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The Human Eye and the Colourful World – Class 10 Science Chapter 10 Notes & NCERT PDF

Class 10ScienceChapter 10NCERT book: Science - Class 10

Notes and a simple summary of Chapter 10 of the NCERT Class 10 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 10: The Human Eye and the Colourful World

Introduction

The human eye has a lens in its structure and works like a camera: its lens system forms an image on a light-sensitive screen, the retina. In this chapter we use what we learnt about lenses to understand how the eye works, how defects of vision are corrected with spectacles, and why we see rainbows, twinkling stars and a blue sky.

Key Concepts

1. Structure of the Human Eye

  • Cornea – thin transparent membrane forming the bulge at the front. Most of the refraction of light entering the eye happens at its outer surface.
  • Iris – dark muscular diaphragm behind the cornea that controls the size of the pupil.
  • Pupil – regulates and controls the amount of light entering the eye.
  • Crystalline (eye) lens – provides only the finer adjustment of focal length to focus objects at different distances.
  • Ciliary muscles – change the curvature (and hence focal length) of the eye lens.
  • Retina – delicate membrane with an enormous number of light-sensitive cells; they generate electrical signals when illuminated.
  • Optic nerve – carries these signals to the brain, which interprets them.

    • The eyeball is roughly spherical, about 2.3 cm in diameter. The eye lens forms a real, inverted image on the retina.

    2. Power of Accommodation

    The ability of the eye lens to adjust its focal length is called accommodation.
  • Looking at distant objects: ciliary muscles relax → lens becomes thin → focal length increases.
  • Looking at near objects: ciliary muscles contract → lens becomes thicker (more curved) → focal length decreases.
  • Near point (least distance of distinct vision) – for a young adult with normal vision, about 25 cm.
  • Far point – infinity for a normal eye. So a normal eye sees clearly from 25 cm to infinity.
  • Cataract – the eye lens becomes milky and cloudy in old age, causing partial or complete loss of vision; vision can be restored by cataract surgery.

    • 3. Defects of Vision and Their Correction

    (a) Myopia (near-sightedness)

  • Can see near objects clearly but not distant ones; far point is nearer than infinity.
  • Image of a distant object forms in front of the retina.
  • Causes: (i) excessive curvature of the eye lens, (ii) elongation of the eyeball.
  • Correction: concave lens of suitable power.

    • (b) Hypermetropia (far-sightedness)

  • Can see distant objects clearly but not nearby ones; near point is farther than 25 cm.
  • Light from a close object focuses behind the retina.
  • Causes: (i) focal length of the eye lens too long, (ii) eyeball too small.
  • Correction: convex lens of appropriate power — it adds the extra converging power needed.

    • (c) Presbyopia

  • Power of accommodation decreases with age; the near point recedes.
  • Cause: gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
  • A person with both myopia and hypermetropia may need bi-focal lenses — the upper part concave (distant vision), the lower part convex (near vision).

    • Refractive defects can also be corrected with contact lenses or surgery.

    4. Eye Donation (box in the chapter)

  • Donors can be of any age or sex; people who wear spectacles or had cataract surgery can donate.
  • Eyes must be removed within 4–6 hours after death; removal takes only 10–15 minutes and causes no disfigurement.
  • One pair of eyes can give vision to up to four corneal-blind people.
  • People who had AIDS, Hepatitis B or C, rabies, acute leukaemia, tetanus, cholera, meningitis or encephalitis cannot donate.

    • 5. Refraction Through a Glass Prism

    A triangular prism has two triangular bases and three rectangular faces; the angle between its two refracting faces is the angle of the prism (A). In Activity 10.1, the ray bends towards the normal entering the prism and away from it leaving the prism. Because the faces are not parallel, the emergent ray is bent at an angle to the incident ray — the angle of deviation (D). (Terms: angle of incidence i, angle of refraction r, angle of emergence e.)

