Keep the Curiosity Alive — Chapter Exercises — "Light: Mirrors and Lenses"

1. 12 marksCuriosity Grade 8, Chapter 10, page 166

   A light ray is incident on a mirror (Fig. 10.21). The angle made by the incident ray with the normal is 40°. What is the angle made by the reflected ray with the mirror (not the normal)? (i) 40° (ii) 50° (iii) 45° (iv) 60°

   Hint. Careful — the question asks for the angle from the mirror surface, not from the normal.

   Answer: (ii) 50°.

   Working. The angle of incidence is measured from the normal: i = 40°. By the first law of reflection, the angle of reflection r = i = 40°, also measured from the normal.

   The normal is at 90° to the mirror. So the angle the reflected ray makes with the mirror surface is:

   90° − r = 90° − 40° = 50°

   ✦ This question is built to catch anyone who reads 'angle of reflection' automatically as the answer. The law gives you 40° measured from the normal; the question asks for the angle from the mirror, which is the complement of that. Since the two reference lines (normal and mirror surface) are always 90° apart, always check which one a question is measuring from before writing down a number — here, writing 40° would answer a question that was not asked.

2. 25 marksCuriosity Grade 8, Chapter 10, page 166

   Fig. 10.22 shows three situations: (i) the ray falls along the normal; (ii) the mirror is tilted but the ray still falls along the normal to the tilted surface; (iii) the mirror is tilted and the ray falls at 20° from the normal. Draw the reflected ray in each case and state the angle of reflection.

   Hint. In two of the three cases, the ray simply retraces its own path.

   | Case | Angle of incidence | Angle of reflection | What the reflected ray does | |---|---|---| | (i) Ray along the normal, flat mirror | 0° | 0° | Reflects straight back along the same line | | (ii) Mirror tilted, ray still along the (tilted) normal | 0° | 0° | Reflects straight back along the same line, at the new tilt | | (iii) Mirror tilted, ray at 20° from the normal | 20° | 20° | Reflects at 20° on the other side of the normal from the incident ray |

   ✦ Cases (i) and (ii) look different — one mirror flat, one tilted — but give the identical result, and that is the whole point of including both. Let the incident beam fall on the mirror along the normal ... both the angles would be zero in this case. Since 'along the normal' means i = 0° regardless of which way the mirror itself is tilted, tilting the mirror in case (ii) changes the direction the ray must travel to still be along the normal, but does not change the law: it still reflects straight back the way it came.

   In case (iii), draw the reflected ray on the opposite side of the normal from the incident ray, at the same 20° angle — the normal always sits exactly between the incident and reflected rays.

3. 34 marksCuriosity Grade 8, Chapter 10, page 166

   In Fig. 10.23, the cap of a sketch pen is placed in front of three mirrors. Match each image to plane, convex, or concave, and justify from the size of the reflected cap in each photo.

   Hint. Compare each reflected cap's size against the real cap sitting beside the mirror in the same photo.

   Matching, based directly on the photographs:

   Image	Mirror	Justification
   (i)	Plane mirror	The reflected cap is the same size as the real cap beside it — a plane mirror always gives an image the same size as the object
   (ii)	Concave mirror	The reflected cap is clearly larger than the real cap — the image is erect but larger than the object in size, that is, enlarged, which Activity 10.3 found for a concave mirror with the object close to it
   (iii)	Convex mirror	The reflected cap is clearly smaller than the real cap — the image is always erect and smaller than the object, that is diminished, exactly the convex-mirror result

   ✦ The printed answer table lists the three mirror names in a scrambled order, not lined up with the photo numbers — this is a genuine matching exercise, and the match has to come from looking at the photos, not from reading down the table. Since size alone distinguishes all three mirrors here (same / larger / smaller), comparing each reflected cap to the real cap standing next to it is sufficient to identify every mirror correctly.

4. 44 marksCuriosity Grade 8, Chapter 10, page 167

   In Fig. 10.24, the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece, all at the same distance. Match each image to the correct type, and justify from the photographs.

   Hint. The same size-comparison method works here as for the mirrors.

   The same size-comparison method, now applied to the three lens/glass photographs:

   Image	Type	Justification
   (i)	Convex lens	The cap seen through it looks distinctly larger — matching Activity 10.9's the object appears erect and enlarged in size for a convex lens at a small distance
   (ii)	Concave lens	The cap seen through it looks distinctly smaller — matching an object placed behind a concave lens ... always appears erect and diminished in size
   (iii)	Flat transparent glass piece	The cap looks unchanged, the same size as it would appear with nothing in the way — matching Activity 10.10's the light beam passes through the thin glass plate as it is

   ✦ As with the mirrors, the printed table's order is a scrambled list of options, not a pre-aligned answer key. Since a flat piece of glass has no curved surface to converge or diverge light, it is the one item in this trio that should show no size change at all, which is the easiest of the three to confirm at a glance and a useful anchor for judging the other two against.

