What is the shape of the interference fringes formed on the screen due to a point source $P$,in the case shown here?

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):

Choose the correct statement.

The beautiful iridescent (like a rainbow) colors on the wings of a tropical or morpho butterfly are due to

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

$A$ ray of light of intensity $I$ is incident on a parallel glass slab at a point $A$ as shown in the figure. It undergoes partial reflection and refraction. At each reflection,$25\%$ of the incident energy is reflected. The rays $AB$ and $A^1B^1$ undergo interference. The ratio $I_{\text{max}} / I_{\text{min}}$ is (in $: 1$)