A thin film of water $(\mu =\frac{4}{3})$ is $3100 \AA$ thick. If it is illuminated by white light at normal incidence. The colour of film in the reflected light will be

In Young's double slit experiment shown in figure. $S_1$ and $S_2$ are coherent sources and $S$ is the screen having a hole at a point $1.0 \mathrm{mm}$ away from the central line. White light ($400$ to $700 \mathrm{nm}$ ) is sent through the slits. Which wavelength passing through the hole has strong intensity?

In Young's double-slit interference experiment, the slit separation is made $3$ fold. The fringe width becomes 

In Young's experiment, the distance between the slits is reduced to half and the distance between the slit and screen is doubled, then the fringe width

Shape of interference fringes formed on the screen due to point source $P,$ in the case shown here

In the Young's arrangement, screen starts moving towards right with constant speed $v$. Initial distance between screen and plane of slits is $x$. At $t=0$, 1st order maxima is lying at point A. After how much time first order minima lies at point $A$ ?