In a Fresnel's biprism experiment,the fringe width is $1 \,mm$. The distance of the nearest dark fringe from the central bright fringe is .......$mm$.

Assertion: At the first glance,the top surface of the Morpho butterfly's wing appears a beautiful blue-green. If the wind moves the colour changes.
Reason: Different pigments in the wing reflect light at different angles.

When exposed to sunlight,thin films of oil on water often exhibit brilliant colours due to the phenomenon of

When white light is incident normally on an oil film of thickness $10^{-4} \,cm$ and refractive index $1.4$,the wavelength which will not be seen in the reflected system of light is ........... $\mathring{A}$.

$A$ parallel beam of light of intensity $I_0$ is incident on a coated glass plate. If $25 \%$ of the incident light is reflected from the upper surface and $50 \%$ of the light that enters the glass is reflected from the lower surface of the glass plate, the ratio of maximum to minimum intensity in the interference region of the reflected light is

Light is incident on a glass plate $(\mu = 1.5)$ such that the angle of refraction is $60^\circ$. $A$ dark band is observed corresponding to a wavelength of $6000 \, \mathring A$. If the thickness of the glass plate is $1.2 \times 10^{-3} \, mm$,find the order of the interference band.