![]() But some people disagreed with him, most notably Isaac Newton. As we discussed in the atom about the Huygens principle, Christiaan Huygens proved in 1628 that light was a wave. The double-slit experiment, also called Young’s experiment, shows that matter and energy can display both wave and particle characteristics. Explain why Young’s experiment more credible than Huygens’.The direction of propagation is perpendicular to the wavefront, as shown by the downward-pointing arrows. The tangent to these wavelets shows that the new wavefront has been reflected at an angle equal to the incident angle. The wavelets shown were emitted as each point on the wavefront struck the mirror. Reflection: Huygens’s principle applied to a straight wavefront striking a mirror. The ray bends toward the perpendicular, since the wavelets have a lower speed in the second medium. Huygens’s Refraction: Huygens’s principle applied to a straight wavefront traveling from one medium to another where its speed is less. shows visually how Huygens’s Principle can be used to explain reflection, and shows how it can be applied to refraction. The principle is helpful in describing reflection, refraction and interference. This principle works for all wave types, not just light waves. The new wavefront is tangent to the wavelets. The emitted waves are semicircular, and occur at t, time later. Where s is the distance, v is the propagation speed, and t is time.Įach point on the wavefront emits a wave at speed, v. Another way to describe this situation is that the larger the NA, the larger the cone of light that can be brought into the lens, and so more of the diffraction modes will be collected.\] Lenses with larger NA will also be able to collect more light and so give a brighter image. A lens with a large NA will be able to resolve finer details. In a microscope, NA is important because it relates to the resolving power of a lens. It can be shown that, for a circular aperture of diameter D, the first minimum in the diffraction pattern occurs at \theta=1.22\frac\\ Just what is the limit? To answer that question, consider the diffraction pattern for a circular aperture, which has a central maximum that is wider and brighter than the maxima surrounding it (similar to a slit) (see Figure 2a). How far away can you be and still distinguish the two lines? What does this tell you about the size of the eye’s pupil? Can you be quantitative? (The size of an adult’s pupil is discussed in Physics of the Eye.) Take-Home Experiment: Resolution of the Eyeĭraw two lines on a white sheet of paper (several mm apart). Telescopes are also limited by diffraction, because of the finite diameter D of their primary mirror. ![]() So diffraction limits the resolution of any system having a lens or mirror. Thus light passing through a lens with a diameter D shows this effect and spreads, blurring the image, just as light passing through an aperture of diameter D does. Be aware that the diffraction-like spreading of light is due to the limited diameter of a light beam, not the interaction with an aperture. The acuity of our vision is limited because light passes through the pupil, the circular aperture of our eye. There are many situations in which diffraction limits the resolution. ![]() This limit is an inescapable consequence of the wave nature of light. ![]() If they were closer together, as in Figure 1c, we could not distinguish them, thus limiting the detail or resolution we can obtain. The pattern is similar to that for a single point source, and it is just barely possible to tell that there are two light sources rather than one. How does diffraction affect the detail that can be observed when light passes through an aperture? Figure 1b shows the diffraction pattern produced by two point light sources that are close to one another. (c) If they are closer together, they cannot be resolved or distinguished. (b) Two point light sources that are close to one another produce overlapping images because of diffraction. (a) Monochromatic light passed through a small circular aperture produces this diffraction pattern. ![]()
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