While Interference of Reflected Waves from Thin Film for Normal Incidence, the path difference is twice the thickness of film .

Transmission through thin layers. In the figure given below light is incident perpendicularly on a thin layer of material $2$ that lies between (thicker) materials $1$and $3 .$ (The rays are tilted only for clarity.) Part of the light ends up in material $3$ as ray ${\mathrm{r}}_{\mathit{3}}$ (the light does not reflect inside material $2$) and ${\mathrm{r}}_{\mathit{4}}$ (the light reflects twice inside material $2$). The waves of ${\mathrm{r}}_{\mathit{3}}$ and ${\mathrm{r}}_{\mathit{4}}$ interfere and here we consider the type of interference to be either maximum (max) or minimum (min). For this situation, each problem in the table refers to the indices of refraction ${\mathrm{n}}_{\mathit{1}}, {\mathrm{n}}_{\mathit{2}}$ and ${\mathrm{n}}_{\mathit{3}},$ the type of interference, the thin-layer thickness $L$ in nanometers, and the wavelength $\lambda$ in nanometers of the light as measured in air. Where $\lambda$ is missing, give the wavelength that is in the visible range. Where $L$ is missing give the second least thickness or the third least thickness as indicated.

Direction: The question has a paragraph followed by two statements, Statement$-1$ and statement$-2$. Of the given four alternatives after the statements, choose the one that describes the statements. A thin air film is formed by putting the convex surface of a plane-convex lens over a plane glass plate. With monochromatic light, this film gives an interference pattern due to light reflected from the top (convex) surface and the bottom (glass plate) surface of the film.

$\mathrm{Statement}-1:$ When light reflects from the air-glass plate interface, the reflected wave suffers a phase change of $\mathrm{\pi }$.

$\mathrm{Statement}-2:$ The centre of the interference pattern is dark. 

In the below figure, a broad beam of light of wavelength $420 \mathrm{nm}$ is incident at $90°$ on a thin, wedge-shaped film with an index of refraction $1.70$. The transmission gives $10$ bright and $9$ dark fringes along the film's length. What is the left-to-right change in the film thickness?