Diffraction Grating vs. Prism

Difference Between Diffraction Grating And Prism

Diffraction Grating

The diffraction grating is a precise optical component of spectroscopes and various measuring machines. Used like a prism, which separates light into its constituent wavelengths, it may consist of a surface ruled with thousands of accurately spaced, parallel slits, each slit being only about a single light wavelength wide. These are termed transmission gratings. Some gratings consist of a series of similarly spaced parallel reflecting surfaces (reflection gratings). Gratings used in the visible and ultraviolet regions have at least 6,000 to 18,000 lines per centimeter; in the infrared region they have from less than 500 to 3,000 lines per centimeter. Gratings with as many as 50,000 lines per centimeter have been developed for use in high-resolution instruments.

The separation of light into its constituent wavelengths with a grating is the result of constructive and destructive wave interference after the light is diffracted by the grating rulings (see diffraction). A prism, on the other hand, separates light on the basis of differing indices of refraction. When a grating is placed at a proper angle to an incident beam of light, the light beam will be dispersed—that is, it will be divided into its constituent wavelengths. The result is a spectrum of the entering light.

In measuring machines, diffraction gratings are used in pairs, because the relative movement of two gratings creates optical patterns (moiré patterns) or fringes. These patterns are detected by photocells, and their outputs are fed into a computer to provide a digital readout of movement, with a resolution of about 0.00127 cm (0.0005 in).


In optics, prism refers to any transparent medium having two or more plane surfaces. A familiar example is the triangular prism, usually made of glass, used to split a beam of white light into its component colors. The ability of the prism to do this stems from the fact that the index of refraction of any optical medium depends on the wavelength (color) of the light, a property called dispersion. In all ordinary media—glass, water, and so on—the refractive index increases as the wavelength becomes shorter. Thus the rays in the violet end of the visible spectrum (corresponding to the shorter wavelengths) are more sharply refracted by a prism than are the longer wavelengths, in the red end of the spectrum.

Another common type of prism is the right-angle, total-reflecting prism. This prism does not disperse light into its colors; rather, it reflects the light by what is called total internal reflection (see refraction). Retroreflecting prisms are used in the making of binoculars and other optical instruments.




Category: VS  |  Tags: