The Human Eye and the Colourful World — Question 12
Back to all questionsQuestion 12
Why does the sky appear dark instead of blue to an astronaut?
The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths i.e blue. When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes and the sky appears blue. But there are no particles, hence no scattering of light in space. Therefore, the sky appear dark to an astronaut.
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