The screen should be as far away from the source as possible.Ģ. Since bright and dark fringes are of same width, they are equi−spaced on either side of central maximum.Ĭondition for obtaining clear and broad interference bandsġ. Similarly, it can be proved that the distance between two consecutive dark bands is also equal to Dλ/d. Waves from A and B meet at P in phase or out of phase depending upon the path difference between two waves. P is a point at a distance x from O, as shown in Fig 5.17. O is a point on the screen equidistant from A and B. A screen XY is placed parallel to AB at a distance D from the coherent sources. Let d be the distance between two coherent sources A and B of wavelength λ. This shows clearly that the bands are due to interference. When one of the slits is covered, the fringes disappear and there is uniform illumination on the screen. These two waves constructively interfere and bright fringe is observed at P. At P on the screen, waves from A and B travel equal distances and arrive in phase.
Using an eyepiece the fringes can be seen directly. These are called interference fringes or bands. When a screen XY is placed at a distance of about 1 metre from the slits, equally spaced alternate bright and dark fringes appear on the screen. So A and B act as coherent sources.Īccording to Huygen’s principle, wavelets from A and B spread out and overlapping takes place to the right side of AB.
Since A and B are equidistant from S, light waves from S reach A and B in phase. The width of each slit is about 0.03 mm and they are about 0.3 mm apart. Light from a narrow slit S, illuminated by a monochromatic source, is allowed to fall on two narrow slits A and B placed very close to each other.
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