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We have started our discussion on transmission photoelasticity in the last class, and I mention
light is used as a sensor in photos elasticity. Then we moved on to find out what is a nature
of light. We recall that light is nothing but an electromagnetic disturbance. So you
have an electric vector and magnetic vector, which are mutually perpendicular and in phase
are to be looked at for the purpose of simplified mathematical treatment. We would take the
electric vector as the basis to represent the light vector.
And what we saw the natural light was a not convenient to be used as a sensor then we
said that we need polarization, and we also looked at because we use polarization optics
we call the equipment used for photoelasticity as the polariscope. The name indicates that
you are using polarization; you have a plan polariscope, you have a circular polariscope
and for you to understand what happens in photo elasticity, you need to build up certain
back ground. The first concept is, what is polarization;
the second concept is, what is birefringence - birefringence is natural to crystals. And
what we do in photo elasticity is certain polymeric substances become birefringence
when they are trust - that is the advantage. When the loads are remove the phenomenon of
birefringence no long their exits. And in order to understand, how do you get the stress
field, you need to understand what is birefringence. So you understand birefringence we look at
crystal optics. And another important aspect is when you have
fringe patterns, you have phenomena interference, one of the common examples given to understand
interference in the first in level course is, you go to a pond take two stones and then
drop it. What happens? You have spherical waves form and they interfere. When there
is a crust and crust meets you have a constructive interference and when you have the destructive
interference where like to such conditions, the wave is.
And what happens in photoelasticity’s slightly different; photoelasticity you have relative
retardation which causes the formation of fringes and in the pond when you put two bubbles
both the waves are in the same plane. And in photoelasticity, you have one wave, the
other wave are mutually perpendicular; there is the phase difference between these two.
And where do these two waves come from - that is what we have to understand. And a difference
from other optimal technique is vibration requirements are not that stringent in photoelasticity
- that is the greatest advantage. And why it is so? All that we will understand when
we look at what happens in a crystal? And if you look at the polarization, birefringence,
and also formation of light ellipse, they are interrelated concepts.
You know you have to go back and forth and then try to understand and then get on to
solid mechanics aspects. And then look at how to relate the optical phenomena to stress
information.
And first look at what is polarization. So I have a natural light which is so random
the magnitude as well as the direction keeps on changing. And a moment you put polaroid
sheet you have plane polarized light coming out after the polaroid sheet. And this is
only a statement, and what I have attempted to show in this course is for any statement
I need, I try to provide you some kind of an experimental justification. To large extended,
most of statement, I will try to provide you experimental justification that gives you
certain level of confidence. And we also saying polarization can be affected
by polaroid sheets, prism polarizer, reflection or scattering. And we said that prism polarizer,
the polarization quality is very high, but the field of use very small. On the other
hand polaroid sheets you almost get 99.9 percent of polarization - the greatest advantage is
the field of you can be as large as even one meter you can do. And for certain wave lengths,
you know reflection scattering or ideal and you also know at an appropriate angle called
Brewster angle you have polarization takes place. This also another method of getting
polarized beam of light, and simplest form of polarization is plane polarized light.
And once you have plane polarized light, you could get elliptical and circular polarization.
And what we will now look at is, we will have a justification, yes the light is polarized;
and that is what we saw in this slide last class, we have understanding polarization.
And the moment you send polarized beam of light, the amount of light available on the
screen is diminished, and I show the show the polarizer simply by a line sketch and
this indicates direction of polarization. So what I have here I said that by putting
this sheet only the vertical component is transmitted - that is what only a statement.
Now I have to understand myself this is indeed show. So how do I get it? I put another element
which also allows light only in one direction when I keep it exactly perpendicular, the
light should be cut off. And what I am doing is, the second element functions as analyzing
the light that is coming out, that is impinging on it. Both physically or same the second
one helps in analyzing the light coming on it, so you call that is an analyzer.
