The Human Eye and the Colourful World — Question 8
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Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
When focusing on a nearby object, the ciliary muscles strain to adjust the eye lens, reducing its focal length. However, there's a limit to this adjustment. Beyond approximately 25 cm, the eye struggles to bring the image into clear focus, resulting in blurred vision for objects placed closer. This limitation is due to the eye's finite ability to accommodate and maintain sharp focus.
Chapter 10: The Human Eye and the Colourful World — Quick Revision Guide
Introduction
The human eye is a natural optical instrument. This chapter explains how the eye works, defects of vision and their correction, and beautiful phenomena like dispersion, atmospheric refraction, and scattering of light.
Key Points at a Glance
- Eye: cornea (refraction) → iris/pupil (light control) → lens (fine focus) → retina (image) → optic nerve → brain
- Accommodation: eye lens changes focal length using ciliary muscles; near point = 25 cm, far point = ∞
- Myopia: can't see far; image before retina; corrected by concave lens
- Hypermetropia: can't see near; image behind retina; corrected by convex lens
- Presbyopia: age-related; corrected by bifocal lens; Cataract: opaque lens; corrected by surgery
- Dispersion: white light splits into VIBGYOR through prism; violet deviates most, red least
- Atmospheric refraction: twinkling of stars, advanced sunrise/delayed sunset (~2 min each)
- Scattering: intensity ∝ 1/λ4; blue sky (short λ scattered more); red sunrise/sunset (blue scattered away); white clouds (all λ scattered equally by large droplets)
- Tyndall effect: scattering by colloidal particles; visible beam in dusty room, fog
Real-World Connections
Eye donations restore sight; LASIK surgery reshapes cornea; blue colour of sky and red sunsets explained by scattering; danger signals are red because red light travels farthest.
Quick Self-Test (5 Questions)
- What is the most important concept you learned from this chapter?
- Can you write three key equations/formulae from this chapter from memory?
- Draw a labelled diagram relevant to this chapter without looking at your notes.
- Explain one real-world application of a concept from this chapter.
- What is one common mistake students make in this chapter, and how can you avoid it?
Further Study
- NCERT Textbook Chapter 10
- NCERT Exemplar Problems
- Bright Tutorials Detailed Notes: ch10-human-eye.html
- Bright Tutorials Practice Questions: ch10-human-eye.html
- Previous Year CBSE Board Papers