Question 15
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 13.19. Draw the reflected ray from the second mirror.
Let MM' and M'M'' be the two plane mirrors at right angles.
The reflected ray from the second mirror is shown in the figure below:
Given that the ray is incident on mirror MM' at 30°,
∴ ∠AOX = 30°
According to the law of reflection,
∠i = ∠r
∴ ∠XOO' = ∠AOX = 30°
As OX ⊥ MM' and O'X ⊥ M'M''
∴ OX ⊥ O'X
∴ ∠OXO' = 90°
In ΔOXO', by angle sum property.
∠XOO' + ∠OXO' + ∠XO'O = 180°
⇒ 30° + 90° + ∠XO'O = 180°
⇒ ∠XO'O = 180° - 90° - 30°
⇒ ∠XO'O = 60°
∴ Angle of incidence of the ray on M'M'' = 60°
∴ Angle of reflection of the ray from M'M'' = 60° [∵ ∠i = ∠r]
Chapter Overview: Light
This optics chapter covers the laws of reflection, regular vs diffused reflection, multiple images in mirrors, dispersion of white light, the human eye anatomy and function, and the Braille system for visually impaired persons. Ray diagrams and the multiple image formula are key exam topics.
Board Exam Weightage: 5-7 marks | Difficulty: Moderate to High
Key Formulas & Concepts
| Concept | Details |
|---|---|
| Laws of Reflection | 1. Angle of incidence = Angle of reflection (i = r); 2. Incident ray, reflected ray, and normal lie in the same plane |
| Multiple Images | Number of images in mirrors at angle θ: n = (360/θ) - 1 |
| VIBGYOR | Violet, Indigo, Blue, Green, Yellow, Orange, Red (spectrum colours) |
| Lateral Inversion | Left and right reversal in mirror images (AMBULANCE written reversed) |
Must-Know Concepts
- Angles of incidence and reflection are measured from the NORMAL, not the surface
- Regular reflection (smooth surface) gives clear images; diffused reflection (rough surface) scatters light
- Diffused reflection also follows laws of reflection at each individual point
- Two parallel mirrors (0°) produce infinite images
- Kaleidoscope uses 3 mirrors to create symmetrical patterns
- White light disperses into VIBGYOR when passing through a prism
- The eye works like a camera: cornea → pupil → lens → retina → optic nerve → brain
Regular vs Diffused Reflection
| Feature | Regular Reflection | Diffused Reflection |
|---|---|---|
| Surface | Smooth, polished | Rough, uneven |
| Reflected Rays | Parallel (same direction) | Scattered (different directions) |
| Image | Clear, well-defined | No clear image |
| Examples | Mirror, still water, polished metal | Wall, paper, road, chalk, wood |
Common Mistakes to Avoid
- Measuring angles from the surface instead of from the normal
- Thinking diffused reflection does not follow laws of reflection (it does, at each point)
- Errors in the multiple image formula (n = 360/θ - 1)
- Confusing parts and functions of the human eye
Scoring Tips
- Always draw the normal first when drawing ray diagrams
- Practice the formula with common angles: 90° → 3 images, 60° → 5 images
- Learn the path of light through the eye in order
- Draw and label the human eye from memory for guaranteed marks
Frequently Asked Questions
Q: How many marks does this chapter carry in the exam?
A: Approximately 5-7 marks in the annual exam.
Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).
Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.