So what I do here is, I put an analyzer, and I keep rotating it. So when I keep rotating
it, I find that light gets diminish and when it becomes perpendicular you have light is
totally strict. And another circle concept was also introduced while discussing this,
I had an natural light - this is the white light source. And we also looked at monochrome
light source. What is the difference between white light
source and the monochrome light source? In a white light source, you have play of colors
you have VIBGYOR. The may be with different proportion depending on the light source,
so it is a multi-wavelength source. And I said for all numerical development, a single
wave length is simple and convenient. Right now people also have develop methodologies
to find out even in a multi wave length as light source, how to interpreted data, but
for this initial development we will look at single wave length.
And what you saw here was for the multi wave length also when the polarizer and analyzer
are cut or crossed the light is extinguished. When I have a mono chrome source also the
light is completely extinguish, and you see block. And this is the very important aspect
that you have to keep in mind, because when we go and put the model in between when I
gone put the model in between the polarizer and analyzer, we will also look at qualitatively
how a fringe gets form. We will not get in to mathematics, first we will look at purely
based on qualitatively arguments. What could be the light extension condition and we will
find out a logically develop you would see contours of this nature, you would see contours
of second nature and so on and so forth. So currently what we are having is, we are
having only a polarizer and analyzer. And we saw that the entire field could be dark
or the field could be bright.
Next we move on to what happens in an isotropic media, because when you want to go in to crystal
it is better that a re-capitulate, what happens in an isotropic media first. And this all
of you have seen in your school physics curriculum, but we will look at the same thing with our
interest in mind. We will look at refractive index but we will look at refractive index
with the different perspective, and we also look at from the point of view of polarization
what happens. So when I take an isotropic media, I have
an incident ray and I have a medium one shown by one color; medium two shown by another
color. So the light gets refracted also gets reflected, and what you have to note here
is the representation of un polarized light - that is nothing but natural light is shown
with dot and a straight line. Suppose you have only a dots, it is understood
as linear polarization in the horizontal direction, and you show only vertical lines it is linear
polarization in the vertical direction. And what you see here, I send an un polarized
beam of light and this comes out as un polarized beam of light, it also gets reflected as un
polarized beam of light. Suppose you adjust the angles of incidents, you may get the reflected
ray at a particular angle for glass it is about 56 degrees, you will get only a polarized
beam of lights - so that is called a Brewster angle that we are not getting in to. Our interested
is to see in an isotropic medium, when I send a natural light you get natural light getting
transported with in the medium - this is one observation. The other observation is, we
will define refractive index in terms of velocities appropriations - that is very crucial from
photo elasticity point of view looking for that perspective is crucial and very famously
that these are all Snell’s laws of refraction and reflection.
We will just look at those statements. These statements you know already, you are only
going to have a relook at that. So what you saw, the normal to the incident wave, the
normal to the interface, and the normal to the reflected and refracted waves all lie
in one plane. It is the first observation, this you all have understood from your course
in physics. And the second observation is, the angle of
incidence is equal to the angle of reflection. So angle I equal to capital R - that is symbol
that we have used. Then what we have the ratio of the sine of the angle of incidence to the
sine of the angle of refraction is a constant and it depends on the given isotropic media.
So you have only a relative refractive index, you should not go to absolutely refractive
index one medium should be so we will see both. And you all know sine I by sine R is
the refractive index - that is also known from your earlier course.
