Applied Physics I: Chapter 8: Applied Optics - Interference
Application of Air Wedge - Testing of Flat Surfaces
Principle, Theory, Explanation
If two surfaces are perfectly flat, then the air film gradually varies in thickness.
APPLICATION OF AIRWEDGE
‒ TESTING OF FLAT SURFACES
Principle
If two surfaces are
perfectly flat, then the air film gradually varies in thickness.
The fringes will be of equal thickness because each fringe is the locus of the
points at which the thickness of the film has constant value. If the fringes
are not equal in thickness, it means the surfaces are not flat.
Theory
We
know the condition for the formation of bright fringes due to the reflected
light is
2μt
cos r = (2n+1) λ/2
When
r = 0
2μt
= (2n+1) λ/2
From
the above equation, a fringe will be characterised by a particular value of 'n' and if the thickness of the air film
or flatness of the surface is constant, we get straight line fringes. When
white light is incident on the film, then the colour of the fringes will
indicate the varying thickness of the air film.
Explanation
Let
us consider two surfaces OA and OB. The surface OB which is to be tested is
placed in contact with optically flat glass plate (i.e) surface OA, forming an
airwedge as shown in Fig 8.6.
The
Fringes are observed in the field of view. The fringe pattern depends upon the
flatness of the surfaces OA and OB.
i)
If the fringes are of equal thickness as shown in Fig. 8.6, then the surface OB
is said to be flat.
ii)
If the fringes observed are of unequal thickness, it implies that the surface
OB is not flat. For this we have two cases. viz.
a)
If the fringes are curved towards the contact edge, the surface is concave as
shown in Fig. 8.7.
b)
If the fringes are curved away from the contact edge, the surface is convex as
shown in Fig. 8.8.
Thus,
to make the surface OB to be flat, the surface OB has to be polished and the
same nom process can be repeated until the fringe pattern is obtained as
straight lines.
NOTE: When
white light is incident on the film, then the colour of the fringes will
indicate the varying thickness of the air film.
Applied Physics I: Chapter 8: Applied Optics - Interference : Tag: Applied Physics : Principle, Theory, Explanation - Application of Air Wedge - Testing of Flat Surfaces
Applied Physics I: Chapter 8: Applied Optics - Interference
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