![]() ![]() Is a wave phenomenon and is also observed with water waves in a ripple tank. The light spreads around the edges of the obstacle. The other side of the opening resembles the wave front shown on the right. Wavelength λ of the light, in an otherwise opaque obstacle, the wave front on When light passes through a small opening, comparable in size to the Replace any wave front by a collection of sources distributed uniformly over the When studying the propagation of light, we can Huygens' principle also holdsįor electromagnetic waves. With each other to produce the traveling wave. The wavelets emitted by all points on the wave front These wavelets propagate outward with the characteristic Lets us treat wave propagation by considering every point on a wave front to be a secondary source of Diffraction results from the interference of an infinite number of wavesĮmitted by a continuous distribution of source points in two or three dimensions. Passing through a finite aperture to spread out as it propagates. (We already encountered interference when studyingĭiffraction is the tendency of a wave emitted from a finite source or Mathematical treatment is much more involved.ĭiffraction and interference are phenomena observed with all waves.ĭiffraction can only be observed with waves traveling in two or threeĭimensions. ![]() Wave optics contains all of ray optics, but the If the wavelengths of the light become comparable to the dimensions of theĮquipment, then we study optical phenomena using the classical theory of radiation, or wave optics. So, as far as visible light is concerned, matter is quasi-continuous. Individual atoms in a solid are separated by distance on the order of 0.1 nm. So a laser beam with a diameter of 1 mm has a diameter of 2000 wavelengths. Wavelengths in the middle of the visible band are on the order of 500 nm. Consider the wavelength scale of light waves. Under these conditions we can make an approximation called In many situations, the wavelengths of the light being studied are very smallĬompared to the dimensions of the equipment used to study the light. Light is a transverse electromagnetic wave. In classical physics, we can classify optical phenomena into one of two categories: ray optics and wave optics. By destructive interference, the light from one half of the slit completely cancels the light from the other half.Single slit diffraction Single slit diffraction If we now have N sources spread out across the space between the two sources, we have have half the sources giving path length differences between 0 and l/2, and the other half giving path length differences between l/2 and l. Let's go to a place on our screen that was one wavelength ( l) further from one source than the other. Why do we have destructive interference occuring for the single slit at the same angles where we had constructive interference occuring for two sources? We call this a diffraction pattern, but it still comes from interference of waves. This is why we can treat the opening as containing a large number of sources. Bringing in Huygen's Principle, every point in the opening can be treated as a source of wavelets, waves that spread out spherically. Our situation above is essentially what happens when light shines on a narrow opening, or when sound or any other wave encounters an opening comparable in size to the wavelength. We still get a bright spot at the center of the screen due to constructive interference, but we find that at all the other places where we had constructive interference taking place for the two sources, we now have destructive interference.Ĭondition for destructive interference for a single slit: a sin( q) = m l, where m is any integer other than zero. Now we remove the barrier between the sources, which changes the interference pattern. Let's return to our two-source situation, two sources separated by a distance that we'll now call a. Increasing the size of the opening reduces the spread in the pattern. The smaller the object the wave interacts with, the more spread there is in the interference pattern. This is true for single slits, double slits, and diffraction gratings. If we increase the width of the slit, what happens to the central maximum in the diffraction pattern? We shine red laser light through a single slit, and we see a diffraction pattern on a screen some distance from the slit.
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