#a
This is because red color has higher wavelength and it can clear universe and our atmosphere better,
This has the same reason as how we can see only red color during sunset.
#b
[tex]\boxed{\sf z=\dfrac{ \lambda-\lambda_o}{\lambda_o}}[/tex]
lambda_o be x
Put values
[tex]\\ \rm\Rrightarrow 7.6=\dfrac{2\times10^{-6}-x}{x}[/tex]
[tex]\\ \rm\Rrightarrow 7.6=\dfrac{2\times 10^{-6}}{x}-1[/tex]
[tex]\\ \rm\Rrightarrow 8.6=\dfrac{2\times 10^{-6}}{x}[/tex]
[tex]\\ \rm\Rrightarrow x=0.23\times 10^{-6}m[/tex]
[tex]\\ \rm\Rrightarrow x=0.23\mu m[/tex]
