Definition- It is a device used for observing the phenomenon of diffraction in the lab.
Construction- It consists of a large number of narrow slits placed side by side. The slits are separated by opaque spaces. Nowadays, gratings are prepared by ruling equidistant lines on the glass surface. The lines are drawn with a fine diamond point. The space in between any two lines is transparent to light and the lined portion is opaque to light waves. Diffraction grating contains nearly 10,000 lines per cm.
It should be noted that gratings with originally ruled lines are a few. For practical purposes, only replicas of the original gratings are used.
Theory- A parallel beam of monochromatic light falling on the grating sends waves in all forward directions from each slit.
Let us consider the parallel rays making an angle ‘θ‘ with the normal ‘OC’ to the grating. The rays are brought to focus on the screen at P, by lens L. If the ray AP travel a distance ‘λ‘ more than ray BP, then waves from A & B will give rise to constructive interference at P.
From the right angle △ ABK,
Sin θ = λ/AB
⇒ λ = AB Sin θ
⇒ λ = b Sin θ [Where b = AB = width of slit]
Thus the condition for reinforcement in the direction ‘θ‘ is as under-
b Sin θ = λ
or In general, a grating equation can be written as-
b Sin θn = nλ
Here ‘b’ is the grating space and ‘n’ is the order of the spectrum.