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๐ฌ Think about it
Shine a torch at a mirror at a slant and the light bounces off at the same slant the other way — never randomly. There are rules that light always obeys when it reflects. Let's discover the two laws of reflection.
What are the incident ray, reflected ray, and normal?
- Light is shown as straight lines with arrows, called rays.
- The ray that falls on the mirror is the incident ray.
- The ray that comes back is the reflected ray.
- The line at 90° to the mirror at that point is the normal.
Example:
A torch beam hitting a mirror is the incident ray; the bounced beam is the reflected ray.
In this Activity, we will measure the angle of incidence and angle of reflection on a plane mirror.
Materials needed
A plane mirror with stand, a torch, a comb, a paper clip, a sheet of white paper, a strip of black paper, a protractor.
Procedure
1. Make a thin slit by covering the comb with black paper, leaving one middle opening.
2. Place the plane mirror upright on white paper.
3. Use the slit and torch to shine a thin beam onto the mirror.
4. Draw the incident ray, reflected ray, and the normal at point O.
5. Measure the angle of incidence (i) and angle of reflection (r) and record in Table 10.1.
6. Repeat several times by changing the angle of incidence.
2. Place the plane mirror upright on white paper.
3. Use the slit and torch to shine a thin beam onto the mirror.
4. Draw the incident ray, reflected ray, and the normal at point O.
5. Measure the angle of incidence (i) and angle of reflection (r) and record in Table 10.1.
6. Repeat several times by changing the angle of incidence.
Table 10.1 — Measuring angles of incidence and reflection
| S.No. | Angle of incidence (i) | Angle of reflection (r) |
|---|---|---|
| 1. | 30° | 30° |
| 2. | 45° | 45° |
| 3. | 60° | 60° |
| 4. | 0° | 0° |
Observation
For every reading, the angle of incidence equals the angle of reflection. When light falls along the normal, both angles are zero.
Explanation
The experiment shows that the angle of incidence (i) equals the angle of reflection (r). This is the first law of reflection.
โ Summary
- Beam hits mirror
- Angles measured
- i equals r
- First law found
What is the first law of reflection?
- The angle of incidence is between the incident ray and the normal.
- The angle of reflection is between the reflected ray and the normal.
- These two angles are always equal.
- So the angle of incidence equals the angle of reflection.
Example:
A beam hitting a mirror at 40° to the normal reflects at 40° to the normal.
In this Activity, we will show that the incident ray, the normal, and the reflected ray all lie in the same plane.
Materials needed
The setup from Activity 10.4, plus a stiff sheet of chart paper.
Procedure
1. Place the chart paper flat so part of it extends beyond the table edge.
2. Shine a beam on the mirror and see the reflected beam on the extended portion.
3. Bend the extended part down along the table edge.
4. Check whether the reflected beam still shows on it.
5. Flatten the paper again and observe.
2. Shine a beam on the mirror and see the reflected beam on the extended portion.
3. Bend the extended part down along the table edge.
4. Check whether the reflected beam still shows on it.
5. Flatten the paper again and observe.
Observation
The reflected beam disappears when the sheet is bent and reappears when it is flattened.
Explanation
The reflected beam lies in the same plane as the incident beam. Bending the sheet creates a new plane and breaks the alignment. So the incident ray, the normal, and the reflected ray all lie in the same plane — the second law of reflection.
โ Summary
- Beam on flat sheet
- Sheet bent
- Beam disappears
- Same-plane law
What is the second law of reflection?
- It is about the incident ray, the normal, and the reflected ray.
- All three lie in the same plane.
- This holds at the point of incidence.
- Bending the plane makes the reflected beam vanish.
Example:
When the paper is flat, all three rays sit on it; bend it and the reflected ray leaves the surface.
๐ก A step further
- In the two cases, the incident rays come in at different directions.
- But both fall at the same point, so the normal direction is the same.
- The incident ray, the normal, and the reflected ray all lie in the same plane.
- This holds true in both cases.
๐ Our scientific heritage
- Over 800 years ago, in the time of Bhāskara II, astronomers used water bowls.
- They viewed reflected images of stars and planets through angled tubes.
- This let them measure the positions of stars and planets.
- Their methods suggest they understood reflection in practice.
What happens when parallel rays hit spherical mirrors?
- Parallel rays on a plane mirror stay parallel after reflection.
- On a concave mirror, reflected rays converge (come together).
- On a convex mirror, reflected rays diverge (spread apart).
