What is the shape of the interference fringes formed on the screen due to a point source $P$,in the case shown here?

Imperfections in optical lenses can be observed with the help of

In a biprism experiment,monochromatic light of wavelength $\lambda$ is used. The distance between two coherent sources is kept constant. If the distance between the slit and the eyepiece $(D)$ is varied as $D_{1}, D_{2}, D_{3}$ and $D_{4}$,and the corresponding measured fringe widths are $z_{1}, z_{2}, z_{3}$ and $z_{4}$,then:

In a biprism experiment, the distance between $4^{th}$ and $13^{th}$ bright band on the same side is '$y$' when light of wavelength $6000 \text{ Å}$ is used. For a light of wavelength '$\lambda$' the distance between $6^{th}$ and $16^{th}$ bright band on the same side is again '$y$'. The value of '$\lambda$' in $\text{Å}$ units is

$A$ thin oil film of refractive index $1.2$ floats on the surface of water $(\mu = \frac{4}{3})$. When light of wavelength $\lambda = 9.6 \times 10^{-7} \ m$ falls normally on the film from air,it appears dark when seen normally. What is the minimum change in its thickness for which it will appear bright in normally reflected light by the same light?

White light reflected from a soap film (Refractive Index $\mu = 1.5$) has a maxima at $600 \ nm$ and a minima at $450 \ nm$ with no minimum in between. Then the thickness of the film in units of $10^{-7} \ m$ is: