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Monochromatic light (single wavelength) falls on two narrow slits S1 and S2 which are very close together acts as two coherent sources, when waves coming from two coherent sources (S1, S2) superimposes on each other, an interference pattern is obtained on the screen. In YDSE alternate bright and dark bands obtained on the screen. These bands are called Fringes.
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(i) Central fringe is always bright, because at central position Φ=0 or Δ=0
(ii) The fringe pattern obtained due to a slit is brighter than that due to a point.
(iii) If the slit widths are unequal, the minima will not be complete dark. For very large width uniform illumination occurs.
(iv) If one slit is illuminated with red light and the other slit is illuminated with blue light, no interference pattern is observed on the screen.
(v) If the two coherent sources consist of object and it's reflected image, the central fringe is dark instead of bright one.
Useful Results:
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1. Path difference: Path difference between the interfereing waves meeting at a point P on the screen is given by Δ=Δi+Δf ; where Δi initial path difference between the waves before the slits and path Δf difference between the waves after emerging from the sits. In this case Δi =0 (Commonly used condition). So  ; where x is the position of point P from central maxima.
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For maxima at P :Δ =nλ; where n=±0,±2... and For minima n=±0,±2...at P : ; where
2. Location of fringe : Position of nth bright fringe from central maxima
Position of nth dark fringe from central maxima, 
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3. Fringe width (b) : The separation between any two consecutive bright or dark fringes is called fringe width. In YDSE all fringes are of equal width. Fringe width  .
and angular fringe width 
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4. In YDSE, if n1 fringes are visible in a field of view with light of wavelength l1, while n1 with light of wavelength l2 in the similar field, then .
5. Separation (Δx) between fringes
(i) Between nth bright and mth bright fringes (n > m)
(ii) Between nth bright and mth dark fringe
6. Identification of central bright fringe: To identify central bright fringe, monochromatic light is replaced by white ray. Due to overlapping central maxima will be white with red edges. On the other side of its we shall get a few colored band and then uniform illumination.
If the entire YDSE set up is given in another medium then l changes so b changes
e.g. in water 
Shifting of FringePateern in YDSE:
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If a transparent thin surface of glass or mica is put in the path of one of the waves, then the whole fringe pattern gets shifted towards the slit in front of which glass plate is placed.
(i) Fringe shift = 
(ii) Additional path difference =(μ-1)t
(iii) If shift is equivalent to n fringes then n = (μ-1)t/λ
or 
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(iv) Shift is independent of the order of fringe (i.e. shift of zero order maxima = shift of nth order maxima.
(v) Shift is independent of wavelength.
missing wavelength in front of one slit:
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Suppose P is a point of observation infront of slit S1 as shown
Missing wavelength at P
By putting n = 1, 2, 3....
Missing wavelengths are 
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