So what you have you have a concept of relative refractive index and also have a concept of
absolute refractive index. And you know sin i by sin r and if the medium is air you will
call this as a relative refractive index, and you have the symbol small n is used and
you should not confuse this with your direction cosines. That is also we use n 1, n 2, n 3,
and here it denotes the refractive index. And what we look at specially here is, I see
this sin i by sin r as a ratio of velocity v 1 and v 2. So what does this say? When you
look at this has ratio velocity what do infer from this, v all low the velocity of the light
in vacuum is c So when you travels in different medium there
is a slight change in the velocity, and what we will look at later we are going to have
some kind of a phase difference which is being initiated into the wave, because of the stress
information. And you need to have some phase difference developed, and what you find here
is when I have these two velocities are v 1 and v 2; when the refractive index is different
the velocities are different in the media. So I have a relative refractive index which
is given as v 1 by v 2, and absolute refractive index from the medium one is given as c by
v 1, and for the medium two is given as c by v 2. And what I have here is for one incident
ray, I have one refracted ray, and this totally changes when I go to crystals. And you know
in photoelasticity, you also call this as common path interferometers so they are less
stringent in vibration isolation requirement - that will be understood when you look at
crystal optics.
And what you have here what happens when light passes through a crystalline media. The first
observation is, the crystalline media are optically anisotropic. So how do you define
isotropic, if the property say along all directions then you call that as isotropic; if the property
is the function of direction then it is anisotropic. So the first observation is the crystalline
media are optically anisotropic, because it is optically anisotropic you also have another
interesting thing happening a single incident ray will give rise to two refracted rays.
And there also named in the conventional sense you know, you have one ray which is called
ordinary ray labeled as o, another ray is called extraordinary and this happens because
of double refractions. So when you say why the label it is an extraordinary?
Even in our common life, you know somebody’s are extraordinary, you should have special
qualities and sometimes you also say somebody does not follow rules he is an extraordinary
person. So what you find here is the ordinary ray faithfully follows Snell’s law, and
a suitable condition extraordinary ray violates Snell’s law that is why we read Snell’s
law first. So one ray same as what you seen in an isotropic
medium, in addition you have another ray which violate this law, and this violation is useful
to us. This is useful to us in photoelasticity and we exploited that is the advantage. So
you have for a single incident ray two refracted rays ordinary and extraordinary. First we
will have a look at the ray diagram and then as I mention I will provide you an experimental
justifications for all these concepts.
We will just look at the ray diagram and what happens here. So I have medium one and medium
two now is crystal; it is not a isotropic media - medium two is a crystal. And you look
at the draw the diagram make a neat sketch of it take your time, and I can replay how
the rays come, so I have a ray which impinges on this. Carefully looks at how the incident
ray is draw and how the refracted rays are drawn.
First observation is for one incident ray I have two refracted rays - I have one as
labeled as ordinary ray another is labeled as extraordinary ray. And if you look at,
how do these rays are depicted. Can you tell me the difference between the incident ray
and the refracted rays? The incident rays is un-polarized but within the medium because
the medium does not end here, medium ends only here within the medium I find the rays
are polarized - that is the represented by only straight lines in this and only dots
in this. And the planes of polarization are mutually perpendicular and this is very pertinent
and important observation that is you should keep in mind.
So I have two simple harmonic motions which are mutually perpendicular. The planes of
polarization of mutually perpendicular, and in this case you see them as two different
rays; in photo elasticity we will develop it and see that they will travel in the same
direction and they will have planes of polarization mutually perpendicular. And when I have this
angle change I have r 2 and r 1 what do you infer, you said refractive index different
- that is not what is sufficient for v. When I go to photoelasticity what I had looked
at? I look at refractive index as different velocities so that gives you the information.
Say when you look at holography what happens light impinges on the model, if you are looking
at metrology application because of the three dimensional shape the depths are different,
so you have a light which impinges and comes back so there is a phase difference. So you
need to have some form of phase difference and that is not what happen in photoelasticity;
photoelasticity it penetrates the model and it gets modify within the model, it acquires
the phase difference all that you can understand. So what you find here is the model behaves
like the crystals the moment you have a crystal a generic understanding is for one incident
ray, I have two refracted beams which are travelling at different velocities. Our interest
is not just r 1 and r 2 are different, we do not want to look at it that way; we want
that is why we introduce definition of refractive index more in terms of velocities. So you
have for one incident ray there are two refracted beams first observation is they move with
different velocities. Their plane of vibration is mutually perpendicular, but in this case
they travel in difference direction. They travels in difference direction that is not
going to be convenient photo elasticity point of view, but this is used.
