Lecture Notes (Applications of Diffraction) Intro: - the iridescent colors seen in many beetles is due to diffraction of light rays hitting the small groovess of its exoskeleton - these ridges are only a few hundred nanometers apart, so they act as a series of slits that diffract light - this will result in an interference pattern similar to that of the double-slit experiment - the wings of moths and butterflies may be plain-colored or may have striking and colorful patterns; the wings themselves don't have much color; it is the presence of scales on the wings that produce the patterns Butterfly wing magnified about 75 times.
- each wing may have hundreds or thousands of scales that contribute to the wing color pattern - when you catch a butterfly or moth you may notice a powder that rubs off of them; this powder is a bunch of tiny scales - in the same way, the grooves on a CD diffract many colors light and produce Diffraction Gratings s: - diffraction grating consists of a block of glass grooves cut into it by a diamond lathe with many - these grooves are very close together and, once cut, become opaque; therefore the block of glass acts like many double slits - typically these gratings have thousands of grooves per centimeter, which makes the distance between the slits very small - for example, if theree are 10,000 grooves per centimeter distance between each pair of slitss will be 10 4 cm then the - this resultss in interference patterns being very incident on a screen spread out when
- at observatories, diffraction gratings are used in spectrographs so that astronomers may study the spectra of light from celestiall sourcess - a spectrograph takes light from a source and separates it by wavelength, so that the red light goes in one direction, the yellow light in another direction, the blue light in another direction, and so forth - there are two basic ways of dispersing light; you can pass it throughh a prism; or you can bounce it off (or pass it through) a diffraction grating - most astronomers these days use gratings, not prisms because light must passs through a prism, some of the light rays will be absorbed or scattered -- in other words, lost - light needs only bounce off the surface of a grating, so that a largerr fraction of the light reaches the detector - astronomers often place a slit over thee focal plane of the telescope, centered on the object of interestt
- only light which passes through this slit will strike the grating (or prism), giving the spectrum a characteristicc shape: vertical lines on a long, horizonta al canvas Resolving Power: - the interference and diffraction effectss have consequences for the development of optical instruments - even if a telescope were of perfect optical quality, it would not produce perfectly focused images because of the nature of light itself - a telescope s resolving power is its ability to produce sharp, detailedd images under ideal observing conditions - resolving power depends directly on the size of the aperture and inversely on the wavelength of thee incoming light; for the same light, a 6-inch telescope has twice the resolving power of a 3-inch telescope - starlight travels in straight lines through empty space, but when waves of starlight pass close to thee edge of a lens or mirror, they spread out, due to diffraction, and come to a focus at different spots
- because of diffraction, the image of a star formed by a lens or mirror appears under magnification as a tiny, blurred disk surrounded by faint rings, called a diffraction pattern, instead of as a single point of lightt - diffraction limits resolving power - if two starss slightly separated in the sky are observed, overlapping diffraction patterns will be seen - if the sources of light move closer together the diffraction fringes confuse viewing of the sources; eventually you may not be able to identify the two separate sources as the bright central maximaa merge - the resolving power of an instrument is the angular separation of the sources, which enables you to tell you are viewing two sourcess
- this is given by the following formula, known as the Rayleigh Criterion θ = 2.5 10 5 λ/ /d where θ is the angle subtended between the sources measured in seconds of arc; λ is the wavelength of the light used in meters; d is the diameter off the aperture in meters - similar effects are produced when viewing two specimens using microscopes - the Hubble telescope's location abovee the Earth's atmosphere, allows it to produce high resolution images of astronomical objects - ground-based telescopes can seldom provide resolution better than 1.0 arc-seconds, except momentarily under the very best observing conditions; Hubble's resolution is about 10 times better, or 0.1 arc-secondss - in the photographs below, the image on the right is a portion of the first image returned by the Wide Field Planetary Camera on the Hubble Space Telescope; on the leftt is a ground based image of the same area of the sky - the object shown in these images is a double star: the pair of stars is well separated in the Hubble image but blurred together in the ground based image
- th he object shown s in the imag ges below w is a spiraal galaxy called M100: the indiv vidual starrs are ablee to be seeen in the Hubble b are bllurred tog gether in tthe grounnd based iimage image but