CBSE Class 10 · Science · board exam 2027
Chapter 10: The Human Eye and the Colourful World — notes
The chapter’s 7 parts in the book’s order — the first idea of each and the points Medha Ma’am writes on the board — and the summary at the end. The full lesson and a check question for each part are in the app.
1. The human eye: its parts and their work
The figure.
- • Human eye ≈ a camera: its lens forms the image on the retina
- • Eyeball: nearly spherical, diameter ≈ 2.3 cm
- • Cornea: thin transparent front membrane; light enters through it
- • Most of the refraction happens at the outer surface of the cornea
- • Iris: dark muscular diaphragm, controls the size of the pupil
- • Pupil: controls the amount of light entering the eye
- • Bright light → pupil small; dim light → pupil large
- • Eye lens: convex; real, inverted image on the retina
- • Retina: light-sensitive cells → electrical signals
- • Optic nerve: carries the signals to the brain
2. Power of accommodation; near point and far point
The figure, two eyes.
- • Distant object: ciliary muscles relax → lens thin → focal length increases
- • Near object: ciliary muscles contract → lens thick → focal length decreases
- • Image distance (lens to retina) stays the same
- • Accommodation: the ability of the eye lens to adjust its focal length
- • Near point (least distance of distinct vision) ≈ 25 cm
- • Far point of a normal eye: ∞
- • Range of vision of a normal eye: 25 cm to ∞
3. Defects of vision and their correction
Sometimes the eye slowly loses its power of accommodation, and vision becomes blurred.
- • Three defects: (1) myopia (2) hypermetropia (3) presbyopia
- • Corrected by suitable spherical lenses
- • Myopia (near-sightedness): near clear, far blurred
- • Far point nearer than ∞
- • Image of a distant object: in front of the retina
- • Causes: (i) excessive curvature of the eye lens
- • (ii) elongation of the eyeball
- • Correction: concave (diverging) lens of suitable power
- • Rays seem to come from the far point F → image on the retina
- • Hypermetropia (far-sightedness): far clear, near blurred
- • Near point farther than 25 cm
- • Image of a near object: behind the retina
- • Causes: (i) focal length of the eye lens too long
- • (ii) eyeball too small
- • Correction: convex (converging) lens of suitable power
- • Rays from N′ seem to come from N → image on the retina
- • | Point | Myopia | Hypermetropia |
- • | Sees clearly | near objects | distant objects |
- • | Image falls | in front of the retina | behind the retina |
- • | Causes | lens too curved; eyeball too long | focal length too long; eyeball too small |
- • | Corrected by | concave lens | convex lens |
- • Presbyopia: with age, ciliary muscles weaken, lens less flexible
- • Power of accommodation decreases; near point recedes
- • Bifocal lens: upper part concave (distant), lower part convex (near)
- • P = 1/f (f in metres); unit: dioptre (D)
- • Example: P = −0.5 D. Lens? Defect? f = ?
- • P negative ⇒ concave lens ⇒ myopia
- • f = 1/P = 1/(−0.5) m
- • 1/(−0.5) = −2
- • f = −2 m (concave lens)
4. Refraction of light through a glass prism
The figure, a triangular glass prism ABC. It has two triangular bases and three rectangular faces.
- • Prism: 2 triangular bases, 3 rectangular faces
- • Angle of the prism: ∠A, between the refracting faces AB and AC
- • PE: incident ray; ∠i: angle of incidence
- • Air → glass: bends towards the normal
- • EF: refracted ray; ∠r: angle of refraction
- • Glass → air: bends away from the normal
- • FS: emergent ray; ∠e: angle of emergence
- • ∠D: angle of deviation, between PE produced and FS produced back
- • Glass slab: emergent ray ∥ incident ray (lateral shift only)
- • Prism: faces inclined ⇒ emergent ray deviated by ∠D
- • Path reversed ⇒ ray retraces it ⇒ ∠D unchanged
5. Dispersion of white light, Newton's prisms and the rainbow
The figure.
- • White light → prism → band of seven colours
- • VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red
- • Spectrum: the band of the coloured components of a light beam
- • Dispersion: the splitting of light into its component colours
- • Cause: each colour bends through a different angle
- • Red bends the least, violet the most
- • Refractive index of glass: least for red, most for violet
- • Newton: second identical prism, inverted
- • Colours recombine → white light emerges
- • Conclusion: sunlight is made of seven colours
- • Rainbow: a natural spectrum after a rain shower
- • Droplet: refraction + dispersion → internal reflection → refraction
- • Always seen opposite to the Sun (Sun behind the observer)
6. Atmospheric refraction: twinkling stars
The hot air above it is lighter, less dense, and bends light differently from the cooler air around it.
- • Atmospheric refraction: refraction of light by the earth's atmosphere
- • Starlight: rarer air → denser air, bends towards the normal
- • Apparent position of a star: slightly higher than the real one
- • Atmosphere keeps changing → path of starlight keeps changing
- • Star = point source → light entering the eye flickers → twinkling
- • Planet = extended source → variations average to zero → no twinkling
- • Sun seen ≈ 2 minutes before sunrise and ≈ 2 minutes after sunset
7. Scattering of light: Tyndall effect, blue sky, red signals
The figure.
- • Scattering: fine particles spread light in all directions
- • Tyndall effect: the path of a beam made visible by scattering from fine (colloidal) particles
- • Very fine particles → mainly blue scattered
- • Large particles → scattered light may look white
- • Air molecules: smaller than the wavelength of visible light
- • Blue scattered more strongly than red (λ of red ≈ 1.8 × λ of blue)
- • So the clear sky looks blue
- • No atmosphere → no scattering → dark sky
- • Very high altitude: hardly any air → sky looks dark
- • Danger signals are red: red is least scattered by fog or smoke
Summary
Done! We began with the human eye, a camera with the cornea, iris, pupil, eye lens and retina. Then we saw accommodation, the near point at 25 cm and the far point at ∞. After that came myopia, corrected by a concave lens, hypermetropia, corrected by a convex lens, and presbyopia, which needs a bifocal lens. Then the prism. It deviates light, and it splits white light into VIBGYOR, because each colour bends through a different angle. Newton joined the colours again with a second prism, and raindrops make a rainbow. Atmospheric refraction makes the stars twinkle, and scattering makes the sky blue and keeps the danger signals red. Draw the two defect diagrams with their correction once in your notebook without looking, with arrows on every ray, because the board asks them every year. Today's homework is on your screen. If anything was not clear, press Ask and ask me. Very good work today!
AskMedha · Medha Ma’am — CBSE Class 10 Science, Chapter 10. Written from the textbook, checked twice.
Questions parents and students ask
- How do I get the "The Human Eye and the Colourful World" notes as a PDF?
- Press "Save as PDF" — the phone’s print sheet opens; choose "Save as PDF" there. The PDF holds the notes only.
- Where are these notes written from?
- From the chapter’s own textbook text, part by part, in the book’s own terms; a second, independent model then checked every part against the book. The full lesson, a check question for each part and the practice are in the AskMedha app.
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