Which of the following phenomena produces the colour in a soap bubble?

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:

On a rainy day,a small oil film on water shows brilliant colours. This is due to:

Assertion: Newton's rings are formed in the reflected system when the space between the lens and the glass plate is filled with a liquid of refractive index greater than that of glass; the central spot of the pattern is bright.
Reason: This is because the reflection in these cases will be from a denser to rarer medium and the two interfering rays are reflected under similar conditions.

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 $: 1$)