As shown in arrangement waves with identical wavelengths and amplitudes and that are initially in phase travel through different media, Ray $1$ travels through air and Ray $2$ through a transparent medium for equal length $L,$ in four different situations. In each situation the two rays reach a common point on the screen. The number of wavelengths in length $L$ is $N_2$ for Ray $2$ and $N_1$ for Ray $1.$ In the following table, values of $N_1$ and $N_2$ are given for all four situations, The order of the situations according to the intensity of the light at the common point in descending order is :

Situations	$1$	$2$	$3$	$4$
$N_1$	$2.25$	$1.80$	$3.00$	$3.25$
$N_2$	$2.25$	$2.80$	$3.25$	$4.00$

In the figure shown in a $YDSE,$ a parallel beam of light is incident on the slits from a medium of refractive index $n_1.$ The wavelength of light in this medium is ${\lambda }_1.$ A transparent slab of thickness $\text{'}t\text{'}$ and refractive index $n_3$ is put in front of one slit. The medium between the screen and the plane of the slits is $n_2.$ The phase difference between the light waves reaching point $O$ (Symmetrical, relative to the slits) is :

In the figure shown if a parallel beam of white light is incident on the plane of the slits then the distance of the nearest white spot on the screen from $O$ is. [assume $d<<D, \mathrm{\lambda }<<d],$

In the figure shown, a parallel beam of light is incident on the plane of the slits of Young's double slit experiment. Light incident on the slit, $S_1$ passes through a medium of variable refractive index $\mathrm{\mu }=1+\mathit{ax}$ (where $\text{'}x\text{'}$ is the distance from the plane of slits as shown), upto a distance $\text{'}l\text{'}$ before falling on $S_1.$ Rest of the space is filled with air. If at $\text{'}O\text{'}$ a minima is formed, then the minimum value of the positive constant a (in terms of $l$ and wavelength $\text{'}\mathrm{\lambda }\text{'}$  in air) is:

$M_1$ and $M_2$ are plane mirrors and kept parallel to each other. At point $O$ there will be a maxima for wavelength $\mathrm{\lambda }.$ Light from monochromatic source $S$ of wavelength $\mathrm{\lambda }$ is not reaching directly on the screen. Then $\mathrm{\lambda }$ is : $[D>>d>>\mathrm{\lambda }]$