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Young's double slit experiment -1, Young's double slit experiment- 2 are considered the most difficult concepts.
118 Questions around this concept.
a YDSE: D = 1 m, d = 1 mm and l = 5000 n m. The distance of 100th maxima from the central maxima is:
Young's double-slit experiment -1-
This experiment is performed by British physicist Thomas Young. He used an arrangement as shown below. In this he used a monochromatic source of light S . He made two pinholes S1 and S2 (very close to each other) on an opaque screen as shown in the figure Each source can be considered as a source of the coherent light source.
So, we can see that the monochromatic light source ‘s’ kept at a considerable distance from two slits s1 and s2. The arrangement is such that the S is equidistant from S1 and S2. S1 and S2 behave as two coherent sources, as both bring derived from S.
Let d be the distance between two coherent sources A and B having wavelength λ. A screen XY is placed parallel to an opaque screen at a distance D. O is a point on the screen equidistant from A and B. P is a point at a distance x from O
From the above figure, we can see that the waves from A and B meet at P. It may be in phase or out of phase depending upon the path difference between the two waves, which we will calculate.
The assumption in this experiment -
1. D> d: Since D > > d, the two light rays are assumed to be parallel.
2. d/λ >> 1: Often, d is a fraction of a millimetre and λ is a fraction of a micrometre for visible light.
For Bright Fringes -
By the principle of interference, the condition for constructive interference is the path difference = nλ
For Dark fringes -
By the principle of interference, the condition for destructive interference is the path difference =
Here, n = 1,2,3 … indicate the order of the dark fringes.
So,
The above equation gives the distance of the nth dark fringe from the point O.
So, we can say that the alternate dark and bright fringe will be obtained on either side of the central bright fringe.
The distance between any two consecutive bright or dark bands is called bandwidth.
Take the consecutive dark or bright fringe -
Angular fringe width -
Young's double slit experiment- 2 -
Intensity of Fringes In Young’s Double Slit Experiment-
For two coherent sources S1 and S2, the resultant intensity at point P on the screen is given by-
where
The intensity of wave from S1
The intensity of wave S2
Putting (Becasue d<<<D)
So the intensity variation from maximum to minimum depends on the phase difference. Let us discuss one by one -
The phase difference between the waves at the point of observation is . Path difference between the waves at the point of observation is
Resultant intensity at the point of observation will be maximum
The phase difference between the waves at the point of observation is
Path difference between the waves at the point of observation is
Resultant intensity at the point of observation will be maximum
As we know that the position of nth order maxima on the screen is -
Value of 'n' cannot be taken as infinitely large, because it violates the assumption of the Young's double slit experiment (Mentioned in the last concept) which means that the is small or we can write x<<D. So,
So the above formula is only applicable for -
But when, , which means that the n is comparable with . Then the above formula is not applicable, then we have to go with the basic and we will equate path difference as -
The above represents box function or greatest integer function.
Similarly, the highest order of interference minima
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