two slits in a piece of card, illuminating with a laser pointer, and observing the diffracted light at a distance of 1 m. If the slit separation is 0.5 mm, and the wavelength of the laser is 600 nm, then the spacing of the fringes viewed at a distance of 1 m would be 1.2 mm. Semi-quantitative explanation of double-slit fringes The difference in phase between the two waves is determined by the difference in the distance travelled by the two waves. If the viewing distance is large compared with the separation of the slits (the far field), the phase difference can be found using the geometry shown in the figure. The path difference between two waves travelling at an angle is given by When the two waves are in phase, i.e. the path difference is equal to an integral number of wavelengths, the summed amplitude, and therefore the summed intensity is maximal, and when they are in anti-phase, i.e. the path difference is equal to half a wavelength, one and a half wavelengths, etc., then the two waves cancel, and the summed intensity is zero. This effect is known as interference. The interference fringe maxima occur at angles where is the wavelength of the light. The angular spacing of the fringes is given by When the distance between the slits and the viewing plane is , the spacing of the fringes is equal to and is the same as above: Diffraction by a grating A grating is defined in Born and Wolf as "any arrangement which imposes on an incident wave a periodic variation of amplitude or phase, or both". A grating whose elements are separated by diffracts a normally incident beam of light into a set of beams, at angles given by: This is known as the grating equation. The finer the grating spacing, the greater the angular separation of the diffracted beams. If the light is incident at an angle , the grating equation is: The detailed structure of the repeating pattern determines the form of the individual diffracted beams, as well as their relative intensity while the grating spacing always determines the angles of the diffracted beams. The image on the right shows a laser beam diffracted by a grating into = 0, and ±1 beams. The angles of the first order beams are about 20°; if we assume the wavelength of the laser beam is