![]() ![]() The black bands between the blobs of light show that a wave is associated with the light. Rotate each object while you look through it. Look at the light through a piece of cloth, a feather, a diffraction grating, or a piece of metal screen. Rotate the hair and watch the line of blobs rotate. Move the hair until it is between your eye and the light source, and notice that the light is spread into a line of blobs by the hair, just as it was by the slit. Stretch a hair tight and hold it about 1 inch (2.5 cm) from your eye. Notice that the blobs have blue and red edges and that the blue edges are closer to the light source. As you squeeze the slit together, the blobs of light grow larger and spread apart, moving away from the central light source and becoming easier to see. If you look closely you may see that the line is composed of tiny blobs of light. While looking through the slit, rotate the pencils until they are horizontal, and notice that the line of light becomes vertical. Notice that there is a line of light perpendicular to the slit. Squeeze the pencils together, making the slit smaller. Hold both pencils close to one eye (about 1 inch away) and look at the light source through the slit between the pencils. The tape wrapped around one pencil should keep the pencils slightly apart, forming a thin slit between them, just below the tape. Hold up the two pencils, side by side, with the erasers at the top. I just want to understand this, by using huygens principle ).Place the light on a stable surface at least one arm’s length away from you. ( I know how the fringes appear using integration over a wave and then finding conditions of maxima and minima. However, this would cause a diffraction patter on any screen irrespective of the presence of an obstacle. But using this logic, if we just consider the simple spreading out of light from a point source, then each point on this spherical wavefront should also act as secondary sources which should interfere with each other. However, after passing through an obstacle, these secondary sources interfere with each other to create bright and dark patterns. In any scenario, each point on a wavefront, acts as a secondary source. Else, we would get a bright and dark band pattern by simply shining light on an object. However, we don't consider the interference of the waves from these secondary sources in the absence of a slit. However, if there was no slit, even then each point on the spherical wavefront would act as secondary source. The case with the slit is explained by saying that after the wave comes out of the slit, each point acts as a secondary source, and the waveforms from these sources interfere constructively and destructively. However, if I now put a single slit between the source and the screen, the intensity pattern would show fringes. There would be an uniform intensity distribution on the screen. Now suppose, there is a screen in front of the source. In case of the simple double slit experiment, the idea of two separate wavefronts interfering with each other to create bright and dark bands seem to make sense.īut now, imagine a single source from which the wavefront spreads in all directions i.e. This self interaction doesn't seem to make sense to me. ![]() Many books suggest, this is because the secondary sources on this new 'semi-circularish' wave-front, produce spherical waves that interfere with one another, to create points of constructive and destructive interference. Huygen's theory seems to explain why light bends around obstacles, but not why we should get a fringe pattern in case of a single slit. Now, all this is well and good, but I don't seem to understand where do the 'fringes' come from. I know this can also be explained using the uncertainty principle, but my goal is to understand Huygens' theory properly.Īnyway, as we can see, the wavefront spreads into the geometric shadow. Hence, our wavefront now looks more like this: When part of this wave is blocked by an obstacle,the 'top-most or bottom-most' part of the new wave would propagate spherically as there is no longer anything above or below, to flatten the wavefront. Normally, each point on the plane wave would act as a spherical wavefront, and the common envelope of all these wavefronts would also be a plane wave. Using Huygens' theory, one explains this by imagining a plane wave hitting a slit. I understand that diffraction is the bending of light around sharp edges. ![]()
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