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