If instead of two slits, several equidistantly spaced slits are illuminated by the wavefront, the interference maxima become much sharper and, when light is normally incident on the grating, the maxima at angles θ m are given by ( m = 0, ☑, ☒, … ) : The sinc function includes the effects of diffraction due to the width of the slits. Where the sinc function is defined as sinc( x) = sin( x)/( x) for x ≠ 0, and sinc(0) = 1. If the spacing between the slits is d and the width of the slits b is greater than the wavelength, the Fraunhofer diffraction equation gives the intensity of the diffracted light as: This is the classical experiment of Thomas Young (1800). If instead of a single slit, two slits are illuminated by a plane wavefront, a series of interference fringes parallel to the slits will appear on a far screen, as shown in the image below. The diffraction phenomena has been treated in the post Light as a Wave : Slit Diffraction. Thus, for visible light, apertures in the range 10-100 μm produce easily resolved diffraction patterns. The angle of of diffraction is of order λ / d with λ the wavelength and d the dimension of the aperture. It can be understood by considering the interference between different parts of the wavefront, which was altered in passing through the aperture. This effect is called diffraction, and it is characteristic of all wave phenomena. When a collimated beam of light passes through an aperture, or if it encounters an obstacle, it spreads out and the resulting pattern contains bright and dark regions. We propose, in particular, to measure the pitch of the grating through the measurement of the diffraction produced on the He-Ne laser beam. With the new Laser He-Ne (described in the Laser He-Ne post), you can easily test the physical properties of the diffraction grating.
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