    6. Dispersion of White Light

  • A prism splits white light into a band of colours — the spectrum: VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
  • Dispersion is the splitting of light into its component colours. It happens because different colours bend through different angles: red bends the least, violet the most.
  • Newton first obtained the spectrum of sunlight with a prism. A second identical prism placed inverted recombined the colours into white light, showing that sunlight is made of seven colours.
  • Rainbow – a natural spectrum formed after rain, always opposite to the Sun. Water droplets act like tiny prisms: they refract and disperse sunlight, reflect it internally, and refract it again as it leaves the drop. You can also see one through a waterfall or fountain with the Sun behind you.

    • 7. Atmospheric Refraction

  • Objects seen through hot air above a fire appear to waver: hot air is less dense and has a slightly lower refractive index, and conditions keep changing.
  • Twinkling of stars: starlight is refracted continuously in the atmosphere (refractive index changes gradually). The star's apparent position is slightly higher than its real position near the horizon, and it keeps fluctuating. Stars are so far away that they act as point sources, so the amount of light entering the eye flickers.
  • Planets do not twinkle: they are closer and act as extended sources — a collection of many point sources whose variations average out to zero.
  • Advance sunrise and delayed sunset: the Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset. The apparent flattening of the Sun's disc at sunrise and sunset is due to the same effect.

    • 8. Scattering of Light

  • Tyndall effect: the path of a light beam becomes visible when it passes through colloidal particles (smoke in a room, mist in a dense forest) because the particles scatter light.
  • The colour of scattered light depends on particle size: very fine particles scatter mainly blue; larger particles scatter longer wavelengths; large enough particles make the scattered light look white.
  • Blue sky: air molecules and fine particles are smaller than the wavelength of visible light and scatter shorter (blue) wavelengths more strongly than red. Red light has a wavelength about 1.8 times that of blue light.
  • No atmosphere → no scattering → dark sky. The sky looks dark to passengers at very high altitudes and to astronauts.
  • Danger signals are red because red is least scattered by fog or smoke, so it is seen in the same colour from far away.

    • Worked Numericals

    1. Distant and near vision (Exercise 5). Lens powers −5.5 D and +1.5 D.

    f = 1/P: distant vision f = 1/(−5.5) m ≈ −18.2 cm (concave); near vision f = 1/(+1.5) m ≈ +66.7 cm (convex).

    2. Myopia (Exercise 6). Far point = 80 cm. The concave lens must form the image of a distant object at the far point, so f = −80 cm = −0.80 m. P = 1/f = −1.25 D (concave).

    3. Hypermetropia (Exercise 7). Near point = 1 m; normal near point = 25 cm. Object at u = −25 cm must appear at v = −100 cm.

    1/f = 1/v − 1/u = −1/100 + 1/25 = 3/100 → f ≈ +33.3 cm; P = 100/33.3 = +3.0 D (convex).

    4. Myopic eye, far point 1.2 m (Question 2). A concave lens with f = −1.2 m, P ≈ −0.83 D.

    Formula Sheet

  • P = 1/f (f in metres) — negative for concave (myopia), positive for convex (hypermetropia)
  • Lens formula (used for correction problems): 1/v − 1/u = 1/f
  • Myopia: f of corrective lens = −(far point)
  • Normal eye: near point ≈ 25 cm, far point = infinity

    • Board Exam Focus

  • Labelled diagram of the eye and functions of each part.
  • Ray diagrams: myopic eye and its correction; hypermetropic eye and its correction.
  • 'Give reasons': stars twinkle but planets don't; sky is blue; sky looks dark to an astronaut; danger signals are red; Sun visible 2 minutes before sunrise.
  • Prism: define angle of deviation; explain dispersion and recombination (Newton).

