Human Eye and the Colourful World Class 10 Notes (2026-27) — CBSE
Class 10 Science Chapter 10 notes: the human eye and accommodation, defects of vision and correction, dispersion through a prism, atmospheric refraction and scattering.
The Human Eye and the Colourful World — Class 10 Science Notes
Chapter Snapshot
This chapter applies the optics of Chapter 9 to the human eye and to natural phenomena. You learn how the eye focuses light and adjusts (accommodation), the three common defects of vision and their corrective lenses, how a prism disperses white light into a spectrum, and how atmospheric refraction and scattering explain the twinkling of stars, the blue sky, and red sunrises/sunsets.
Board relevance: reliably gives a defects-of-vision question (identify the defect and the correcting lens/power) and a scattering/dispersion reasoning question.
Key Concepts & Definitions
The human eye works like a camera, forming a real, inverted image on the light-sensitive retina.
Part Function
Cornea Transparent front; most refraction of light happens here
Iris Coloured ring; controls the size of the pupil
Pupil Opening that regulates the amount of light entering
Eye lens Fine-focuses light; a flexible convex lens
Ciliary muscles Change the curvature (focal length) of the lens
Retina Screen with light-sensitive cells; forms the image
Optic nerve Carries the image signal to the brain
Accommodation — the ability of the eye lens to change its focal length (by the ciliary muscles changing its curvature) to focus objects at different distances.
- To see a near object: ciliary muscles contract, lens becomes thicker (shorter focal length).
- To see a distant object: muscles relax, lens becomes thinner (longer focal length).
Near point = 25 cm (least distance of distinct vision for a normal eye). Far point = infinity. The eye cannot focus objects closer than the near point.
Defects of Vision
Defect Cause Image forms Correction
Myopia (short-sightedness) Eyeball too long, or lens too curved; far point < infinity In front of the retina Concave (diverging) lens
Hypermetropia (long-sightedness) Eyeball too short, or lens too flat; near point 25 cm Behind the retina Convex (converging) lens
Presbyopia Ciliary muscles weaken and lens loses flexibility with age Both near & far blurred Bifocal lens (concave upper + convex lower)
- A myopic person sees near objects clearly but not distant ones; a hypermetropic person sees distant objects clearly but not near ones.
- Power of the corrective lens P = 1/f (in metres); concave lens has negative power, convex positive.
- Cataract (clouding of the lens) is not corrected by lenses — it needs surgery.
Refraction and Dispersion Through a Prism
A glass prism refracts light at both surfaces. The angle of deviation is the angle between the incident ray's direction and the emergent ray's direction. Because a prism's faces are not parallel (unlike a glass slab), the emergent ray is not parallel to the incident ray — it is deviated.
Dispersion of white light — the splitting of white light into its seven colours (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red). Different colours bend by different amounts: violet bends the most, red the least (violet has the shortest wavelength). The band of colours is a spectrum.
- Newton showed a second, inverted prism recombines the spectrum back into white light.
- A rainbow is a natural spectrum: sunlight is dispersed and internally reflected by water droplets after rain, with the red on the outer edge.
Atmospheric Refraction
Light bends as it passes through air layers of changing density (temperature). This causes:
- Twinkling of stars — starlight refracts through fluctuating air, so the star's apparent position and brightness keep changing. Planets don't twinkle (they are closer, seen as extended sources).
- Advance sunrise and delayed sunset — atmospheric refraction lets us see the Sun about 2 minutes before it actually rises and after it actually sets.
- The Sun appears flattened/oval near the horizon at sunrise/sunset due to refraction.
Scattering of Light
Tyndall effect — the scattering of light by tiny particles (smoke, fog, colloids) that makes the path of a light beam visible. The amount of scattering depends on the wavelength — shorter (blue) wavelengths scatter more than longer (red) ones.
- Blue sky: air molecules scatter blue light much more than red, so scattered blue reaches us from all directions.
- Reddish Sun at sunrise/sunset: light travels through more atmosphere, so most blue is scattered away and mainly red reaches us. At noon the Sun looks white (least atmosphere to travel through).
- Danger signals are red because red light scatters the least and travels the farthest, so it is visible from a distance even in fog.
- The sky looks dark/black to an astronaut because there is no atmosphere to scatter light in space.
Key Facts
Quick facts boards ask directly:
Topic Fact to remember
Most refraction in the eye At the cornea
Image on retina Real and inverted
Focal-length adjustment By ciliary muscles (accommodation)
Near point 25 cm
Far point Infinity
Myopia correction Concave (diverging) lens, negative power
Hypermetropia correction Convex (converging) lens, positive power
Presbyopia correction Bifocal lens
Colour deviated most by a prism Violet (shortest wavelength)
Colour deviated least Red (longest wavelength)
Spectrum order VIBGYOR
Colour scattered most Blue (short wavelength)
Colour scattered least Red — used for danger signals
Sky colour in space Black (no atmosphere to scatter)
Two definitions to quote: Dispersion is the splitting of white light into its component colours; scattering is the redirection of light in many directions by small particles, more for shorter wavelengths.
Important Question Patterns
1. Defects of vision (3 marks): identify the defect from a description or ray diagram, name the correcting lens, and compute its power (P = 1/f). Draw the defective and corrected eye.
2. Accommodation (2 marks): how the eye focuses near vs distant objects; near point and far point values.
3. Dispersion (2–3 marks): why a prism produces a spectrum but a slab does not; order VIBGYOR; recombination; formation of a rainbow.
4. Scattering (2–3 marks): why the sky is blue, the Sun red at sunset, danger lights red, the sky dark in space; Tyndall effect.
5. Atmospheric refraction (2 marks): twinkling of stars; why the Sun is visible before actual sunrise.
⚡ Quick Revision
- Eye: cornea (most refraction) → pupil (iris controls) → convex lens → real inverted image on retina. Accommodation by ciliary muscles. Near point 25 cm, far point infinity.
- Myopia (far point near): image before retina → concave lens (−P). Hypermetropia (near point far): image behind retina → convex lens (+P). Presbyopia (age): bifocal.
- Prism deviates light (faces not parallel). Dispersion → VIBGYOR; violet deviates most, red least; rainbow = dispersion + internal reflection in droplets.
- Atmospheric refraction: twinkling of stars; sunrise ~2 min early, sunset ~2 min late.
- Scattering: blue scatters more (short wavelength). Blue sky; red Sun at sunrise/sunset; red danger signals (scatter least, travel far); black sky in space (no atmosphere). Tyndall effect = scattering makes a beam visible.
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