Caption: A comparison of the spectra obtained by dispersion from a diffraction grating and a prism.

Features:

1. A diffraction grating is just a screen with many finely spaced slits through which light diffracts.

   The slit shape means that diffraction is strong perpendicular to the slit because the slit WIDTH is comparable to the wavelength of the electromagnetic radiation (EMR). On the other hand, diffraction is usually negligible parallel to the slit because the slit LENGTH is large compared to the wavelength of the EMR.

2. A diffraction grating is, in fact, designed so that the diffraction pattern for monochromatic light is a set of widely spaced bright fringes (comparable in LENGTH to the slit LENGTH) separated by very broad dark fringes.

   The bright fringes for monochromatic light look like lines, and so are called spectral lines or just lines for a shorthand.

3. Diffraction is wavelength-dependent: the bigger the wavelength, the bigger the dispersion (i.e., the more the EMR will spread from rectilinear propagation) for any single spectral line in a diffraction pattern of spectral lines.

   There is always a central bright fringe (i.e., spectral line) for all wavelengths---which is the white beam in the image.

   The other bright fringes are numbered in order of angular distance from the central bright fringes: i.e., 1st order, 2nd order, etc. as illustrated in the image. The fringes brightness decreases with order, and so the 1st order is brightest and usually the only one used in spectroscopy.

   The central bright fringes can be called the 0th order fringes.

4. If the incident EMR consists of a continuum of wavelengths, each order of the diffraction pattern gets spread into a continuous band of dispersed light. This situation is illustrated in the image.

   On the other hand, if the incident EMR consists of a discrete set of narrow wavelength bands, each order of the diffraction pattern gets spread into a line spectrum consisting of a set of spectral lines each one formed by one of the narrow wavelength bands. This is situation is NOT illustrated in the image.

   A dilute gas gives rise to line spectrum and so we say it HAS a line spectrum.

   Usually, when one says spectral line one means one from line spectrum. It can also be called a transition line since it usually results from a transition between energy levels in a microscopic particle (e.g., atom, molecule, ion, or nucleus).

   A spectral line has a line wavelength (AKA central line wavelength) which is the central wavelength of the narrow wavelength band that makes up the spectral line. It also has linewidth which is a characteristic width for the narrow wavelength band. Such narrow wavelength bands do NOT have sharp edges, and so the linewidth follows from a conventional definition.

5. A prism creates dispersion by wavelength-dependent refraction.

   As one can see from the figure, with refraction the shorter the wavelength, the greater the angular deviation of the light ray (i.e., the greater the refraction).

6. In practice, diffraction gratings can give much more dispersion than prisms. Thus, diffraction gratings are the main dispersion optical devices used in spectroscopy (i.e., in spectroscopes).

7. There is an optical device that combines a diffraction grating and a prism, and that is used for special purposes---it's called a grism---which is NOT named after astronaut Gus Grissom (1926--1967)---who named a star after himself---Navi (γ CAS)---which is his middle name Ivan spelled in reverse---it's an ananym.