And we summaries this points; the first point is, an isotropic medium can transmit common
light which is nothing but a natural light so un polarized beam will go as un polarized
beam, while the travelling through a crystal is always polarized and this is very key and
important point. So within a crystal, whatever the light that
travel as through it is polarized. A crystal cannot sustain and polarized light. And what
happens in photo elasticity, the model behaves like a crystal when it is stressed, - so that
is the reason why we are able to look at modification the light as a function of stress, because
a natural crystal will always have birefringence. If you look, we are also go to look at wave
planes there are natural crystals we will have two vibration and we will use it in an
advantages way. So a crystal can transmit light within the crystal it is always polarized,
so first and foremost observation that we make.
And we have seen very clearly the ordinary and extraordinary rays are plane polarized,
and their planes of polarization are perpendicular to each other. And this is again only a statement,
you have only taking the statement because I am a teacher and you are listing to it.
If you are a true student, you will have to question is it really, show. I have do an
experiment you provide an opportunity to the experiment and let me get convince myself
yes there are two beam seen and they are also polarized, and the planes of polarization
is mutually perpendicular do not you think that knowledge you should get, because you
learning science, you are not learning other pseudoscience you hear we need proof for every
statement we make - that is what we look at.
We look at the next slide. I again come back to the later beam so I have a image of the
word beam written on a paper and you view it normally. Then what I do? I go on put a
crystal on top of it; this is in normal light you view it, the moment I put a crystal I
clearly see two sets of revered beam appearing. And we have seen within the crystal you have
an ordinary and extraordinary rays, you have a two rays are being seen, and you see two
images. I also made one more statement that these ordinary and extraordinary rays are
polarized. And you are already looked at what is polarization,
if there is a light beam is polarized I can always analyzed by a polarizer keep on rotating
it when the light is cut off I will say that the original light was polarized. So I can
do that analysis by taking a polaroid sheet, and here we also seen that the ordinary and
an extraordinary rays are polarized in perpendicular direction. So that means what I want a suppose
I take a polaroid sheet place it on this image and rotate it appropriately what should I
anticipate, at particular orientation one of the images should not be seen, only one
image should be seen. And another orientation, I should see the other image, because if you
understood what is polarization, we also understand the polarization can be analyzed and you take
a polaroid sheet and then move over it, I should be able to get that - we will just
perform that. So what I will do is I am going to a put polaroid
sheet on top of it and the process is intermediate process is not shown, the final process is
shown here. So I have a analyzer and I oriented in this direction what do I see? I see only
one of the images what I saw as two in the earlier case. So this goes to prove two things
- you find these rays are polarized - that is why I am able to cut off one image; the
other image vibration is perpendicular to this so that images cut off, because it will
allow only one component of light. So what you see here is the first case you
see when you put a crystal I see two images. So this indicates for a single incident ray
you have two refracted beams and I have shown by putting a polarizer because it analyses
the light it named as an analyzer. And when I place it appropriately, one of the images
is eliminated, this shows that the extinguished image is plane polarized perpendicular to
the analyzer. You can interpreted in any way you can look at this image and then discuss
it.
And suppose I keep this analyzer perpendicular to this, what I should see? I should see the
other image. Let us put rotate it, and then keep it; let us see what happens. This is
exactly show, so I could see here I have two images seen and you could also see there is
a slight shift - the shift is same as like what you see here. This is the image on the
top and this is the image of the bottom. So I could filter out from this one of the
images by employing an analyzer, so this shows that the beams are polarized, and because
I get these two images when I keep them at mutually perpendicular positions of the analyzer.
We also have establish that these two beam beams are polarized in mutually perpendicular
direction. So that is what is summaries here.
