Interference diffraction and polarization

1. DEPARTMENT OF
BASIC SCIENCES &
HUMANITIES
SHARAD INSTITUTE OF TECHNOLOGY
COLLEGE OF ENGINEERING, YADRAV
Interference, Diffraction and
Polarization
1

2. Interference of light
Interference is a phenomenon in which two waves superpose to form a
resultant wave of greater, lower, or the same amplitude.

4. a1
a2
A
a1
a2
A

7. Interference of light in thin parallel film
N’

13. Newton’s rings
When Plano convex lens is placed with its convex surface on a plane
glass plate and a beam of monochromatic light is made incident
normally on the lens, then a set of concentric circular, bright and
darks rings are formed. Such a rings are called as Newton’s rings.
Plane glass plate
Plano convex lens
Incident monochromatic light

14. Experimental setup of Newton’s rings

15. Formation of Newton’s rings

16. Applications of Newton’s rings for determination of
wavelength of monochromatic light
rn rn
S
P
H
E
C
O
R
t
R-t
G
Planoconvex lens

18. rn rn
S
P
H
E
C
O
R
t
R-t
G
Planoconvex lens

21. Polarization of light
Polarization is the process in which the vibrations of materials
particles are restricted to one plane

22. Plane of vibration
Plane of polarization

23. Unpolarized light Plane polarized light

29. Methods for production of polarized light
Polarization
1)
Polarization
by reflection
2)
Polarization
by refraction
3)
Polarization
by double
refraction

30. 1) Polarization by reflection
ɸ
ɸ

31. 2) Polarization by refraction

32. 2) Polarization by double refraction

33. Huygen's theory of Double refraction

35. 1) Parallel to the surface

36. 2) Perpendicular to the surface

37. 3) Inclined to the surface

38. LAURENT’S HALF SHADE POLARIMETER
Optical activity

39. Specific rotation

40. LAURENT’S HALF SHADE POLARIMETER

41. Diffraction

42. Diffraction grating construction theory
An arrangement consisting of a large number of equidistant parallel narrow slits
of equal width separated by equal opaque portions is known as a diffraction grating.
The modern commercial form of grating contains about 15000 lines per inch. The
rulings act as obstacles having a definite width ‘b’ and the transparent space
between the rulings act as slit of width ‘a’. The combined width of a ruling and a slit
is called grating element (d). Points on successive slits separated by a distance
equal to the grating element are called corresponding points.

43. Theory
MN represents the section of a plane transmission grating. AB, CD, EF … are the
successive slits of equal width a and BC, DE … be the rulings of equal width.
Let d = a + b.
Let a plane wave front of monochromatic light of wave length λ be incident normally
on the grating. According to Huygen’s principle, the points in the slit AB, CD … etc act
as a source of secondary wavelets which spread in all directions on the other side of the
grating.
Let us consider the secondary diffracted wavelets, which makes an angle θ with the
normal to the grating.
The path difference between the wavelets from one pair of corresponding points A and
C is CG = (a + b) sin θ. It will be seen that the path difference between waves from any
pair of corresponding points is also (a + b) sin θ
The point P1 will be bright, when
(a + b) sin θ = m λ where m = 0, 1, 2, 3
In the undiffracted position θ = 0 and hence sin θ = 0.

44. a + b) sin θ = 0, satisfies the condition for brightness for m = 0. Hence
the incident rays will produce maximum intensity at the centre O of the
screen. This is called zero order maximum or central maximum.
If (a + b) sin θ1 = λ, the diffracted wavelets inclined at an angle θ1 to
the incident direction, reinforce and the first order maximum is obtained.
Similarly, for second order maximum, (a + b) sin θ2 = 2λ
On either side of central maxima different orders of secondary
maxima are formed at the point P1, P2.
In general, (a + b) sin θ = m λ is the condition for maximum intensity,
where m is an integer, the order of the maximum intensity.
When white light is used, the diffraction pattern consists of a white
central maximum and on both sides continuous colored images are
formed.

46. Determination of wavelength of light using a
plane diffraction grating
For monochromatic light
For Polychromatic light

47. Initially all the preliminary adjustments of the spectrometer are made.
The slit of collimator is illuminated by a monochromatic light, whose
wavelength is to be determined. The telescope is brought in line with
collimator to view the direct image. The given plane transmission grating
is then mounted on the prism table with its plane is perpendicular to the
incident beam of light coming from the collimator. The telescope is
slowly turned to one side until the first order diffraction image coincides
with the vertical cross wire of the eye piece. The reading of the position
of the telescope is noted.
Similarly the first order diffraction image on the other side, is made to
coincide with the vertical cross wire and corresponding reading is noted.
The difference between two positions gives 2θ. Half of its value
gives θ, the diffraction angle for first order maximum. The wavelength of
light is calculated from the equation λ = (a+b) sin θ. Here (a+b) is the
grating element.