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NCERT Solutions Curiosity Chapter 10 .3 What Are the Laws of Reflection? — Activity 10.4: Let us experiment

Book page 15710 Updated on2026-09-05

Q1.
Now, move the slit and torch slightly so that the beam of light falls at a different angle on the mirror (Fig. 10.8b). Does the reflected beam of light also shift?
Answer

Yes. The moment the incoming beam falls at a new angle, the reflected beam swings to a new direction as well — and it does so immediately, not gradually.

Why it happens: the mirror has not moved, so the normal at the point of incidence still points the same way. Only the incident ray has been turned. Since the reflected ray must always make the same angle with that normal as the incident ray does, turning the incident beam through some angle forces the reflected beam to turn through the same angle — but to the other side of the normal.
Check it yourself: bring the torch closer to the normal and the reflected beam also comes closer to the normal. Take the torch nearly parallel to the mirror and the reflected beam skims away nearly parallel to it too.
Q2.
Make the beam of light fall on the mirror at different angles and observe how the direction of the reflected beam changes.
Answer

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.

i = 40°r = 40°50°Incident rayReflected rayNormalMirrorO
Both i and r are measured from the normal, never from the mirror. If i = 40°, then r = 40°, and the angle between the reflected ray and the mirror surface is 90° − 40° = 50°.
Incident beam close to the normal → reflected beam close to the normal
Incident beam far from the normal → reflected beam far from the normal
In every case, angle of reflection = angle of incidence
Why the normal, and not the mirror, is the reference: the normal is the one line that is fixed by the mirror surface itself at that point. Measuring from the mirror would work for a plane mirror but would be useless on a curved one, where 'the mirror' slopes differently at every point. The normal, drawn at 90° to the surface at the point of incidence, is defined the same way everywhere — which is why the law is stated in terms of it.
Q3.
On your drawing, measure the angle of incidence and the angle of reflection and note it in Table 10.1. Repeat the activity several times by changing the angle of incidence.
Answer

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.

S.No.Angle of incidence (i)Angle of reflection (r)
120°20°
230°31°
345°44°
460°60°
570°69°

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

i = r, the first law of reflection
Where the one-degree gaps come from: the beam from the slit has a real width, so the pencil line you draw along it may be a little off; the mirror line may not be drawn exactly where the mirror stood; and a protractor can only be read to about half a degree. None of these can be removed completely, which is why the readings are 'nearly equal' rather than exactly equal.
Tip: mark two points far apart on the beam before you remove the mirror, then join them with a ruler. A long line gives a far more accurate angle than a short one.
Q4.
Finally, let the incident beam fall on the mirror along the normal and observe the direction of the reflected beam. What would be the angle of incidence and angle of reflection in this case?
Answer

Both angles are zero.

Incident ray lies along the normal → angle between them = i = 0°
By the law of reflection, r = i = 0°
So the reflected ray also lies along the normal

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

Why this case matters: it is the one direction in which a mirror sends light exactly back to its source. It is also a useful check on the definition of the angles — if you had wrongly measured them from the mirror surface you would have got 90° here instead of 0°, and the law i = r would still have 'worked', which is why the book insists on measuring from the normal.
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