Class 10 Ch 10 The Human Eye

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The Human Eye – 5-Page Study Notes
Ch-10: The Human Eye 👁️
1. Introduction (परिचय)
  • The Human Eye is like a camera. It forms images on a light-sensitive screen called the Retina.
  • मानव नेत्र एक कैमरे की भाँति है। इसका लेंस निकाय एक प्रकाश-सुग्राही परदे, जिसे रेटिना कहते हैं, पर प्रतिबिंब बनाता है।
2. Main Parts and Functions
  • Cornea (स्वच्छ मंडल): Transparent layer covering the front. Most refraction occurs here.
  • Iris (परितारिका): Dark muscular diaphragm that controls the size of the pupil.
  • Pupil (पुतली): Regulates amount of light entering the eye.
  • Crystalline Lens: Fibrous jelly-like material. Adjusts focal length.
  • Ciliary Muscles: Adjust the curvature of the eye lens to see distant/near objects.
  • Retina (दृष्टिपटल): Contains rod & cone cells; sends signals via Optic Nerve.
Retina Cornea Ciliary Muscles
3. Power of Accommodation (समंजन क्षमता)
  • Ability of eye lens to adjust its focal length.
  • Near Point: Minimum distance of distinct vision = 25 cm.
  • Far Point: Farthest point the eye can see clearly = Infinity ($\infty$).
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4. Defects of Vision (दृष्टि दोष)

जब नेत्र अपनी समंजन क्षमता खो देता है, तो दृष्टि दोष उत्पन्न होते हैं।

I. Myopia (निकट-दृष्टि दोष) – Near-sightedness
• Person can see nearby objects but NOT distant ones.
• Image forms In Front of the retina.
• Causes: Excessive curvature of lens OR elongation of eyeball.
• Correction: Use a Concave Lens of suitable power.

Correction of Myopia

Retina
II. Hypermetropia (दूर-दृष्टि दोष) – Far-sightedness
• Person can see distant objects but NOT nearby ones.
• Image forms Behind the retina.
• Causes: Focal length too long OR eyeball too small.
• Correction: Use a Convex Lens.
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5. Presbyopia (जरा-दूरदृष्टिता)
  • Aging causes ciliary muscles to weaken & lens loses flexibility.
  • Difficult to see nearby objects clearly. Often needs Bi-focal lenses (Upper part Concave, Lower part Convex).
6. Refraction through a Prism (प्रिज्म)
  • Angle of Prism (A): Angle between two lateral faces.
  • Angle of Deviation (D): Peculiar shape of prism makes the emergent ray bend at an angle to the incident ray.

Refraction through Glass Prism

A D
7. Dispersion of White Light (विक्षेपण)
  • Splitting of white light into its component colours.
  • VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
  • Newton’s Experiment: Combined 7 colours back into white light using an inverted prism.
  • Rainbow: Internal reflection & dispersion by water droplets. Red at top, Violet at bottom.
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8. Atmospheric Refraction (वायुमंडलीय अपवर्तन)

वायुमंडल की परतों का घनत्व अलग-अलग होने के कारण प्रकाश मुड़ जाता है।

  • I. Twinkling of Stars: The starlight, on entering the earth’s atmosphere, undergoes refraction continuously. The apparent position of the star changes slightly, leading to twinkling.
  • II. Planets do NOT twinkle: They are much closer and act as extended sources; variations nullify each other.
  • III. Advanced Sunrise & Delayed Sunset: Sun is visible 2 minutes early and 2 minutes late due to atmospheric refraction.

Apparent Position of Star

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9. Scattering of Light (प्रकाश का प्रकीर्णन)
  • Tyndall Effect: Scattering of a beam of light by colloidal particles. Visible path of light (e.g., in a dusty room).
  • Why is the sky Blue? Fine particles in atmosphere scatter Shorter wavelengths (Blue) more than longer ones (Red).
  • If there was NO atmosphere, the sky would look Black (no scattering).
  • Red color of Sun (Sunrise/Sunset): Light travels longest distance. Blue is scattered away, leaving only Red/Orange light to reach our eyes.
  • Danger Signals: Always Red because red is scattered the least by fog or smoke.
10. Quick Summary
Myopia $\to$ Concave Lens (-)
Hypermetropia $\to$ Convex Lens (+)
Dispersion $\to$ VIBGYOR (Red least deviated)
Scattering $\to$ Blue Sky / Red Sun
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Rainbow 🌈
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