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Question

# (a) Consider two coherent sources S1 and S2 producing monochromatic waves to produce interference pattern. Let the displacement of the wave produced by S1 be given by Y1=a cos ωt and the displacement by S2 be Y2=a cos(ωt+ϕ). Find out the expression for the amplitude of the resultant displacement at a point and show that the intensity at that point will be I=4a2 cos2ϕ2. Hence, establish the conditions for constructive and destructive interference. (b) What is the effect on the interference fringes in Young's double slit experiment when (i) the width of the source slits is increased; (ii) the monochromatic source is replaced by a source of white light.

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Solution

## (a) Let S be a narrow slit illuminated by a monochromatic source of light and S1 and S2 two similar parallel slits very close together and equidistant from S. Displacement of the wave produced by S1 is given by y1=a cos ωt and the displacement of the wave produced by S2 is given by y2=a cos (ωt+ϕ) The resultant displacement is given by y=y1+y2 y=a cos ωt+a cos(ωt+ϕ) y=2a cos ϕ/2 cos(ωt+ϕ/2) The amplitude of the resultant displacement is 2a cos ϕ/2. The intensity of light is directly proportional to the square of amplitude of the wave. The resultant intensity is given by I=4a2 cos2 ϕ/2 For constructive interference the intensity of light at point P is maximum, if cos ϕ=max=+1:ϕ=0, 2π, 4π.... ϕ=2nπ where n=0, 1, 2, ..... Path difference =λ2π×2nπ=nλ For distructive interference the intensity of light at point P is minimum, if cosϕ=−1 ∴ϕ=π, 3π, 5π, ..... or ϕ=(2n−1)π where n=1, 2, 3, .... The corresponding path difference between the two waves x=λ2πϕ=λ2π(2n−1)π=(2n−1)λ2 (b) (i) As the width of the slits is increased, the fringe width decrease. It is because, β∝1d (ii) The different colours of white light will produce different interference patterns but the central bright fringes due to all colours are at the same positions. Therefore, the central bright fringe is white in colour. Since the wavelength of the blue light is smallest, the fringe closed on the either side of the central white fringe is blue and farthest is red. Beyond a few fringes, no clear fringe pattern will be visible.

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