In a biprism experiment,the $6^{\text{th}}$ bright band with wavelength $\lambda_1$ coincides with the $7^{\text{th}}$ dark band with wavelength $\lambda_2$. Then the ratio $\lambda_1 : \lambda_2$ is (other settings remain the same):

$A$ 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

The colors seen in a soap bubble are due to .......

$A$ 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

In a biprism experiment using sodium light $\lambda_1 = 6000\, \mathring{A}$,an interference pattern is obtained in which $20$ fringes occupy $2\, cm$. On replacing sodium light by another source of wavelength $\lambda_2$ without making any other change,$30$ fringes occupy $2.7\, cm$ on the screen. What is the value of $\lambda_2$ in $\mathring{A}$?

Light of wavelength $6000 \, \mathring{A}$ is incident on a thin glass plate of refractive index $1.5$ such that the angle of refraction in the plate is $60^\circ$. Find the thickness of the plate so that it appears dark in reflection.