The ordinary and extraordinary rays are plane polarized. How do you established the plane
polarized, I just use an analyzer which only cut off light perpendicular to it. So that
shows when I extinguish one image the plane of polarization is perpendicular to it, so
we indirectly show that it is the plane polarized beam of light, and because I keep these two
perpendicular to each other. I also understand the planes of polarization of ordinary and
extraordinary rays are mutually perpendicular. You know this helps in understanding photo
elasticity but in photo elasticity we do not want to see two images we want to see it in
a particularly different fashions we will also bring in the concept of optic axis, we
will look at the incident ray in relation to the optic axis. And we will find out one
of this combination is advantages for photo elasticity - that is what we will learn today.
And what you have here is, I said that when you have natural crystals; it has two refractive
beams ordinary and extraordinary ray. And you also have tables available which give
you the ordinary ray refractive index and the extraordinary ray refractive index. And
this is return for a large list you write it for an ice; you write it for calcite - these
two are sufficient. So that gives you an indication that you have
two refracted beams which are different refractive indices. And what is important in photoelasticity,
we look at this different refractive indices more from the point of view of velocities.
We are not interested this is 1.6 and that is 1.4; we are only interested that these
two waves will travels with different velocities - that is the key point; that is why we define
we looked at the definition of refractive index not only a sine i by sine r but also
as ratio of velocities. So a crystal you have this naturally happening and the crystal behavior
is introduced because of the stresses, we are in position to relay stresses to optical
behavior that is see crux of photo elasticity.
And now what we will had look at is what is the direction of incident of light, which
is suitable for photoelastic analysis. And when you take a crystal you have what is known
as optical axis. I have a crystal here and I have a ray of light impinging on it and
how is this rays shown this is an unpolarized beam; you have a dot and straight line. And
this direction is parallel to optic axis what do you define, what is this diagram show.
I have the ray travel in as such like in an isotropic medium. In an isotropic medium,
you will have the unpolarized beam transmitted as unpolarized beam. There is no specialty
about crystal behavior here. So what you find here is when the incident rays are parallel
to the optic axis the ordinary and extraordinary rays have the same refractive index.
Now we look at another case. We will have this axis impinging at a different direction.
For convenient, I shown but that I have cut the crystal in a manner that optic axis is
cut an arbitrary position like this. And what I have here, I have an incident ray and I
have two refracted beams. And the animation is also very carefully shown, what does this
shown, one ray travels faster than the other. And you find this is polarized; this is also
polarized, and the planes of polarization is mutually perpendicular. This is nice to
illustrate that I see two images and you see two images without any hallucinations. You
are not on the power of intoxicants to see two images then also you see two images. And
you are alert in a morning class you are very bright and you see two images, you see two
images because of a physical phenomenon not because of a hallucinations.
So what I find here is, when the incident rays at some angle from the optic axis, the
extraordinary ray will deviate from the ordinary ray, because of the different indices with
direction. Because we are already seen crystal is optically anisotropic and that is dictated
by the incident ray in relation to the optic axis direction. If it is same as optic axis
direction then it behaves like an optical isotropic medium; if it is at an angle, you
see two refracted beams which travel with different velocities and the planes are vibration
is mutually perpendicular. Even this is not used in photoelasticity;
this is not use in photoelasticity neither this is useful in photo elasticity. This is
only to understand that within a crystal you have double refraction you see two images
and so on and so forth. And what you will have to look at in photoelasticity is, the
incident ray is perpendicular to the optic axis, then what happens, when it is perpendicular
to the optic axis, both the ordinary and extraordinary ray travel in the same direction, but within
the crystal what will they do, they will acquire the phase retardation, because they have different
refractive indices. And we have all emphasis that we look at in photo elasticity different
refractive indices more from the point of view of velocity of propagation and here you
have the magic. So within the crystal, the two waves acquires
the phase retardation. In fact if you understand this aspect photoelasticity. This is all you
require from photoelasticity’s where many thing you can understand from this slide.
