- A repetitive array of diffracting elements either apertures or obstacles,
that has the effect of producing periodic alterations in the phase,
amplitude or both of an emergent wave is said to be a diffraction grating.
- Grating is a set of equally spaced narrow parallel sources.
- The Grating equation is $$ a\sin\theta = m\lambda $$
Where
\(a = \) width of one spacing in grating,
\(N = \) number of lines per inch,
\(\theta = \) angle between white light and different colours,
\(m = \) order of the various principle maximum,
\(\lambda =\) wavelength of the corresponding light.
-
For a source having a broad continuous spectrum such as tungsten filament,
the \(m = 0\) or zeroth order the image corresponds to the un-deflected \(\theta_{0} = 0\),
i.e.;
we will see white light view of the source.
-
For the image of other colours the \(\theta_{0}\) will be different. so, \(m \neq 0\)
-
The first order spectrum \(m = \pm 1\) appears on
either side of \(\theta = 0\) and is followed,
along with alternate intervals of darkness.
-
The smaller the \(a\) (width of one spacing), the fewer will be the number of visible orders.