The diffraction grating
Young’s demonstration of the wave nature of light was the prelude to further experimental and theoretical work on the subject which continued during the nineteenth century.
Within a few years, the German physicist, Joseph von Fraunhofer, invented a more satisfactory w~y of measuring the wavelength of light. Instead of two slits, he used as many close parallel slits as it was then possible to obtain. Such a device is called a diffraction grating. Some he made by ruling as many lines as he could on a piece of smoked glass: others were constructed from parallel wires, kept equidistant by locating them in between the threads of two fine screws. Better gratings were made by later workers who used accurate temperature controlled dividing engines to rule parallel lines on sheets of glass. Replicas of such
gratings can be made by coating them with a layer of collodion (a solution of cellulose nitrate) which, when dry, is carefully peeled off and attached to a piece of glass. Modern gratings may have 500 or more lines per millimetre. It is now possible to obtain large gratings on cellulose acetate sheet. For elementary work this may be cut up into pieces about 50 mm square and sandwiched between thin glass plates for protection and to keep it fiat.
We shall first explain the action of a diffraction grating and then go on to show how wavelengths of light may be measured with it. Note that all our diagrams have been simplified by showing only a few grating elements but it must be realized that, in practice, there are some hundreds per millimetre.
Fig. 26.16 shows the cylindrical diffracted wavelets emerging from the slits of a
diffraction grating when a beam of parallel light is incident upon it. The whole effect is similar to that obtained with water waves in a ripple tank when straight waves are incident on a barrier with a number of equidistant narrow openings.
Now the slits of a diffraction grating are only a few wavelengths wide: very much finer than those used in Young’s experiment. This enables the light to diffract over an angle of practically 1800 but the reduced light energy passing through in any particular direction is, of course, compensated by the sum-total effect of the large number of slits.
Let us suppose that the arcs of the circles drawn represent the wavefronts of he
diffracted wavelets and that these are spaced one wavelength apart. The tangent planes drawn to touch these cylindrical wavefronts suggest that there are at least three main directions along which all the wavelets combine in step with one another, and experiment shows that this is so (see Fig. 26.16). Incidentally, this way of regarding a plane wavefront as being composed of a vast number of spherical or cylindrical wavelet fronts was first used by Huygens in his original wave theory of light.
The resultant wavefront emerging parallel to the grating is the zero order wavefront, so-called because all the wavelets have travelled the same distance (i.e., zero path difference between them). The two plane wavefronts emerging at an angle on either side are called the first order diffracted wavefronts, a name derived from the fact that each wavelet has to travel exactly one wavelength further than its next-door neighbour in order to be in step along the combined wavefront.