And what is summarized here, when the incident rays are perpendicular to the optic axis,
the extraordinary ray will travel faster than the ordinary ray because of its lower refractive
index, but it will travel in the same direction, will travel in the same direction. This is
very, very important and you have already seen from a solid mechanics point of view
photoelasticity provides difference in principle stresses, and I also mention in one of the
earlier classes that refractive index is a tensor of rank two stresses are also a tensor
of rank two, so whatever for happens refractive index I can match it on stress tensor. And
here what I am showing for a single incident ray, in general you have two refracted beams,
but where in it is perpendicular to the optic axis they travel the same direction and acquire
the phase retardation.
So this is what happens in photo elasticity. The phase retardation is not because of depth
change, it is because of the way the material behaves like a crystal. Crystal behavior is
perceived as n 1 and n 2 at the point of interest. And this n 1 and n 2 could be related to what,
sigma 1 and sigma 2. If I have a crystal at every point n 1 and n 2 is same, if you have
this as a stressed model n 1 and n 2 also change as from point to point; n 1, n 2 does
not remain same at every point. In a crystal plane n 1, n 2 remain same; in a stressed
model n 1 and n 2 is dictated by the stresses apply; this is the observation number 1.
Other observation is, I also said photo elasticity gives you direction of principles stress where
does this comes, this comes from the optics axis and it is perpendicular to it and we
are looking at only planer problems, we are only looking at planer problems.
So now you understand, how optics is related to stresses in a very simplified fashion.
Here again I will say you can say sorry, I shown me that two refracted beams but one
of shown me when I rotate the crystals I have only one beam; this I can show by an demonstration.
In photoelasticity one beams travels faster another beams trails behind it - that can
I show only in photoelasticity. If a perform a separate experiment where I rotate the crystal
when a certain beam of a light by rotating the crystal what I should see, in for different
orientation I should see different things. At particular orientation, you will see only
one beam coming out at different orientation you have to beams coming now that is what
you have to anticipate. I have also said experimental is anticipate
the result before performing an experiment. In many cases you are anticipation may match
with observation, in some cases anticipation does not match with our observation and that
is where the scope for the search. Why does not match what have we missed and then you
learn and then go deeper in to the subject. So this is what I have said in this course
the powerful multimedia presentation. I would bring in experiments that we have conducted
in the lab which is recorded. Most of these experiments are done in digital photo mechanics
lab IIT Madras; some of these experiments are specially commission for this book. Many
of my students are participated my thanks to all of them and you will see that experiment.
Because what I want to emphasis here is you learn the theory you do not take it because
a teachers says, you also observe. You observe re-convince yourself this is indeed so; it
is not fiction.
I laser is experiment. So what I have here is I have this crystal is moving. Crystal
it getting rotated, I have laser beam and I will replay this. What you have to see this
is I have a right spot here and what you had looks for is when a rotate the crystal you
will see two dots and merge into one dot. See when the light is coming out of the crystal
then they will interfere suppose I have a the light ray which is parallel to the optic
axis or perpendicular to the optic axis, both will appears one dot only. Only when I go
to photo elasticity, I can show fringes. In fact if you recall the last class I show two
plates when a applied load I saw beautiful patterns, why do I see beautiful patterns,
instead of black background I saw beautiful patterns we will reason it out. So that shows
that light travels with different velocities. I cannot show that in this animation; in this
animation you have to look for one dot and two dots that will convince the optic axis
has in influence on the behavior of the crystal. The incident light in relation to the optic
axis behaves differently. So I replay it and I also have a magnified
picture, so you could see two dots and then that merges into one, and what I will do is
I will have to go and once this is observes you can repeat it again and then see. You
see that the crystal is rotated; I have one dot becoming two dots. Have you all seen it?
Have you been able to observe it? This that is a key point.
And once you understand this there is joy in doing photoelasticity because you know
the in and outs of what happens within the model. Now we have get into mathematics, see
what you have here is in normal interferometer both waves travels in the same direction.
