Human Eye and Colourful world SSLC 10 Science Notes

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  • Human eye :
Part of eye Identification Function  
Eyeball Round part of eye, diameter 2.3 cm Organ allows us to see objects, housing for various parts eye
Cornea Transparent , thin membranous, bulged structure in front of eye   Refracts light to fall on eye lens  
Crystalline lens Eye lens made of tissues. Refract light rays, provide finer adjustments of focal length, forms clear, real and virtual image on retina.
Pupil Black part of an eye Regulates and controls the amount of light entering in eye
Iris Dark muscular diaphragm Controls size of pupil on basis of amount of light entering the pupil
Retina Light sensitive screen of  eye Contains light sensitive cells which generate electrical impulses
Optic nerves Nerve fibres in eye Sends signals from light sensitive cell to brain
  • Power of Accommodation : Ability to adjust focal length.
Vision Action of Ciliary muscle Curvature of eye Size of lens Focal length Distance of clear vision
Distant object Relax Decrease Thin Increase Upto infinity (far point of eye)
Closer object Contracts Increase Thick Decrease Minimum 25 cm (Near point of eye)
  • Cataract :
    • Lens of eye becomes milky and cloudy.
    • May cause loss of vision
    • Cured with Cataract surgery.
  • Defects of vision / Refractive defects :
Myopia ( Nearsightedness) Hypermetropia (Farsightedness) Presbyopia
  1. Difficulty to see far object.
  1. Image is formed in front of retina.
  2. Cause due to :   Increase in curvature of eye lens or elongation of eyeball.
  1. Correction : Use of concave lens.
  1. Difficulty to see near objects.
  1. Image is formed behind the retina .
  2. Cause due to: Decrease in curvature of eye lens or shortening of eyeball.
  1. Correction : Use of convex lens.
  1. Difficulty to see far and near objects.
  2. Decrease in power of accommodation
  3. Causes due to : Weakening of ciliary muscles, decrease in flexibility of eye lens
  4. Correction : Use of  bifocal lens (Convex and Concave lens)
  • Refraction through a glass prism :
    • Refraction : Change in direction of light from one medium to another.
    • Prism : Piece of glass in which two triangular bases and three rectangular lateral surfaces are inclined to each other.
    • Angle of deviation : Angle made by emergent ray with incident ray.
    • Dispersion of light through glass prism : Splitting of light in different color components while passing through glass prism is called dispersion. It occurs as different component bent with different angle.
    • Determination of sunlight component : [Refer the video to understand in detail] When sunlight passes through a vertically upward prism it splits into seven different colors, Violet, Indigo, Blue, Green, Yellow, Orange, Red. This band of colors is visible spectrum. If this spectrum is allowed to fall on other prism which is placed vertically downward, again white light is obtained. Hence, it was concluded that, sunlight is made up of seven colors.
    • Rainbow: [Refer the video to understand in detail] Rainbow is visible due to phenomenon of refraction and reflection and dispersion. Incident light when fall on a tiny droplet first get refracted inside the droplet.  Inside the droplet it strikes the opposite surface and get reflected again. During this light components undergo different bends i.e. dispersion occurs. Cumulative effect of all rain drops forms rainbow.
  • Atmospheric Refraction : [Refer the video to understand in detail]
    • Wavering / Flickering : Fluctuation of objects is observed through hot air / steam. Cold air is denser than hot air.  Hence, refractive index of cold air is more than hot air. When light rays from the object passes from hot air to cold air it undergoes refraction. Since, air is not stationary, fluctuations are observed.
    • Twinkling of star : Star are present at long distance hence are considered to be point source. Light rays from stars passes through different layers of earth, which have different refractive indexes. Starlight entering eye seems to be flickering, hence we observe twinkling of stars.
    • Stars twinkle while planets don’t? : Planets are closer to earth, hence are not point source. Large number of light rays from plants, enters earth’s atmosphere, but these are not observable, because they nullify each other.
    • Advanced sunrise and delayed sunset : Due to earth’s refraction sun is visible two minutes before the actual sunrise and two minutes later the actual sunset. Sun appears to be flattened during sunrise and sunset.
  • Scattering of light : Light travelling through any medium undergo random variations.
    • Tyndall effect : Earth’s atmosphere consist of various minute particles (Colloidal particles : smoke, dust, etc). When light ray strikes with these particles, path of lights becomes visible. This is tyndall effect.
    • Scattering in sky : Scattering of light depends on size of particles. Atmosphere contains very small molecules that scatters sunlight. Size of particle is smaller than the wavelength hence, only blue light scatters. Therefore, sky appears to be blue.
    • Color of sun at sunrise and sunset : At sunrise and sunset, sun is at larger distance,  hence , light rays travels through thick layer of atmosphere. Hence, only red light is visible. So sun appears red in color. In afternoon, sun is at shorter distance hence, it appears white.

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