5. 51 markCuriosity Grade 8, Chapter 10, page 167

   When light is incident along the normal on the mirror, which statement is true? (i) Angle of incidence is 90° (ii) Angle of incidence is 0° (iii) Angle of reflection is 90° (iv) No reflection of light takes place

   Hint. 'Along the normal' has a precise meaning stated elsewhere in the chapter.

   Answer: (ii) Angle of incidence is 0°.

   Let the incident beam fall on the mirror along the normal ... What would be the angle of incidence and angle of reflection in this case? Both the angles would be zero in this case.

   Why the others fail. (i) and (iii) both claim 90°, which would describe a ray travelling along the mirror's surface, not along the normal — the opposite case. (iv) is simply false: reflection still occurs when the ray strikes along the normal; it just bounces straight back the way it came instead of at an angle.

   ✦ 'Along the normal' is a coordinate, and it is easy to mix up which one. Since the normal is defined at 90° to the mirror, a ray travelling along the normal is travelling perpendicular to the mirror surface — the two phrases describe the same direction from two different reference lines, and confusing 'along the normal' with 'along the mirror' is precisely how options (i) and (iii) are constructed to be chosen wrongly.

6. 64 marksCuriosity Grade 8, Chapter 10, page 167

   In Fig. 10.25, three mirrors — plane, concave and convex — reflect a graph sheet held above them. Identify each mirror from the reflected grid, and explain your method.

   Hint. The method is to compare the size of the reflected grid squares with the size of the real grid squares above.

   Identification, from the reflected grid squares in each mirror:

   Mirror (left to right)	Reflected grid squares	Identity
   Left	About the same size as the real grid	Plane mirror
   Middle	Larger than the real grid — visibly magnified	Concave mirror
   Right	Smaller than the real grid — visibly compressed	Convex mirror

   ✦ The method is exactly the one Activity 10.3 already established, applied to a grid instead of a toy. A plane mirror leaves size unchanged; a concave mirror enlarges (the image is erect but larger than the object in size); a convex mirror diminishes (always erect and smaller than the object). Since a fine grid pattern makes even a small change of scale easy to see and measure, reflecting a graph sheet is a more sensitive way to compare the three mirrors than a single small object would be — every square is a built-in ruler for judging magnification.

7. 73 marksCuriosity Grade 8, Chapter 10, page 167

   In a museum, a woman walks TOWARDS a large concave mirror. Which of the following describes what she will see? (i) her erect image keeps decreasing in size (ii) her inverted image keeps decreasing in size (iii) her inverted image keeps increasing in size and eventually becomes erect and magnified (iv) her erect image keeps increasing in size

   Hint. She is walking towards the mirror — so her distance from it is decreasing, not increasing.

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

   Why, from Activity 10.3's own results, read in reverse. The activity describes what happens as an object moves away from a concave mirror: when the object is placed close to the mirror, the image is erect but larger ... when the object is moved farther away, the image becomes inverted. Initially, the image is enlarged in size and then keeps getting smaller.

   The woman is doing the opposite — walking towards the mirror, starting far away. Reversing the sequence: far away, her image is inverted; as she approaches, that inverted image grows; and once she is close enough, the image switches to erect and enlarged, matching the close-up case the activity describes.

   ✦ This is the same technique used elsewhere in the chapter — running a stated rule backwards to predict the opposite situation. Since the activity only describes the object receding, correctly answering a question about an object approaching requires recognising that it is simply the same sequence read in the other direction, not a new fact to memorise separately.

8. 83 marksCuriosity Grade 8, Chapter 10, page 167

   Hold a magnifying glass over text and find the distance at which the text looks bigger than it is. Move it away from the text. What do you notice, and which type of lens is a magnifying glass?

   Hint. This is Activity 10.9's convex-lens result, performed with a real magnifying glass instead of a small toy.

   What you should notice. Held close to the text, the letters appear erect and enlarged. As you slowly move the magnifying glass farther from the text, at some point the enlarged, erect view is lost — the image can become blurred, and with a true convex lens moved far enough, it eventually appears inverted and changes in size, exactly as Activity 10.9 describes.

   A magnifying glass is a convex lens.

   Have you seen a magnifying glass ... It is also a lens that helps in reading small print by making the letters appear bigger.

   ✦ This links directly back to the water-drop lens of Activity 10.8 and to Activity 10.9's stated result for a convex lens. When an object is placed behind a convex lens at a small distance from it and seen through the lens, the object appears erect and enlarged in size. Since a magnifying glass is always used at a small distance from the text — that is the whole point of using it — you are seeing precisely the close-up half of the convex lens's two-part behaviour, and moving it farther away is what starts to reveal the second half.

9. 94 marksCuriosity Grade 8, Chapter 10, page 168

   Match: (i) Concave mirror (ii) Convex mirror (iii) Convex lens (iv) Concave lens with (a) reflecting surface curves inwards (b) image always erect and diminished (c) object may appear inverted at some distance (d) object always appears diminished in size.

   Hint. One option is a definition of shape; the rest describe behaviour, and two of the behaviours overlap closely.

