Question

In Young's double slit experiment with monochromatic light, the central fringe will be

• A. coloured
• B. white
• C. bright
• D. black

Answer 1

Answer: (C)

In Young's double slit experiment, for maximum intensity (bright fringe)
$x=m\frac{D\lambda }{d}$
where $m$ is path difference, $D$ the distance between screen and coherent sources, $d$ the distance between coherent sources and $k$ the wavelength.
Putting $m=0,$ we get the position of the central bright fringe (which is called zero order fringe).
Hence, at point $O$ the path difference between two wavelets is zero.
Hence, at $O$ there is always a bright fringe. This is called the central fringe.