The Human Eye and Colourful World

• The Human Eye

  • Structure & Parts
    • Cornea: Transparent front part, refracts light
    • Iris: Dark muscular diaphragm, controls pupil size
    • Pupil: Regulates amount of entering light
    • Crystalline Lens: Focal adjustment for clear vision
    • Ciliary Muscles: Modify lens curvature (accommodation)
    • Retina: Light-sensitive screen, contains rods and cones
    • Optic Nerve: Transmits electrical signals to the brain
  • Power of Accommodation
    • Ability of the eye lens to adjust its focal length
    • Near Point (Least distance of distinct vision): 25 cm for normal eye
    • Far Point: Infinity for normal eye
  • Defects of Vision & Correction
    • Myopia (Near-sightedness)
      • Can see nearby objects clearly, distant objects blurred
      • Cause: Elongation of eyeball or excessive curvature of lens
      • Correction: Concave lens
    • Hypermetropia (Farsightedness)
      • Can see distant objects clearly, nearby objects blurred
      • Cause: Focal length too long or eyeball too small
      • Correction: Convex lens
    • Presbiopia
      • Age-related loss of accommodation
      • Correction: Bifocal lens
    • Cataract
      • Milky and cloudy eye lens
      • Correction: Surgical removal

• Refraction of Light through a Prism

  • Glass Prism
    • Two triangular bases and three rectangular lateral surfaces
    • Angle of Prism ($\angle A$)
    • Angle of Deviation ($\angle D$)
  • Dispersion of White Light
    • Splitting of white light into its 7 constituent colours (VIBGYOR)
    • Caused by different speeds of different colours in glass
    • Spectrum: Band of coloured components
  • Recombination of White Light
    • Newton's experiment using two identical prisms in inverted positions

• Atmospheric Refraction

  • Apparent Position of Stars
    • Starlight bends towards the normal due to gradient refractive index
    • Stars appear slightly higher than actual position
  • Twinkling of Stars
    • Due to fluctuating atmospheric refraction and changing refractive index
  • Advanced Sunrise and Delayed Sunset
    • Sun appears about 2 minutes earlier and 2 minutes later
    • Caused by atmospheric refraction of sunlight

• Scattering of Light

  • Tyndall Effect
    • Phenomenon of scattering of light by colloidal particles
    • Visible path of light beam (e.g., sunlight through canopy in a dense forest)
  • Why is the Colour of the Clear Sky Blue?
    • Molecules of air scatter shorter wavelengths (blue) more effectively than longer wavelengths (red)
  • Colour of the Sun at Sunrise and Sunset
    • Light travels longer distance through atmosphere at horizon
    • Most blue light is scattered away, leaving red/orange light to reach our eyes