Diffraction
It is a matter of common experience that the path of light entering a dark room through a hole in the window illuminated by sunlight is straight. Similarly if an opaque obstacle is placed in the path of light, a sharp shadow is cast on the screen, indicating thereby that light travels in straight lines. (Rectilinear propagation of light can be easily explained on the basis of Newton’s corpuscular theory. But it has been observed that when a beam of light passes through a small opening (a small circular hole or a narrow slit) it spreads to some extent into the region of the geometrical shadow also. If light energy is propagated in the form of a beam of light round the edges of an opaque obstacle or illumination of the geometrical shadow.
Each progressive wave, according to Huygens wave theory produces secondary waves, the envelope of which forms the secondary wavefront. S is a source of monochromatic light and MN is a small aperture. XY is the screen placed in the path of light. AB is the illuminated portion of the screen and above A and below B is the region of the geometrical shadow. Considering MN as the primary wavefront, according to Huygens’s construction, if the light in the geometrical shadow. Thus, the shadows formed by small obstacles are not sharp. This bending of light round the edges of an obstacle or the encroachment of light within the geometrical shadow is called diffraction. Similarly, if an opaque obstacle MN is placed in the path of light, there should be illumination in the geometrical shadow region AB also. But the illumination in the geometrical shadow of an obstacle is not commonly observed because the light sources are not point sources and secondly the obstacles used are of very large size to the wavelength of light.
If a shadow of an obstacle is cast by an extended source, say a frosted electric bulb, light from every point of the surface fo the bulb, light from every point of the surface of the bulb forms its own diffraction pattern (bright and dark diffraction bands) and these overlap such that no single pattern can be identified. The term diffraction is referred to such problems in which one considers the resultant effect produced by a limited portion of a wavefront.
Diffraction phenomena are a part of our common experience. The luminious border that surrounds the profile of a mountain just before the sun rises behind it, the light streaks that one sees while looking at a strong source of light with half shut eyes and the coloured spectra (arranged in the form of a cross) that one sees while viewing a distant source of light through a fine piece of cloth are all examples of diffraction effects.
Augustin Jean Fresnel in 1815, combined in a striking manner Huygens wavelets with the principle of interferences and could satisfactorily explain the bending of light round obstacles and also the rectilinear propagation of light.
Services: - Diffraction Homework | Diffraction Homework Help | Diffraction Homework Help Services | Live Diffraction Homework Help | Diffraction Homework Tutors | Online Diffraction Homework Help | Diffraction Tutors | Online Diffraction Tutors | Diffraction Homework Services | Diffraction