.
SNELLíS LAW
 
    According to Snellís law
"The ratio of the sine of the angle of incidence to the sine of the angle of refraction is always constant. "
    Mathematically,
    Sine <i/sine <r = constant
or
sin< i/sine< r =
m
    where m = Refractive index of the material of medium.
TOTAL INTERNAL REFLECTION
    When light rays enter from one medium to the other, they are refracted. If we increase the angle of     incidence, angle of refraction will also increase. At certain angle of incidence light rays are reflected     back to the first medium instead of refraction. This condition or phenomenon is called Total Internal     Reflection.
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CRITICAL ANGLE
 
    The angle of incidence at which the angle of refraction will become 90o is called Critical Angle. If angle     of incidence further increased then instead of refraction, reflection will take place.
DEFECTS OF VISION
Write down the defects of the vision.
    There are four common defects of vision:
    1.
SHORT SIGHTEDNESS OR MYOPIA
    2.
LONG SIGHTEDNESS OR HYPER METROPIA
    3. ASTIGMATISM
    4. PRESBYOPIA
SHORT SIGHTEDNESS
OR MYOPIA
SYMPTOMS
 
    In Myopia, a person can not see distant objects clearly, but he can see clearly the objects near to him.
REASON
 
    The reason for Myopia is either the focal length of lens of eye is too short or the eyeball is very much     elongated.
WHAT HAPPENS IN MYOPIA
 
    In Myopia, light rays from a distant object are focused in front of the Retina.
CORRECTION OF DEFECT
 
    This defect can be corrected by using a concave lens of suitable focal length
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