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Refraction:-When a pencil of light passes obliquely from one
transparent medium into another, it undergoes some
deflection from its straight path. This change in direction
is called refraction
Law of Refraction:-
1. The incident ray the refracted ray and the normal to the
surface separation at the point of incidence, all lie in the
same plane
2. Irrespective of the obliquity of the incident ray, the
ration of the sine of the angle of incidence to the sine of
the angle of refraction is a constant quality of any two
given media. This is called Snell?s law of refraction
Total internal reflection:-Whenever a ray of light traveling
from a denser into a rare medium is incident at an angle
greater than the critical angle for the two media, the ray
is totally reflected back into the same medium
Mirage:-In this optical illusion inverted images of distant
objects are seen, as if reflects from a pool or as if
suspended from atmosphere. In sandy districts objects are
seen reflected from a pond while in colder regions they are
seen suspended from atmosphere. In sandy deserts the payers
of air near surface of earth get heated so soon that density
adjustments do not take place. Hence densities and
refractive indices of the layers just above sand are lower
than the layers higher up. The rays of light form a distant
object after passing through layers which are gradually less
refracting, bend more and more still they fall on layer at
an angle greater than the critical angle for it and get
totally reflected. These rays then travel and undergo a
series of refractions but in the opposite direction for now
they pass through layers which are gradually more and more
refraction, till they reach the eye of the observer, who
sees an image of the object as though reflected from surface
of the water.
Twinking of Stars:-A ray coming from a star at night has to
pass through layers of air having different refractive
indices. It is therefore bent different till the observer
sees it in a particular position. The densities and hence
the refractive indices of the payers go on changing due ti
wind. Due t this the star appears to be at different place
at the very next moment. The difference of place being very
small, therefore the star appears to be moving in a small
area and hence it twinkles
Planets do not appear to twinkle as due to their being
nearer the amount of light received from them id greater and
so the variation in their brightness is not appreciable
Lena:-It is a portion of a transparent refraction medium
bounded by the surfaces one of which is spherical and the
other is either spherical or plant
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