Answer
140 bright fringes are formed along the film’s length.
Work Step by Step
The first bright fringe on the left end will be a result of the wave bouncing off the lower edge of the air wedge undergoing an additional phase shift of $\lambda/2$ due to the path difference.
I.e., $2d_1={\lambda\over 2}$
Where $d_1$ is the wedge height at the point of incidence of the light ray.
Whereas on the opposite end where the wedge height is $d_2$, the formation of the last bright fringe requires,
$2d_2=(m+{1\over 2}\lambda)$
Where, the total number of bright fringes is $m+1$.
Subtracting the above two equations,
\begin{align*}
2(d_2-d_1)&=m\lambda\\
m&={2(d_2-d_1)\over\lambda}\\
&=2\times {48\times 10^{-6}\over 683\times 10^{-9}}\\
&=140.5
\end{align*}
Hence, a total of 140 bright fringes are formed along the film’s length.
