Question
Wed December 26, 2012 By:
 

what happens in young exp when the monochromatic source is replaced by a source of white light.What fringe pattern is observed. In this context My ncert book says that " The fringe closest on either side of the central white fringe is red and the farthest will appear blue . After a few fringes , no clear fringe pattern is seen ." i am not able to understand this .plz explain me why it happens and how .

Expert Reply
Mon December 31, 2012
If a white light is used in the double slit experiment, the different colours will be split up on the viewing screen according to their wavelengths. The violet end of the spectrum (with the shortest wavelengths) is closer to the central fringe, with the other colours being further away in order. For any component of white light having wavelength ?, the y-coordinate of nth maxima on the screen = n?D/d (where D = Distance between the screen and slits, d = distance between the slits) Hence, for the 1st maxima, y1 = ?D/d. Its easy to see that y1 will be greater for greater wavelengths i.e. the colour having smallest wavelength will have its first maxima closest to the central bright => the fringe next to the white fringe will be violet.
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