Activities 10.4 and 10.5 — The Laws of Reflection — "Light: Mirrors and Lenses"

1. 14 marksCuriosity Grade 8, Chapter 10, pages 157-158

   Describe Activity 10.4's set-up, and define incident ray, reflected ray, normal, angle of incidence and angle of reflection.

   Hint. The comb-and-slit trick from Grade 7 returns, now to measure angles rather than just to see a beam.

   The set-up. Collect a plane mirror on a stand, a torch, a comb, a paper clip, white paper and black paper. Cover all the comb's openings but one in the middle with black paper, so the torch produces a single thin beam. Shine it onto a plane mirror standing on white paper, and note how the reflected beam shifts as the incident beam's angle changes.

   The five terms, from the diagram (Fig. 10.9):

   Term	Definition
   Incident ray	The ray of light that falls on the mirror
   Reflected ray	The ray of light that comes back from the mirror
   Normal	A line at 90° to the mirror at the point of incidence, O
   Angle of incidence (i)	Between the normal and the incident ray
   Angle of reflection (r)	Between the normal and the reflected ray

   ✦ The normal is not drawn on the mirror at all — it is drawn from the mirror. It exists purely as a reference line to measure the two angles against, since 'the angle between two rays hitting a surface at different points' would be meaningless without one fixed line to compare each ray to at its own point of incidence.

2. 23 marksCuriosity Grade 8, Chapter 10, page 158

   How does Activity 10.4 arrive at the first law of reflection? What result does Table 10.1 show?

   Hint. Repeat the measurement several times, not once.

   The method. Draw the incident and reflected rays on paper, remove the mirror, draw the normal at 90° to the mirror line at the point of incidence, then measure the angle of incidence and angle of reflection with a protractor and record them in Table 10.1. Repeat with several different incident angles.

   Do you notice that both angles in Table 10.1 are nearly equal? If done carefully, the experiment shows that the angle of incidence (i) is equal to the angle of reflection (r). This is a law of reflection.

   ✦ A single trial proving i = r would be a coincidence you could not rule out; several trials at different angles are what turns it into a law. Since the rule holds at every angle you try — steep, shallow, anywhere in between — it cannot be an accident of one particular set-up, and that repeatability is exactly why the activity insists you repeat the activity several times by changing the angle of incidence before drawing any conclusion.

   Along the normal is the special case worth stating precisely. Let the incident beam fall on the mirror along the normal ... both the angles would be zero in this case — the rule i = r still holds; it has simply become 0° = 0°.

3. 34 marksCuriosity Grade 8, Chapter 10, pages 158-159

   State the first law of reflection precisely, and explain why 'the angle of incidence equals the angle of reflection' cannot be replaced by 'the incident and reflected rays make the same angle with the mirror'.

   Hint. Both statements sound equivalent — check whether they measure the same thing.

   The law: the angle of incidence (i) is equal to the angle of reflection (r), where both angles are measured from the normal, not from the mirror's surface.

   Why the alternative phrasing is risky rather than wrong. Because the normal is at exactly 90° to the mirror, the angle a ray makes with the normal and the angle it makes with the mirror surface are simply related — one is 90° minus the other. So 'equal angles from the normal' and 'equal angles from the mirror' happen to agree with each other in this particular geometry, which is precisely why the two phrasings are so easy to blur together.

   ✦ But the chapter defines the law using the normal, and that is the version to reproduce exactly. The angle between the normal and the incident ray is called the angle of incidence (i). The angle between the normal and the reflected ray is known as the angle of reflection (r). Since exam answers and diagrams are marked against this specific definition, always measure and label your angles from the normal, and never casually substitute the mirror surface as the reference line even where the numbers would come out consistent.

4. 44 marksCuriosity Grade 8, Chapter 10, page 159

   Describe Activity 10.5 and the second law of reflection it demonstrates.

   Hint. This time the mirror does not move — the paper does.

   The activity. Use the same set-up as Activity 10.4, but lay a stiff sheet of chart paper flat on the table so that part of it extends beyond the table's edge. Shine the beam onto the mirror on the sheet and observe the reflected beam on the extended part. Then bend the extended part down along the table's edge.