    • Key Terms

  • Retina (दृष्टिपटल/रेटिना), Cornea (कॉर्निया/स्वच्छ मंडल)
  • Iris (परितारिका), Pupil (पुतली)
  • Ciliary muscles (पक्ष्माभी पेशियाँ)
  • Accommodation (समंजन क्षमता)
  • Near point (निकट बिंदु), Far point (दूर बिंदु)
  • Myopia (निकट-दृष्टि दोष), Hypermetropia (दीर्घ-दृष्टि दोष), Presbyopia (जरा-दूरदृष्टिता)
  • Cataract (मोतियाबिंद)
  • Dispersion (वर्ण-विक्षेपण), Spectrum (स्पेक्ट्रम)
  • Atmospheric refraction (वायुमंडलीय अपवर्तन), Scattering (प्रकीर्णन)

    • Common Mistakes

  • Saying the eye lens does most of the bending — most refraction happens at the cornea.
  • Mixing up the corrections: myopia → concave, hypermetropia → convex.
  • Writing that ciliary muscles contract for distant objects — they relax for distant objects.
  • Saying red bends the most in a prism — violet bends the most, red the least.
  • Explaining the blue sky by 'reflection from the ocean' — it is scattering by air molecules.
  • Forgetting that planets don't twinkle because they are extended sources, not because they are bright.

    • 💡 Key Learning Points

    • ✓Name the parts of the human eye (cornea, iris, pupil, lens, ciliary muscles, retina, optic nerve) and state the function of each.
    • ✓Explain accommodation, the near point (about 25 cm) and far point (infinity) of a normal eye.
    • ✓Describe myopia, hypermetropia and presbyopia — their causes and correction with concave, convex and bi-focal lenses — and calculate the required power.
    • ✓Explain refraction through a prism, dispersion into VIBGYOR, recombination, and rainbow formation.
    • ✓Use atmospheric refraction and scattering to explain twinkling stars, non-twinkling planets, early sunrise, the blue sky, the dark sky at high altitude and red danger signals.

    👨‍🏫 Teaching Tips

    • →Compare the eye to a camera part by part; use a simple eye model or labelled chart and have students trace the path of light to the brain.
    • →Demonstrate accommodation: ask students to look at a finger near the face, then at a distant wall, and feel the difference in effort.
    • →Draw the three myopia diagrams (normal eye, defective eye, corrected eye) side by side; then repeat for hypermetropia so students contrast them.
    • →Do Activity 10.2 with sunlight and a prism (or a phone torch) and let students record VIBGYOR; add a second inverted prism to show recombination.
    • →Create a rainbow in the playground with a water spray and the Sun behind the students.
    • →Run a 'give reasons' quiz round on twinkling, blue sky and red signals — these are frequent board questions.
    • →Discuss the eye-donation box sensitively and invite students to share awareness posters.
    • →Work the hypermetropia numerical (Exercise 7) step by step with the lens formula before letting students try Exercise 6 alone.

    📋 Assessment Questions

    1. Can the student label a diagram of the eye and explain the roles of the cornea, iris, pupil, ciliary muscles and retina?
    2. Can the student distinguish myopia from hypermetropia (cause, where the image forms, corrective lens) with ray diagrams?
    3. Can the student calculate the power of the corrective lens for a given far point or near point?
    4. Can the student explain dispersion and why red bends least and violet most through a prism?
    5. Can the student give scientific reasons for twinkling of stars, blue sky and advance sunrise?

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    Keep learning

    All chapters in Science - Class 10

    1. 1. Chemical Reactions and Equations
    2. 2. Acids, Bases and Salts
    3. 3. Metals and Non-metals
    4. 4. Carbon and its Compounds
    5. 5. Life Processes
    6. 6. Control and Coordination
    7. 7. How do Organisms Reproduce?
    8. 8. Heredity
    9. 9. Light – Reflection and Refraction
    10. 10. The Human Eye and the Colourful World
    11. 11. Electricity
    12. 12. Magnetic Effects of Electric Current
    13. 13. Our Environment