Q. What is a normal spectrum? Show that grating spectrum is a normal spectrum.
Ans. Normal spectrum and the dispersive power of grating is defined as the rate of change of angle of diffraction with wavelength or light l.
From above expression, it is clear that dispersive power is inversely proportional to cosine of angle of diffraction. Higher value of ? shall have higher dispersion.
However for smaller values of ?, cos ? = 1 and the angular dispersion d? of spectral lines in a given order for a given grating is directly proportional to wavelength separation dl.