$A$ thin film of water $(\mu = 4/3)$ is $3100 \,\mathring{A}$ thick. If it is illuminated by white light at normal incidence,the colour of the film in the reflected light will be:

$A$ mixture of yellow light of wavelength $580 \ nm$ and blue light of wavelength $450 \ nm$ is incident normally on an air film of thickness $2.9 \times 10^{-4} \ mm$. The colour of reflected light is

If prism angle $\alpha = 1^\circ$,refractive index $\mu = 1.54$,distance between screen and prism $b = 0.7\,m$,distance between prism and source $a = 0.3\,m$,and wavelength $\lambda = 180\pi\,nm$,then in a Fresnel biprism,find the value of fringe width $\beta$.

$A$ light of wavelength $5890 \; \mathring{A}$ falls normally on a thin air film. What is the minimum thickness of the film such that the film appears dark in reflected light?

In a biprism experiment,monochromatic light of wavelength $\lambda$ is used. The distance between two coherent sources $d$ 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:

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.