In a Fresnel biprism experiment,which of the following light sources is used to identify the central fringe?

$A$ circular planar wire loop is dipped in a soap solution and after taking it out,held with its plane vertical in air. Assuming the thickness of the film at the top is very small,as sunlight falls on the soap film,the observer receives reflected light. What does the observer see?

Choose the correct statement.

$A$ 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 $: 1$)

In a biprism experiment,$21$ fringes are observed in a given region using light of wavelength $4800 \text{ Å}$. If light of wavelength $5600 \text{ Å}$ is used,the number of fringes in the same region will be:

$A$ very thin transparent film (like a soap bubble) is viewed in white light. The color of the film appears .......