![]() It was able to "invert" only the part of the wave front that it was able to collect, due to its limited size. These intensity changes depend on the WaveLength: the shorter the wavelength, the more rapidly the intensity decays.īut the lens did not trap the whole original wave front to transform it all. The further we go away from the center, the more destructive is the average resulting interference. At any other point away from the center some waves will arrive "almost" in phase, but many others will have "almost" opposite phase, so the interference is not totally constructive, and the resulting intensity decreases. If we put there a screen or a detector, we will see a hight intensity point at the focus. At the other side of the lens (the image side) points of the space are excited and become secondary sources in such a way that they produce an inverted (mirrored) spherical wave front.Īt the central point of that new sphere (the focus, that is at equal distance from all the secondary sources), all the wave fronts from the secondary sources arrive in phase, resulting in a constructive interference. We can see a focusing (convergent) lens as an optical device that, by introducing the appropriate delays in the light paths (more delay in the center, and less in the borders) "inverts" the shape of a spherical wave front coming from a punctual source in the object. The same is true of light passing the edge of an obstacle, but this is not as easily observed because of the shorter wavelength of visible light ( wikipedia ). As far as the second room is concerned, the vibrating air in the doorway is the source of the sound. ![]() For example, if two rooms are connected by an open doorway and a sound is produced in a remote corner of one of them, a person in the other room will hear the sound as if it originated at the doorway. This simple idea helps to understand a lot of phenomena, such as diffraction. An observer on the other side of the wall will detect radiation as being emitted by the hole itself, in a particular wave front that depends on the shape of the hole. If we make a small hole in this wall, only the interior of the hole will get excited and emit to the other side. If the obstacle is an infinite wall, no radiation will cross to the other side. Therefore, the resulting addition of secondary fronts will no longer produce a sphere. If we put now an obstacle in the way of the spherical wave front we avoid the excitation of some of the secondary sources. As we actually have infinite secondary point sources in a spherical disposition, the addition of all their wave fronts makes again a spherical wave front that keeps propagating, again and again, from one sphere to the next one around it. The Huygens-Fresnel principle says that the shape of the wave front a moment later comes from the sum of all the waves arising from these secondary sources. These points behave themselves as new sources of radiation. To describe how this wave propagates in space, how its shape would be a moment later based on its shape at present time, we can imagine the original wave at any moment as if it were "exciting" the points of the space that it reached. Without any obstacle, this radiation is "equal" in all directions, it has a spherical wave front traveling away from the source. ![]() This radiation can be any type of waves, like sound or visible light. It applies to situations like this: imagine a single mathematical point source emiting radiation in all directions. It is just a simplified model to describe the behavior of waves in space. To understand the image formation in a microscope, we will make use of the Huygens Principle.
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