👁️ The Human Eye
- A natural optical instrument that enables us to see.
- Diameter: about 2.3 cm.
- Parts of the Eye:
- Cornea – Transparent front part; refracts most of the light.
- Aqueous humour – Fluid between cornea & lens.
- Lens – Convex, flexible, changes shape (accommodation).
- Iris – Colored part, controls amount of light entering the eye.
- Pupil – Opening in iris, appears black.
- Ciliary muscles – Adjust curvature of the lens.
- Retina – Light-sensitive screen, contains rods & cones.
- Optic nerve – Sends signals to the brain.
- Vitreous humour – Fluid inside eyeball behind the lens.
🌟 Power of Accommodation
- Ability of eye lens to adjust its focal length to focus on near & distant objects.
- Near point of normal eye: 25 cm.
- Far point of normal eye: Infinity.
👓 Defects of Vision
|
Defect |
Cause |
Correction |
|
Myopia (short-sightedness) |
Eye lens too curved / eyeball too long |
Concave lens |
|
Hypermetropia (long-sightedness) |
Eye lens too flat / eyeball too short |
Convex lens |
|
Presbyopia |
Loss of elasticity of lens (with age) |
Bifocal lenses |
|
Cataract |
Clouding of the lens |
Surgery (replace lens) |
🌈 The Colourful World
🌄 Atmospheric Refraction:
- Twinkling of stars: Light bends due to varying air densities in atmosphere.
- Apparent sunrise & sunset: Sun appears earlier & sets later due to bending of light.
- Star appears higher than actual position.
🌈 Dispersion of Light:
- Splitting of white light into its component colors by a prism.
- VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
🌈 Rainbow:
- Formed by dispersion, internal reflection, and refraction of sunlight in water droplets.
🔷 Tyndall Effect:
- Scattering of light by particles in the atmosphere.
- Explains:
- Blue sky (short wavelengths scattered more)
- Red sunsets & sunrises (longer wavelengths dominate).
📋 Topic-wise Previous Year Questions (PYQs)
👁️ Human Eye
- Draw a labelled diagram of human eye. (3 Marks)
- What is the role of ciliary muscles in the eye? (1 Mark)
- Define power of accommodation. (1 Mark)
- What are the near point & far point of a normal eye? (1 Mark)
👓 Defects of Vision
- What is myopia? How is it corrected? (2 Marks)
- What causes presbyopia? How is it corrected? (2 Marks)
- Name two defects of vision & their remedies. (2 Marks)
🌄 Atmospheric Refraction
- Why does the sun appear earlier than actual sunrise & set later? (2 Marks)
- Why do stars twinkle? (2 Marks)
🌈 Dispersion & Rainbow
- What is dispersion of light? Draw diagram showing dispersion through a prism. (3 Marks)
- How is a rainbow formed? Explain. (3 Marks)
🔷 Scattering
- Why is the sky blue? (1 Mark)
- Why does the sun appear reddish at sunrise & sunset? (1 Mark)
Diagram-based Questions
- Labelled diagram of human eye.
- Ray diagram showing dispersion of light through prism.
- Diagram of rainbow formation.
📝 Tips for Board Exam
- Practice labelled diagrams thoroughly.
- Learn all definitions word-perfect.
- Prepare answers for common phenomena: sunrise, sunset, rainbow, sky color.
- Practice at least one numerical on power of lens (if asked).