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• Human eye :

• Power of Accommodation : Ability to adjust focal length.

• Cataract :
• Lens of eye becomes milky and cloudy.
• May cause loss of vision
• Cured with Cataract surgery.
• Defects of vision / Refractive defects :

• 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.