   The correct matching:

   Column I	Column II	Why
   (i) Concave mirror	(a)	This is the definition itself: a spherical mirror, which has a reflecting surface that curves inwards, is called a concave mirror
   (ii) Convex mirror	(b)	The image is always erect and smaller than the object — always erect and diminished
   (iii) Convex lens	(c)	As the distance ... increases, the object appears inverted — the one item in this list that can invert
   (iv) Concave lens	(d)	Always appears erect and diminished in size — diminished, though the word 'erect' is not repeated in (d), it is consistent with it

   ✦ (a) can only match (i), because it is a statement about shape — 'curves inwards' — which is specific to how a concave mirror is defined, not a behaviour shared with any lens. Since only the convex lens, among these four, is ever described as becoming inverted, (c) has only one possible home; once (a) and (c) are placed, (b) and (d) sort themselves by matching 'mirror' language to the mirror and 'diminished' language to the concave lens.

10. 102 marksCuriosity Grade 8, Chapter 10, page 168

    Assertion: Convex mirrors are preferred for observing traffic behind us. Reason: Convex mirrors provide a significantly larger view area than plane mirrors. Choose: (i) both true, R explains A (ii) both true, R does not explain A (iii) A true, R false (iv) both false

    Hint. Check the reason on its own merits before deciding whether it explains the assertion.

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

    The Assertion is true. Look at the side-view mirrors on vehicles. These mirrors are convex.

    The Reason is true. Since the convex mirror is curved outside, it provides a much wider area of the road behind. A convex mirror's outward curve spreads (diverges) the reflected rays, letting it capture a wider field of view than a flat mirror of the same size could.

    And the Reason explains the Assertion. The wider field of view is exactly why convex mirrors are chosen for this job — a driver needs to see as much of the road behind as possible, and a plane mirror of the same size would show a narrower slice of it.

    ✦ The trade-off that comes with this wider view — the image being smaller, and so needing the 'objects are closer than they appear' warning — is not mentioned in the Reason, and does not need to be for this question. Since the Reason only has to explain why convex mirrors are preferred, not describe every consequence of using one, the assertion-reason logic holds even though a fuller discussion (as in the chapter's opening questions) would also raise the size-versus-distance trade-off.

11. 113 marksCuriosity Grade 8, Chapter 10, page 168

    In Fig. 10.27, O stands for object, M for mirror, I for image. In figure (a) the O and I arrows are the same height; in figure (b) the I arrow is clearly taller than the O arrow. Which mirror does each figure represent?

    Hint. Both arrows point the same way in both figures — so orientation is not what distinguishes them here.

    Answer: (i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror.

    Reading the figures. In both (a) and (b), the O and I arrows point in the same direction — both images are erect. The only difference between the two figures is size: in (a), the image (I) is the same height as the object (O); in (b), the image is noticeably taller than the object.

    Figure	Image size relative to object	Mirror
    (a)	Same	Plane — always gives an image the same size as the object
    (b)	Larger	Concave — gives an erect, enlarged image when the object is close (Activity 10.3)

    ✦ A convex mirror is ruled out for figure (b) by the direction of the size change alone. Since a convex mirror is always erect and diminished, never enlarged, an erect image that is larger than the object cannot be a convex mirror's — it has to be a concave mirror with the object placed close to it, which is the one situation in which Activity 10.3 records an erect image that is also bigger than the object.

12. 124 marksCuriosity Grade 8, Chapter 10, page 169

    Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Fill the tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed through the water, and why?

    Hint. Extend Activity 10.8's water-drop reasoning from a small drop to a whole tumbler of water.

    Observation, from the photograph. Through the empty tumbler (Fig. 10.28a), the pencil looks straight and continuous. Once the tumbler is half-filled with water (Fig. 10.28b), the part of the pencil behind the water looks noticeably shifted and enlarged compared with the part still seen through only air above the waterline — the pencil appears to break or bulge at the water's surface rather than continuing as one straight line.

    Why, reasoning from what this chapter establishes. Activity 10.8 showed that a small curved drop of water acts as a simple lens: the curved surface of the water drop made the size of the text look different. The water in the tumbler, held inside a curved, cylindrical glass, presents the same kind of curved transparent surface to anything viewed through it — so it behaves like a (rough) lens over the portion of the pencil that sits behind the water, changing how that part appears, while the part of the pencil above the waterline, seen only through air, is unaffected.

    ✦ Be precise about what this explanation does and does not claim. The chapter never performs this exact tumbler experiment itself, so the explanation above is a reasonable extension of Activity 10.8's water-drop principle to a larger, curved container of water — worth stating as reasoning from the chapter's own idea, rather than as a result the book reports directly. Since both cases share the one feature that matters — a curved transparent surface changing the apparent size and position of what lies behind it — the extension is well founded even without a matching activity to quote.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 10 (hecu110.pdf), Reprint 2026-27, pages 152-169. Questions are referenced from the NCERT textbook for identification.