Question

If we replace the monochromatic source in $YDSE$ by white light, then

• A. fringe pattern in not observed
• B. central fringe is white and the fringe closest on either side of the central white fringe is red
• C. central fringe is white and the fringe closest on either side of the white fringe is blue
• D. central fringe is white and the fringe closest on either side of the white fringe is dark

Answer 1

Answer: (B)

The interference patterns due to different component colors of white light overlap. The central bright fringes for different colors are at the same position. Hence, the central fringe is white. For a point $P$ for which $S_{2}P$ $S_{1}P={\lambda }_b/2$ where ${\lambda }_b(=4000\mathring{A})$represents wavelength of blue light, the blue component will be absent and the fringe will appear red in color. Slightly farther away where
$S_{2}Q-S_{1}Q={\lambda }_b=\frac{{\lambda }_r}{2}$ where ${\lambda }_r(=8000\mathring{A})$
is the wavelength for the red color, the fringe will be predominantly blue. Thus, the fringe closest on either side of the central white fringe is red and the farthest will appear blue.