Question

The figure shows a Young's double-slit experiment where, $P$ and $Q$ are the slits. The path lengths $PX$ and $QX$ are $n\lambda$ and $(n+2)\lambda$ respectively, where $n$ is a whole number and $\lambda$ is the wavelength. Taking, path difference at the central fringe as zero, what is formed at $X$ ?

• A. First bright
• B. First dark
• C. Second bright
• D. Second dark

Answer 1

Answer: (C)

Path difference $=QX-PX=(n+2)\lambda -n\lambda =2\lambda$ ... (i)
For constructive interference or bright band,
Path difference $=\Delta x=n\lambda$ (where, $n=1,2,\dots$ )
From Eq. (i), it is obvious that second bright band is formed as $n=2$.