So if you go and look at books you might have done when you are learn simple harmonics motion,
wave addition, you may have done in on off your earlier mathematics courses without knowing
where this should be applied, see does it is this how many course of structure you will
do a course in mathematics separately then you do a course in engineering very rarely
you find teacher bridges these two and show you what you learned there is what you apply
it here. Because while you learn a course in mathematics you would of found out if I
have waves travels in the same direction; if they there is a phase difference how do
you add them you can add them. Suppose, I have waves travelling mutually
perpendicular and they acquire phase difference, how do you add them you have to add it very
carefully. You cannot do the same addition law like what you have done it when the waves
are travelling in the same plane, when they are mutually perpendicular the mathematics
is slightly different. And we have been same that we have a elliptical polarization, elliptical
polarization can be thought of us circular polarization in one limiting case and another
limiting case are plane polarization. And where does all this comes from, this all
comes from your addition of two simple harmonic vibrations which are mutually perpendicular
- that is what we need in photo elasticity and I have convincingly shown you have indeed
two rays which move with planes of polarization perpendicular to each other. And we are already
seen that this can be return as cos omega t or sine omega t, so you have two simple
harmonic motions which are mutually perpendicular, and I also shown that they acquire a phase
difference all that you know. Now you can do the mathematics with little more physical
understanding, why this is needed and how it is exploited - that is what we will have
a brief look at it; we will continue in the next class but this is what we have look at
it.
When the incident light is perpendicular to the optic axis, the light emerging out of
a crystal plate has two mutually perpendicular plane polarized lights of different phases.
This is what you have to understand, they have different phases is very important. And
you represent the light waves like this. So I have two beams of light - one is given in
the x direction - it call it as a x cos omega t plus alpha 1; and you called the other wave
as E y equal to a y cos omega t plus alpha 2. So this as one amplitude, this as another
amplitude; amplitudes are not same. We are taking very generic situation, we keep the
amplitudes different and we also have some absolute phase whenever you a generic representation
you will also have an absolute phase, alpha 1 can be 0 in a particular case; and you represent
the second wave as omega t plus alpha 2. But what is important in photoelasticity?
We are not interested in absolute phase, so we will only look for difference in phase
because with in the model you have difference in phase is acquired between the two waves
because of this optical behavior and this optical behavior is induced by stresses.
So, it is a fundamentally different from other interferometer techniques. Here you have two
waves which are mutually perpendicular acquire phase retardation within the model, and this
phase retardation is cause by the stresses introduced. So this you can understand it
shuttle and because one ray becomes two whatever happens to the one ray will happen to both
the rays, so vibration oscillation is not a stringent requirement.
So that is how it comes, it is beautiful it is very nice marriage between physics and
stress analysis. If I look at photoelasticity that is why people enjoyed and also to enthuse
you god was so kind he gave you nicely colored fringe patterns. That is you saw only in photoelasticity
you call that as isochromatic beautifully; iso means constant, chromo means color why
you see contours of constant color and this I will see in a white light.
So in this class what we focused was for photo elasticity, you need to understand there are
two beams which travel within the model and they acquire phase retardation, and these
two beams are plane polarized and the planes of polarization are mutually perpendicular,
and this comes from an understanding of crystal optics. Because a crystal by its very nature
behaves like this, by its very nature behaves like this whereas in photoelasticity the model
temporarily behaves like a crystal when loads are apply. When loads are remove, the crystal
behavior is different. In this will see we will develop it in the future classes and
you will also appreciate because if you have this understanding you can go and develop
newer philosophy in data acquisition - that is our research develops.
There are two aspects one is how to use a techniques; how to use the technique may be
having few steps. A technician requires only that that much information and engineer should
know what goes behind the technique and that is what you learn this course. So with this
background, I anticipate that you come out with innovative methods and it helps you to
develop your concepts better and may be come and develop a new experimental technique.
Thank you.