   The reflected beam disappears when the sheet is bent but reappears when it is flattened again. This shows that the reflected beam lies in the same plane as that of the incident beam. Bending the sheet creates a new plane, breaking this alignment.

   The second law: the incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.

   ✦ Making the reflected beam vanish and reappear is a cleaner proof than simply describing the geometry, because it turns an abstract claim into something you can watch happen twice. Since bending the sheet changes nothing about the mirror or the angle of the incident ray — only the flatness of the surface the reflected beam would need to travel across — the disappearance can only be explained by the reflected ray having left the single flat plane the sheet used to provide. Flattening the sheet restores that plane, and the beam is visible again.

5. 53 marksCuriosity Grade 8, Chapter 10, page 159

   The 'A step further' box says that in the two cases of Activity 10.5, the directions of the normal are the same even though the incident rays differ. Explain why.

   Hint. Ask where each incident ray actually strikes the mirror.

   In the two cases, even though the directions of incident rays are different, they fall at the same point on the mirror, and thus, the directions of normal are the same.

   Why the point of incidence decides the normal's direction. The normal is defined at the point of incidence, at 90° to the mirror at that specific spot. For a plane mirror, the surface is flat everywhere, so the normal points the same way regardless of where along the mirror you draw it — so as long as both rays strike the same point, both normals are identical lines, whatever direction the incident rays themselves came from.

   ✦ This is precisely why Activity 10.5 can bend the paper without disturbing the geometry it is testing. Since the mirror, the point of incidence and therefore the normal are all held fixed, bending the extended sheet only changes whether the reflected ray still lies in the flat plane the incident ray and normal define — it cannot be blamed on a shifting normal, which rules out the one alternative explanation that might otherwise be offered for the disappearing beam.

6. 63 marksCuriosity Grade 8, Chapter 10, page 160

   The chapter states that the laws of reflection are valid for all kinds of mirrors — plane and spherical. If that is true, why do a plane mirror and a spherical mirror produce such different-looking images?

   Hint. The difference is not in the law each ray obeys, but in what the mirror's shape does to many rays together.

   The laws of reflection are valid for all kinds of mirrors — plane and spherical. But if multiple parallel rays fall on the spherical mirrors, we observe something interesting.

   The resolution. Every single ray, at every mirror, obeys i = r and stays in the same plane as its own normal — that never changes. What differs is that a plane mirror's surface is flat everywhere, so parallel rays striking it all have parallel normals, and their reflected rays stay parallel too. A spherical mirror's surface curves, so parallel rays strike it at different points with normals pointing in different directions, and each ray obeys the same law relative to its own, differently angled, normal.

   ✦ The result is that the individually identical rule produces a collectively different outcome. When multiple parallel beams of light fall upon a plane mirror, the multiple reflected beams are also parallel. However, when multiple beams of light fall upon a concave mirror, the multiple reflected beams get closer, that is, they converge. Since each ray is still separately following i = r, converging and diverging are not exceptions to the laws of reflection — they are exactly what those laws predict once the mirror's curvature is taken into account.

7. 73 marksCuriosity Grade 8, Chapter 10, pages 157-159

   A beam of light strikes a plane mirror at an angle of incidence of 35°. State the angle of reflection, and describe how you would verify it using Activity 10.4's method.

   Hint. The law gives you the answer directly; the activity gives you the method to check it.

   The angle of reflection is 35°, by the law the angle of incidence (i) is equal to the angle of reflection (r).

   Verifying it, following Activity 10.4:

   1. Set the plane mirror upright on white paper and use the slit torch to send in a thin beam at the chosen angle.
   2. Draw the position of the mirror, and the incident and reflected beams as lines with arrows.
   3. Remove the mirror and draw the normal, at 90° to the mirror line, at the point where the incident ray struck it.
   4. Measure the angle between the normal and the incident ray (i) and the angle between the normal and the reflected ray (r) with a protractor.
   5. Record both in a table like Table 10.1, and repeat at a few different angles to confirm the pattern holds generally rather than only at 35°.

   ✦ A single measurement agreeing with the prediction is good evidence but not yet a demonstrated law. Since the chapter's own activity insists on repeating the trial at several different angles, a complete verification reports readings at more than one angle of incidence, showing r tracks i every time — which is what turns 'this one measurement matched' into 'this is a law'.

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.