← Back 🏠 Home

Science: Class 10 — Chapter 10: The Human Eye and the Colourful World

10.1 The Human Eye

10.1.1 Structure and Function of the Eye
  • Human Eye: Works like a camera, focusing light onto a light-sensitive screen called the retina to form an inverted, real image.
  • Cornea: A thin, transparent membrane forming a bulge at the front of the eye through which light enters.
  • Iris: A dark muscular diaphragm that controls the size of the pupil, regulating the amount of entering light.
  • Crystalline Lens: Adjusts its focal length to focus objects clearly onto the retina.

10.1.2 Power of Accommodation
  • Accommodation: The ability of the eye lens to adjust its focal length using the ciliary muscles.
  • Near Point: The minimum distance to see objects distinctly without strain; about 25 cm for a normal eye.
  • Far Point: The farthest point up to which the eye can see clearly; infinity for a normal eye.
  • Cataract: A condition where the crystalline lens becomes milky and cloudy in old age, treatable by surgery.
10.2 Defects of Vision and Their Correction

10.2.1 Myopia (Near-sightedness)
  • Description: Nearby objects are clear, but distant objects are blurry because the image forms in front of the retina.
  • Correction: Using a concave lens of suitable power to shift the image back to the retina.

10.2.2 Hypermetropia (Far-sightedness)
  • Description: Distant objects are clear, but nearby objects are blurry because the image forms behind the retina.
  • Correction: Using a convex lens of suitable power to provide additional focusing power.

10.2.3 Presbyopia
  • Description: Loss of accommodation power due to aging, caused by the gradual weakening of ciliary muscles and rigidity of the eye lens.
  • Correction: Corrected using bifocal lenses containing both concave (upper part) and convex (lower part) lenses.
10.3 Refraction of Light Through a Prism

10.3.1 Prismatic Refraction
  • Angle of the Prism: The angle between the two lateral faces of a triangular glass prism.
  • Angle of Deviation: The peculiar angle between the direction of the incident ray and the emergent ray.
10.4 Dispersion of White Light by a Glass Prism

10.4.1 Splitting of Light
  • Dispersion: The splitting of white light into its component colors (VIBGYOR) upon passing through a prism.
  • Spectrum: The band of colored components of light; red light bends the least, while violet light bends the most.
  • Rainbow: A natural spectrum caused by the dispersion, internal reflection, and refraction of sunlight by tiny atmospheric water droplets.
10.5 Atmospheric Refraction

10.5.1 Phenomena of Atmospheric Refraction
  • Twinkling of Stars: Caused by continuous atmospheric refraction of starlight traveling through air layers of changing refractive indices.
  • Planetary Twinkling: Planets do not twinkle because they are closer to the earth and act as extended light sources, causing fluctuations to average out to zero.
  • Advance Sunrise and Delayed Sunset: Atmospheric refraction makes the Sun visible 2 minutes before actual sunrise and 2 minutes after actual sunset.
10.6 Scattering of Light

10.6.1 Tyndall Effect
  • Tyndall Effect: The scattering of a light beam by colloidal particles, rendering the path of the beam visible.

10.6.2 Blue Sky and Sunset Reddening
  • Blue Sky: Atmospheric molecules and fine particles scatter short wavelengths (blue light) more strongly than longer red light.
  • Danger Signals: Made red because red light is least scattered by fog or smoke, remaining visible over long distances.