Answer:
It looks yellow because that is the only (major) color reflected.
Visible spectra is from about 4000-7000 Angstroms (10^-10 m).
Red are longer wavelengths and blue are the shorter wavelengths.
The Sodium doublet (yellow) occurs around 5900 Angstroms.
No the substance will remain the same substance as before.
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Explanation:
It is given that,
Wavelength of red laser light, 
The second order fringe is formed at an angle of, 
For diffraction grating,

, n = 2


The wavelength λ of light that creates a first-order fringe at 22 is given by :




Hence, this is the required solution.