$A$ thin film of soap solution $(n = 1.4)$ lies on the top of a glass plate $(n = 1.5)$. When visible light is incident almost normal to the plate,two adjacent reflection maxima are observed at two wavelengths $420 \ nm$ and $630 \ nm$. The minimum thickness of the soap solution is.....$nm$

Dark and colour patterns on thin soap films are due to

Some currency notes (to avoid counterfeits) change their colour as you tilt them. This is due to

In a Fresnel biprism experiment,the eyepiece is placed at a distance of $120 \, cm$ from the source. The distance between the two virtual images is found to be $0.075 \, cm$. If the eyepiece is moved by $1.92 \, cm$ to cross $20$ fringes in the field of view,what is the wavelength of light in $\mathop A\limits^o$?

To ensure almost $100$ per cent transmittivity,photographic lenses are often coated with a thin layer of dielectric material. The refractive index of this material is intermediate between that of air and glass (which makes the optical element of the lens). $A$ typically used dielectric film is $MgF_2$ $(n = 1.38)$. What should the thickness of the film be so that at the center of the visible spectrum $5500 \, \mathring{A}$ there is maximum transmission?

The approximate thickness of an oil film required to observe the interference of light (due to which it appears coloured) is .......