With the use of formula, the wavelength of light is being used is 4.0 x m or 400nm
DIFFRACTION GRATING
Diffraction grating works exactly the same way as a double slit. Instead of two slits, it has thousands of slit much more of shorter fringes.
Given that a grating has 3900 slits per cm produces a third- order fringe at a 28.0° angle. The wavelength of light being used can be expressed as
sin Ф = nλ / a
Make λ the subject of the formula
nλ = a sin Φ
λ = a sin Φ / n
Where
λ = wavelength
n = 3
Φ = 28 degrees
a = 0.01 / 3900
a = 2.56 x m
Substitute all the parameters into the formula
λ = (2.56 x x sin 28) / 3
λ = 1.2 x /3
λ = 4.0 x m
λ = 400 nm
Therefore, the wavelength of light is being used is 4.0 x m or 400nm