The wavelength is 590 nm.
Calculation:
The equation for diffraction is given as,
dsinθ = nλ
λ = dsinθ/n
Given,
θ = 21.1°
1/d = 1/610 lines per mm
n = 1
d = 1/610 lines per mm
= 1.639 × 10⁻⁶
Now, put the values in the equation
λ = dsinθ/n
λ = 1.639 × 10⁻⁶ × sin 21.1°/1
= 590 nm
Hence, the wavelength is 590 nm.
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