📝 Chapter Notes & Revision

The Human Eye and Colourful World

🏫 MP BoardClass 10Science

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Quick Revision Notes & Formula Sheet

Class 10 Science - The Human Eye and Colourful World

MP Board

1. The Human Eye (मानव नेत्र)

The human eye is a natural optical instrument that enables us to see the wonderful colourful world around us. It acts like a camera.

  • Cornea: The transparent spherical membrane covering the front of the eye. It refracts most of the light entering the eye.
  • Iris: A dark muscular diaphragm behind the cornea that controls the size of the pupil.
  • Pupil: The aperture (opening) in the iris through which light enters the eye. It regulates the amount of light entering.
  • Crystalline Lens: A convex lens made of transparent jelly-like protein that focuses the image on the retina.
  • Ciliary Muscles: Muscles that hold the eye lens in place and change its focal length (accommodation).
  • Retina: The delicate membrane behind the eye lens acting like a screen where the image is formed. It contains light-sensitive cells called rods (respond to intensity of light) and cones (respond to colours).
  • Optic Nerve: Transmits electrical signals from the retina to the brain.
  • Power of Accommodation: The ability of the ciliary muscles to adjust the focal length of the eye lens so as to focus objects at varying distances clearly.
    • Near point of eye (Least distance of distinct vision): $25 \text{ cm}$ for a normal human eye.
    • Far point of eye: Infinity ($\infty$) for a normal human eye.

2. Defects of Vision and Their Correction (दृष्टि दोष तथा उनका संशोधन)

There are three common refractive defects of vision:

A. Myopia (Near-sightedness / निकट-दृष्टि दोष)

  • Meaning: A person with myopia can see nearby objects clearly, but cannot see distant objects distinctly.
  • Cause:
    1. Excessive curvature of the eye lens.
    2. Elongation of the eyeball.
  • Correction: Corrected by using a Concave lens (अवतल लेंस) of suitable focal length, which diverges the light rays before they strike the eye lens.

B. Hypermetropia (Far-sightedness / दूर-दृष्टि दोष)

  • Meaning: A person with hypermetropia can see distant objects clearly, but cannot see nearby objects distinctly.
  • Cause:
    1. Focal length of the eye lens is too long.
    2. Eyeball has become too small.
  • Correction: Corrected by using a Convex lens (उत्तल लेंस) of suitable focal length, which converges the light rays before they enter the eye.

C. Presbyopia (जरा-दूरदर्शिता)

  • Meaning: The power of accommodation of the eye decreases with aging. Near point gradually recedes.
  • Cause: Weakening of ciliary muscles and diminishing flexibility of the eye lens.
  • Correction: Corrected by using Bifocal lenses (upper part consists of concave lens for distant vision, lower part consists of convex lens for reading).

3. Refraction of Light Through a Prism (प्रिज्म से प्रकाश का अपवर्तन)

  • Prism: A transparent optical element with flat polished surfaces that refract light.
  • Angle of Prism (A): The angle between two refracting surfaces of a prism.
  • Angle of Deviation ($\delta$): The angle between the incident ray and the emergent ray.
  • Formula relating Refraction through Prism: $$\angle A + \angle D = \angle i + \angle e$$ Where:
    • $\angle A$ = Angle of the Prism
    • $\angle D$ = Angle of Deviation
    • $\angle i$ = Angle of Incidence
    • $\angle e$ = Angle of Emergence

4. Dispersion of White Light by a Glass Prism (श्वेत प्रकाश का वर्ण-विक्षेपण)

  • Dispersion: The splitting of a beam of white light into its component colours when passing through a refracting medium like a glass prism.
  • Spectrum: The band of coloured components of a light beam is called its spectrum.
  • Order of Colours (VIBGYOR): Violet, Indigo, Blue, Green, Yellow, Orange, Red.
  • Key Fact:
    • Red light bends the least (has maximum wavelength, minimum frequency).
    • Violet light bends the most (has minimum wavelength, maximum frequency).
    • Sir Isaac Newton first used a glass prism to obtain the spectrum of sunlight and showed that sunlight is made of seven colours.

5. Atmospheric Refraction (वायुमंडलीय अपवर्तन)

The refraction of light by the earth's atmosphere is called atmospheric refraction.

  • Apparent Position of Stars: Due to atmospheric refraction of starlight, the stars appear slightly higher than their actual position.
  • Twinkling of Stars: Starlight undergoes continuous refraction as it passes through varying refractive indices of the atmosphere, causing the apparent position and amount of light reaching the eye to fluctuate.
  • Advanced Sunrise and Delayed Sunset: The sun is visible about $2$ minutes before actual sunrise and $2$ minutes after actual sunset due to atmospheric refraction.

6. Scattering of Light (प्रकाश का प्रकीर्णन)

The phenomenon in which light is deflected in all directions by minute particles (like dust, smoke, water droplets, and gas molecules) in the atmosphere.

  • Tyndall Effect: The phenomenon of scattering of light by colloidal particles is called the Tyndall effect. (e.g., beam of sunlight entering a dusty room or canopy of a dense forest).
  • Why is the sky blue? Molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
  • Why are danger signals red? Red colour has the maximum wavelength in the visible spectrum. Therefore, it is least scattered by fog or smoke and can be seen from a long distance.
  • Colour of the Sun at Sunrise/Sunset: Light from the sun travels a longer distance through the atmosphere at sunrise and sunset. Most of the blue light and shorter wavelengths are scattered away by the particles, leaving the longer red wavelengths to reach our eyes, making the sun look red/orange.