- Each ray still obeys the laws of reflection.
Parallel Rays on Three Mirrors
Plane mirror
Reflected rays stay parallel.
↓
Concave mirror
Reflected rays converge.
↓
Convex mirror: reflected rays diverge
In this Activity, we will shine many parallel beams on each mirror to see how they reflect.
Materials needed
A plane mirror, a concave mirror, a convex mirror, mirror stands, a torch, a comb, a paper clip.
Procedure
1. Use the Activity 10.4 setup but leave many comb openings uncovered.
2. This gives multiple parallel beams of light.
3. Let the beams fall on the plane mirror and observe.
4. Repeat with the concave mirror, then the convex mirror.
5. Compare the reflected beams in each case.
2. This gives multiple parallel beams of light.
3. Let the beams fall on the plane mirror and observe.
4. Repeat with the concave mirror, then the convex mirror.
5. Compare the reflected beams in each case.
Observation
Plane mirror: beams stay parallel. Concave mirror: beams converge. Convex mirror: beams diverge.
Explanation
Each ray follows the laws of reflection, but the curved surface makes parallel rays converge (concave) or diverge (convex) depending on the mirror's shape.
โ Summary
- Many beams shone
- Plane stays parallel
- Concave converges
- Convex diverges
๐ก A step further
- If we draw what we observed in Activity 10.6, we get the figures shown.
- On a plane mirror, parallel rays reflect as parallel rays.
- On a concave mirror, parallel rays converge after reflection.
- On a convex mirror, parallel rays diverge after reflection.
In this Activity, we will use a concave mirror to focus sunlight and heat a paper.
Safety first
Always perform this under the supervision of a teacher or adult. Do not look at the Sun or into the mirror. Focus the light only on paper, never on anyone's face or eyes.
Materials needed
A concave mirror, a sheet of thin paper or newspaper.
Procedure
1. Hold the concave mirror with its reflecting surface facing the Sun.
2. Direct the reflected sunlight onto the sheet of paper.
3. Adjust the distance until you get a sharp bright spot.
4. Hold steady for a few minutes and watch for smoke.
2. Direct the reflected sunlight onto the sheet of paper.
3. Adjust the distance until you get a sharp bright spot.
4. Hold steady for a few minutes and watch for smoke.
Observation
A bright spot forms on the paper, and after a while the paper begins to burn and smoke.
Explanation
The concave mirror concentrates the Sun's light onto one point. This produces enough heat to ignite the paper.
โ Summary
- Mirror faces Sun
- Light focused
- Bright spot forms
- Paper burns
๐ก A step further
- Devices that concentrate sunlight are called solar concentrators.
- They use mirrors and lenses to focus light into a small area.
- The heat makes steam to generate electricity or for cooking.
- Solar furnaces can even melt steel.
Important Points
- First law: angle of incidence equals angle of reflection.
- Second law: incident ray, normal, and reflected ray lie in one plane.
- Concave mirrors converge parallel rays; convex mirrors diverge them.
โ Test Yourself
-
State the first law of reflection.
View Answer
The angle of incidence is equal to the angle of reflection. -
State the second law of reflection.
View Answer
The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane. -
What is the normal?
View Answer
The line drawn at 90° to the mirror at the point where the ray strikes it. -
What do parallel rays do on a concave mirror?
View Answer
They converge — the reflected rays come together. -
If light falls along the normal, what is the angle of reflection?
View Answer
Zero. The light reflects straight back along the same path.
Important Definitions
- Incident ray — the ray of light that falls on the mirror.
- Reflected ray — the ray of light that comes back from the mirror.
- Normal — the line drawn at 90° to the mirror at the point of incidence.
- Angle of incidence (i) — the angle between the incident ray and the normal.
- Angle of reflection (r) — the angle between the reflected ray and the normal.
- Converge — when rays come together after reflection (concave mirror).
- Diverge — when rays spread apart after reflection (convex mirror).
๐ NCERT Question 1 — A light ray is
A ray hits a mirror at 40° to the normal. What angle does the reflected ray make with the mirror?
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๐ NCERT Question 2 — Fig. 10.22 shows three
Draw the reflected ray for three situations and give the angle of reflection in each.
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๐ NCERT Question 5 — When the light is
When light is incident along the normal, which statement about the angles is true?
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๐ NCERT Question 6 — Three mirrors—plane, concave
Identify plane, concave, and convex mirrors from the images of a graph sheet formed in them.
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