well to one side. That is why the instruction says “to a side above water”, and it is the detail most often ignored.

Activity 9.8

Put a coin in a large shallow bowl on a table. Move away slowly until the coin just disappears behind the rim. Stay there and have a friend gently pour water into the bowl.

What you should observe. The coin reappears without anything having moved.

Why it is the better of the two. Activity 9.7 shows that refraction shifts an apparent position; this shows that it can bring into view something that was geometrically hidden. Nothing about the straight-line path of light can explain a coin becoming visible while everything stays still.

What goes wrong. Pouring too fast and moving the coin, or the observer shifting to keep looking. Both destroy the result. Fix the head position — resting the chin on a stack of books is worth the trouble — and pour down the side of the bowl.

Activity 9.9

Draw a thick straight line in ink on white paper. Place a glass slab over it so that one of its edges makes an angle with the line. Look at the part of the line under the slab from the side.

What you should observe. The line breaks where it enters the glass.

Why the angle matters. Set the slab square to the line and nothing happens — the light enters along the normal and is not bent. Set it at an angle and the ray is refracted at entry and again at exit, emerging parallel to its original direction but displaced sideways. That is why the instruction specifies “makes an angle with the line”.

What goes wrong. Using a thin slab. The displacement grows with thickness, and a microscope slide shows almost nothing. It also needs a thick, sharply drawn line — a pencil line is too fine to see the break.

Activity 9.10

Pin a sheet of white paper to a drawing board. Place a rectangular glass slab in the middle and draw its outline, ABCD. Fix two pins E and F vertically so the line joining them is oblique to edge AB. Look from the opposite side and fix two more pins G and H so that all four appear in a straight line. Remove the pins and slab, join the points, and measure the angles.

What you should observe. The ray bends one way going in and the other way coming out, and ends up parallel to where it started.

Why it comes out parallel. The two faces of the slab are parallel, so the bending at the second surface is equal and opposite to the bending at the first. The direction is restored; only the position is not.

Why four pins and not two. The pins define the ray by sight alone — you are using your own eye as the detector, lining up G and H with the images of E and F seen through the glass. It works because light travels in straight lines everywhere except at the two surfaces.

What goes wrong. Pins not vertical. A leaning pin appears displaced when viewed from a low angle, and the “straight line” you judge is then wrong, which puts an error into every angle measured afterwards. Push them in straight, and sight along the paper rather than down at it.

Activity 9.11

Hold a convex lens and direct it towards the Sun. Focus the light onto a sheet of paper to get a sharp bright image of the Sun. Hold it there for a few minutes. The book repeats its warning: never look at the Sun directly or through a lens.

What you should observe. A brilliant point, and then smoke.

Why it parallels Activity 9.2. Same measurement, different device: the Sun’s rays are parallel, so they converge at the focus, and the mirror-to-spot or lens-to-paper distance is f. The chapter needs this number before Activity 9.12 can be set up.

Why a convex lens can do this and a concave lens cannot. A convex lens converges light; a concave lens diverges it, and can never bring parallel rays to a real point. That is the whole difference, and it is why only one of the two burns paper.

What goes wrong. The same hazard as 9.2, and worse — a lens will set paper alight in seconds. Never leave a convex lens on a sunny windowsill. And never, under any circumstances, look at the Sun through it.

Activity 9.12

Find the convex lens’s focal length as in 9.11. Chalk five parallel lines on a long table, each spaced by that focal length, and stand the lens on the middle line with its optical centre over it — the lines then mark 2F₁, F₁, F₂ and 2F₂. Place a burning candle beyond 2F₁ and catch a sharp image on a screen on the far side. Repeat with the candle just beyond 2F₁, between F₁ and 2F₁, at F₁, and between F₁ and O.

What you should observe. Six cases again, and they mirror the concave mirror’s table almost exactly.

Why the five chalk lines. They are the experiment. Spacing them by exactly one focal length means the positions 2F₁, F₁, F₂, 2F₂ are marked physically on the table, so a student can put the candle at 2F₁ rather than estimate it. Every row of the table depends on getting that placement right.

Why this table looks like Table 9.1. A converging lens and a concave mirror do the same job to light — they bring it together — so the pattern of images is the same, with 2F₁ playing the role that C played for the mirror. Noticing that saves learning the second table from scratch.

What goes wrong. Trying to screen the last row. Between F₁ and O the image is virtual, on the same side as the object, and must be looked at through the lens — this is the lens working as a magnifying glass. Exactly the same trap as Activity 9.3’s last row.

Activity 9.13

Stand a concave lens on a lens stand with a burning candle on one side. Look through it from the other side. Try to catch the image on a screen; if you cannot, observe it through the lens. Note nature, size and position, then move the candle further away.

What you should observe. Nothing you do changes the kind of image you get.

Why it can never be screened. A concave lens diverges the rays passing through it, so they never meet on the far side. The image is where they appear to come from — always virtual, always on the same side as the object.

Why this is the counterpart to Activity 9.5. Concave lens is to convex lens as convex mirror is to concave mirror: the diverging member of each pair gives one kind of image only, and its table has two rows instead of six.

What goes wrong. Persisting with the screen. The instruction says “try to get the image on a screen, if possible” — and the answer is that it is not possible. A student who eventually gives up and looks through the lens has learned the point; one who is told in advance not to bother has not.

Common mistakes

• Reporting the concave mirror’s image in Activity 9.1 as always enlarged. It is enlarged only while your face is inside the focus. Past that it inverts and shrinks, and that transition is what the activity is for.
• Looking at the Sun in Activities 9.2 and 9.11. Both carry the book’s warning. The reflected or focused image is as dangerous as the Sun itself; look only at the paper.
• Hunting for a screen image of a virtual image in Activity 9.3’s last row or Activity 9.12’s. Virtual images cannot be caught on a screen, by definition — they must be viewed through the lens or in the mirror.
• Writing “highly enlarged” where NCERT prints “image would not be formed” for an object at F. The physics is the same — the rays emerge parallel — but it is worth knowing which wording the book uses.
• Viewing the coin in Activity 9.7 from directly above. There is no apparent shift along the normal. The eye must be to one side for refraction to displace anything.
• Leaning pins in Activity 9.10. A pin that is not vertical looks displaced, so the line of sight is judged wrongly and every angle measured afterwards inherits the error.
• Mixing up which way the ray bends. Into a denser medium, towards the normal; into a rarer one, away. Activity 9.10 shows both in a single ray, which is the easiest way to keep them straight.
• Expecting the concave lens of Activity 9.13 to behave like the convex lens of 9.12. Six rows against two. Diverging optics have one behaviour, not a family of them.

Practise next

• The end-of-chapter Exercises for this chapter, where the mirror formula and lens formula turn the tables built in Activities 9.3 and 9.12 into calculations, and where the convex mirror of Activity 9.5 returns as questions on rear-view mirrors.
• The in-text questions of this chapter, which ask for the focal length given a radius of curvature — the R = 2f measured in Activity 9.2 — and why convex mirrors are preferred for wing mirrors.
• Chapter 10, The Human Eye and the Colourful World, where the convex lens of Activities 9.11 and 9.12 becomes the lens of the eye, and its focal length becomes something the body adjusts.