Class 10 Science Notes — Chapter 10: Human Eye and Colourful World
Structure and defects of the eye; refraction of light through prism; atmospheric optical phenomena.
Detailed NCERT notes
- Eye parts: cornea (main refraction), aqueous humour, iris (controls pupil size), pupil, lens (fine focusing via ciliary muscles), vitreous humour, retina (image formation, contains rods and cones), optic nerve.
- Power of accommodation: ability of eye lens to adjust focal length. Least distance of distinct vision (near point) = 25 cm; far point = infinity (for a normal eye).
- Defects of vision:
- Myopia (short-sightedness): far point < ∞; distant objects blur; corrected with concave lens.
- Hypermetropia (long-sightedness): near point > 25 cm; nearby objects blur; corrected with convex lens.
- Presbyopia: age-related loss of accommodation; often needs bifocal lenses.
- Cataract: lens becomes cloudy; corrected by surgical replacement.
- Refraction through a glass prism: light bends toward base; splits (dispersion) into constituent colours (VIBGYOR) because different colours have different refractive indices — violet bends most, red least.
- Newton's experiment: white light passing through a prism separates into 7 colours; passing through an inverted second prism recombines to white.
- Rainbow: sunlight refracts, disperses and reflects internally in raindrops.
- Atmospheric refraction: apparent flattening of Sun near horizon, twinkling of stars (due to changing air density), advance sunrise & delayed sunset (~2 min each due to refraction).
- Scattering (Tyndall effect / Rayleigh): shorter wavelengths scatter more.
- Sky appears blue: blue scatters more than red.
- Sun appears reddish at sunrise/sunset: blue scattered away, red passes through longer atmospheric path.
- Danger signals red: least scattered → visible from far.
Formulas & key results
- P = 1/f (in dioptres, f in metres)
- Normal near point = 25 cm; far point = ∞
Mind map
- Eye structure → accommodation → defects & correction
- Prism → dispersion → rainbow
- Atmospheric refraction & scattering
Tricks & shortcuts
- Myopia → 'my-o' close → concave for far.
- Hypermetropia → 'hyper' far → convex for near.
- Rayleigh scattering ∝ 1/λ⁴.
Common mistakes to avoid
- Confusing which defect uses which lens.
- Thinking stars twinkle due to their own brightness variation (it's atmospheric refraction).
Competency-based questions & answers
- Q. Why does the sky appear blue?A. Rayleigh scattering — shorter wavelengths (blue) are scattered more strongly by atmospheric molecules, so blue light reaches our eyes from all directions.
- Q. Why don't planets twinkle like stars?A. Planets are closer and act as a collection of point sources; averaged refraction cancels twinkling. Stars are point sources whose light gets deflected slightly by moving air pockets.
- Q. A person cannot read a book kept at 25 cm but can read it at 50 cm. Identify the defect and correction.A. Hypermetropia — corrected by a convex lens forming a virtual image of the book at his near point.