Imperfections in optical lenses can be observed with the help of

$A$ thin transparent film with refractive index $1.4$ is held on a circular ring of radius $1.8 \ cm$. The fluid in the film evaporates such that transmission through the film at wavelength $560 \ nm$ goes to a minimum every $12 \ seconds$. Assuming that the film is flat on its two sides, the rate of evaporation is . . . . . . $\pi \times 10^{-13} \ m^3/s$.

Choose the correct statement.

In a biprism experiment, the maximum intensity is $I_0$. If the path difference between the two interfering waves is $\frac{\lambda}{3}$, then the intensity at the point on the screen is [$\sin 30^{\circ} = \cos 60^{\circ} = 0.5, \sin 60^{\circ} = \cos 30^{\circ} = \sqrt{3}/2$].

In a Fresnel biprism experiment,which of the following light sources is used to identify the central fringe?

In a biprism experiment,the fringe width is $0.6 \ mm$. The distance between the $6^{th}$ dark fringe and the $8^{th}$ bright fringe on the same side of the central bright fringe is: (in $mm$)