Presented by:
KARTHIK I.

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POLARISCOPE
POLARISCOPE
 A polariscope is an optical inspection device used to detect internal stresses in glass and other transparent materials such as plastics, synthetic resins, etc.
 The Polariscope is an essential tool for determining strain patterns developed during fabrication and manufacturing. It permits immediate determination of strains in most transparent materials. It is valuable in production or as a laboratory tool wherever glass is fabricated, welded or bent.
 As polarized light travels through strained glass or plastic, it undergoes a retardation proportional to the amount of stress. A Polariscope is an instrument which can be used to qualitatively view this retardation.
 When a photo elastic material is subjected to a load and illuminated with polarized light from the measurement instrumentation (called a reflection polariscope), patterns of color appear which are directly proportional to the stresses and strains within the material. The sequence of colors observed as stress increases is: black (zero stress), yellow, red, blue-green, yellow, red, blue-green, yellow, red, etc. The transition lines seen between the red and green bands are known as "fringes." The stresses in the material increase proportionally as the number of fringes increases. Closely spaced fringes means a steeper stress gradient, and uniform color represents a uniformly stressed area. Hence, the overall stress distribution can easily be studied by observing the numerical order and spacing of the fringes.
LIGHT
Light can be thought of as a wave, that vibrates back and forth as it moves. Individual light waves each have their own wavelength, as well as direction of vibration.
Imagine a light wave vibrating back and forth, at some angle, as the light wave moves across the room.
Angles of vibration can range through a full 360°
 Ordinary visible light is a mixture of many kinds of waves. The light that enters your eye is composed of waves of a multitude of different wavelengths.
Each of these different waves vibrates in many different directions as the light travels.
The beam is made up of waves vibrating in all directions, as shown by the white arrows in this diagram.
 We perceive light waves that vibrate horizontally as 'glare'. As light hits a particular portion of the truck's surface and returns to our eye, the waves get polarized. This means that a higher than normal number of them are vibrating horizontally. When this light enters our eye, the large number of horizontally vibrating waves seem to overpower the other waves, and everything near that part of the truck is hidden by the 'glare' caused by the polarized waves.
 One way is to pass ordinary light, containing waves that are vibrating in all directions, through very thin openings that are aligned in the direction we want the waves to be polarized.
In the picture below, you can see that the some of the waves approaching the barrier are vibrating at oblique angles, and can't get through the vertical slits.
 The result will be that the light which comes out the other side will be vibrating mostly up and down; waves vibrating at all the other angles will have been stopped by the barrier. What comes out the other side is called polarized light.
 Only light vibrating up and down can easily get through the scratches. Light vibrating in other directions has a hard time getting through. In particular, horizontally vibrating waves will be stopped completely. Since they are the cause of glare, polarizing sunglasses will stop the glare waves, and the truck will look like this
POLARISCOPE
The experimental setup varies from experiment to experiment. The two basic kinds of setup used are plane polariscope and circular polariscope.
Plane polariscope
The setup consists of two linear polarizers and a light source. The light source can either emit monochromatic light or white light depending upon the experiment. First the light is passed through the first polarizer which converts the light into plane polarized light. The apparatus is set up in such a way that this plane polarized light then passes through the stressed specimen. This light then follows, at each point of the specimen, the direction of principal stress at that point. The light is then made to pass through the analyzer and we finally get the fringe pattern.
The fringe pattern in a plane polariscope setup consists of both the isochromatics and the isoclinics. The isoclinics change with the orientation of the polariscope while there is no change in the isochromatics.
Circular polariscope
In a circular polariscope setup two quarter-wave plates are added to the experimental setup of the plane polariscope. The first quarter-wave plate is placed in between the polariser and the specimen and the second quarter-wave plate is placed between the specimen and the analyser. The effect of adding the quarter-wave plates is that we get circularly polarised light.
The basic advantage of a circular polariscope over a plane polariscope is that in a circular polariscope setup we only get the isochromatics and not the isoclinics. This eliminates the problem of differentiating between the isoclinics and the isochromatics.
CONSTRUCTIONDETAILS OF POLARISCOPES
Polariscopes may be classified under two broad types
 The diffused-light polariscope
 The lens polariscope
There are also some special types of polariscopes designed to suit the requirements of special methods of experimental analysis. One such special design reflection polariscope which are used to analyse stress by photoelastic coatings.