Answer
a). $(1)$ increase.
b). $0.63cm$
c). $0.94cm$
Work Step by Step
a). Let v be the wavelength.
Separation between adjacent maxima = $y_{n}-y_{n-1}=\frac{nLv}{d}-\frac{(n-1)Lv}{d}=\frac{Lv}{d}$
Thus, $y_{n}-y_{n-1}$ is proportional to L hence when L increases the separation between adjacent maxima will (1) increase.
b). $y_{n}-y_{n-1}=\frac{Lv}{d}=\frac{2\times 550\times10^{-9}}{1.75\times10^{-4}}=0.63cm$
c). $y_{n}-y_{n-1}=\frac{Lv}{d}=\frac{3\times550\times10^{-9}}{1.75\times10^{-4}}=0.94cm$
