Embed Size (px)
Citation preview
Polarizing lenses
By praveen
Polarized light
Light reflected from any smooth non diffusing Surface is said to be polarized or plane polarized .Eg :- snow,water Glare is commonly caused by reflection from
water ,snow ,high waves and metallic surfaces . A special group of filter in addition to absorbing
unwanted radiation also absorbs plane polarized light the polarized light usually having been produced by reflection .
Polarized light is different in character from ordinary light or unpolarized light may be represented by wave motion which is vibrating in every direction .
Polarized light vibration in one direction only .When unpolarized light is reflected by certain
substances or reflected from a non metallic surface ,it becomes plane polarized its vibration being restricted to one meridian only .
Principle
Only if the light rays which are forming a Brewster angle will be total polarized light .
Polarizing filter eliminate horizontal reflected radiation .
In case of reflected light the degree of polarization depends on the incidence and reaches a maximum .
When the angle b/w the reflected and the refracted beam is 900
This critical angle of incidence is known as Brewster’s angle .
A polarizing filter will only transmit plane polarized light if the plane of vibration of light is parallel to the polarizing axis of the filter .
If the plane of vibration is at right angle to the polarizing axis the polarized light is absorbed .
The reflected light is totally plane polarized the plane of vibration being horizontal such that parallel to the reflecting surface .
If the polarizing filter is placed in the path of the reflected light with its polarizing angle at right angle to the reflecting surface such that vertical it absorbs the reflected light .
Manufacturing of polarizing filters
The original polarizing filters were produced by sandwiching microscopic crystals of sulphate of quinine b/w two thin sheets of cellulose acetate .
The axis of crystal being made to coincide by placing the sheet in an electrical field .
The modern filter are however made by stretching a thin sheet of PVA polyvinyl alcohol so that the molecular structure of the sheet is straightened into a chain which are lying parallel to the direction of stretch .
The sheet is then dipped into iodine which is absorbed by the sheet transforming it immediately into a polarizing filter .
The thin polarizing sheet is finally sandwiched b/w two sheets of cellulose acetate and is pressed b/w two ground glass .
Alternatively the thin polarizing filter may be sandwiched between two glass lenses and any desired power can be incorporated into the filter .
Advantanges
Decreased driving fatigue specially during day time .
Fisherman can see more with polarizing lenses because it not only removes reflected or discomfort glare but enables the wearer to see below the surface .
Anglers derive great benefit from this filters because they can be see in water clearly without the distraction of surface distortion .
Comfort at the beach is increased because sand and water are both source of polarized glare marking these lenses especially appropriate .
Color is not bleached. Bright snowy rays are not blinding when viewed
with polarizing lenses . Polarizing lenses are good alternative to
sunglasses because they inhibit U.V radiation .
Application
All lenses such as bifocal ,single vision ,progressives can be made polarized .
Any material of lenses and can be made polarized .
Testing Polaroid /demonstration of polarizing lenses
In polarizing lenses ,the color will appear a normal as against sunglasses.
Can be demonstrated by showing a glossy magazine or books .
Polarizing charts are provided by companies which contain a word which can’t be seen by naked eye and can be seen with polarizing lenses.
It is possible to take two Plano polarizing lenses and by holding one before the other with their polarizing axis crossed at 900 eliminate all incoming visible light rays .
