Answer
λb = 460 nm
λr = 670 nm
Grating formula = m x λ = d (sin of entrance angle + sin of exit angle)
I assume the entrance angle is 0°.
For red:
1 x 670 nm = 10 000 / cm x (sin 0° + sin of exit angle)
sin of exit angle = 0,67.
Meeting at the same point means having the same exit angle for both light waves. So for blue:
2 x 460 nm = ? x (sin 0° + 0,67)
? = 13 731 l / cm
__________________
Örsan Yüksek
