Question 6
Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be ............... m away from his image.
(b) If you touch your ............... ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with ...............
(c) The size of the pupil becomes ............... when you see in dim light.
(d) Night birds have ............... cones than rods in their eyes.
(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.
Reason — When a person stands in front of a plane mirror, their image appears to be as far behind the mirror as the person is in front of it.
So, when the person is 1 m away from the mirror. His image will be formed at 1 m from the mirror on the other side, hence, his image will be 1 + 1 = 2 m from the person himself.
(b) If you touch your left ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your left hand.
Reason — The image formed by a mirror is laterally inverted hence, when we touch our left ear with our right hand in front of a plane mirror, it will be seen in the mirror that our right ear is touched with our left hand.
(c) The size of the pupil becomes large when you see in dim light.
Reason — In dim light conditions, there's less available light for vision. To allow more light to enter the eye and improve visibility in low-light environments, the iris dilates the pupil. When the pupil dilates, it enlarges in size, allowing more light to pass through the eye and reach the retina at the back of the eye.
(d) Night birds have fewer cones than rods in their eyes.
Reason — Having fewer cones and more rods allows night birds to have better night vision. Rods are more sensitive to low levels of light, making them ideal for detecting prey and navigating in the dark. Additionally, having fewer cones means that night birds likely have reduced color vision or even monochromatic vision, which is less important for hunting in low-light conditions.
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.