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?

Four different independent waves are represented by

$$\begin{gathered} i) y_1 = a_1\sin \omega t \\ ii) y_2 = a_2\sin 2\omega t \\ iii) y_3 =a_3\cos \omega t \\ iv) y_4 = a_4\sin \left(\omega t + \pi /3\right) \end{gathered}$$

With which of the two waves interference is possible

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