Diffraction

Bending of light around the sharp corners, a spreading of light within the geometrical shadow of opaque obstacles is called diffraction of light. Diffraction is the general characteristic exhibited by all types of waves.

Diffraction of light can be described as the bending of lights in sharp corners along with spreading of or within the opaque obstacles’ geometrical shadow. The light deviates from the linear path. It becomes way more pronounced in case the aperture or the obstacle’s dimension is comparable to the light’s wavelength.

According to Fresnel, diffraction occurs on account of mutual interference of secondary waveless starting from portions of the wavefront which are not blocked by the obstacle or from portions of the wavefront which are allowed to pass through the aperture.

Diffraction of light at the single slit:

Consider a parallel beam of light with the plane wavefront falling on the single slit LN. The width of the slit LN = a is of the wavelength of light order; hence the diffraction takes place when the light beam reflects through a single slit.

Wavelets from a single wavefront reach the C centre onto the screen in the exact same phase. Therefore, they interfere constructively to give the maxima (bright fringe).

The total path difference between parallel rays will be

                                                      2 x path difference  = LN sinθ,

                     Or                             path difference  = (a sinθ)/2.

therefore, for the nth secondary minima,

                      Path difference = (a/2) sinθ = λ/2,

Or                                            

                                       Sin θn = nλ/a, (n = 1, 2, 3, ….).

Hence, for nth secondary maxima,

                               a sin θn = (2n + 1) λ/2,                 [n= 1, 2, 3, ….]

or           

                                sin θn = (2n + 1) λ/2a.

Hence, the diffraction pattern will be as described ahead:- Point C corresponds to the position of the central maxima. And the positions -3λ, -2λ, -λ, λ, 2λ, 3λ, …… are secondary minima. The above conditions for diffraction maxima and minima are exactly the reverse of mathematical conditions for interference maxima and minima.

Diffraction pattern difference at the single slit because of the monochromatic white light:

For monochromatic light, the diffraction is of alternate bright and dark bands of unequal widths. The central bright fringe has maximum intensity and the intensity of successive secondary maxima decreases rapidly.

If the source is white light, the diffraction pattern is coloured. The central maximum is white, but other bands are coloured. As bandwidth is directly proportional to λ, the red bandwidth is wider than the violet bandwidth.

Difference between diffraction and interference:

1. The interference pattern is the one that consists of the number of equally spaced dark and bright bands. On the other hand, the diffraction pattern is described as the central bright maximum, twice as broad as the maxima. As we move to the successive maxima, the intensity starts to fall from the centre on both sides.

2. We calculate interference patterns while superposing two different waves that originate from the two different narrow slits. A diffraction pattern can be described as the superposition of the wave’s continuous family, which originates from each point on the single slit.

3. For the single slit of width, a, the first-ever interference pattern null takes place at an angle of λ/a. At this angle only, we get a maximum (not a null) for two different narrow slits which are separated by the distance of a.

One must understand that both distances between two slits in Young’s double-slit experiment, d and a width of each slit have to be quite small, to be able to observe good interference and diffraction patterns, respectively. E.g., d must be of the order of a millimetre or so and must be even smaller of the order of 0.1 or 0.2 mm.

Scattering of light:

Crystal, in optics, pieces of glass or other straightforward material cut with exact points and plane countenances, is valuable for dissecting and mirroring light. A common three-sided crystal can isolate white light into its constituent tones, called a range. Each tone, or frequency, making up the white light is bowed, or refracted, an alternate sum; the more limited frequencies (those toward the violet finish of the range) are bowed the most, and the more extended frequencies (those toward the red finish of the range) are bowed the least. Crystals of this sort are utilised in specific spectroscopes, instruments for dissecting light and for deciding the personality and design of materials that discharge or retain light.

Conclusion:

Diffraction is a phenomenon of the spreading of waves around obstacles. Diffraction may take place with sound or electromagnetic radiation like light, X- rays, gamma rays etc., or very small moving particles such as atoms, neutrons or electrons, which show wave-like properties. A diffraction is an important tool that helps science and scientists to unravel the atomic structure of our world. We encountered this phenomenon of diffraction in our everyday life, like the murmuring in background noise or the levels of heat or light in a room.