PowerPoint Presentation

Optics DIFFRACTION

DISPERSION

SCATTERING

DIFFRACTIONDiffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening. However, if the two are closer in size or equal, the amount of bending is considerable, and easily seen with the naked eye.In the atmosphere, diffracted light is actually bent around atmospheric particles -- most commonly, the atmospheric particles are tiny water droplets found in clouds.

Atmospheric DiffractionDiffracted light can produce fringes of light, dark or colored bands. An optical effect that results from the diffraction of light is the silver lining sometimes found around the edges of clouds orcoronas surrounding the sun or moon.

InterferencesOptical effects resulting from diffraction are produced through the interference of light waves.As these waves spread outward in all directions from the float, they interact with other water waves. If the crests of two waves combine, an amplified wave is produced (constructive interference). However, if a crest of one wave and a trough of another wave combine, they cancel each other out to produce no vertical displacement (destructive interference).

Huygens PrincipleAccording toHuygens' principle, every point on a wave front of light can be considered to be a secondary source of spherical wavelets. Thesewaveletspropagate outward with the characteristic speed of the wave. The wavelets emitted by all points on the wave front interfere with each other to produce the traveling wave. Huygens' principle also holds for electromagnetic waves.

Application of DiffractionIn an optical storage device such as a CD-ROM or DVD, laser light is focused by a lens onto the surface of the disk. The lens, while focusing the light, also serves as a circular aperture. Thus, the light coming out will produce an interference pattern on the disk. If more than one bit falls inside the central bright spot, the bits cannot be resolved and information will be lost. This limit of resolvability is described by the Rayleigh criteria: sinq1.22l/d.The bit must be no smaller than a circle of radiusr, wherer/D=tanq~1.22l/d.r~ 1.22lD/d

Youngs ExperimentSuppose we have 2 slits separated by a distance, d and a distance L from a screen. Let point P be a bright fringe.We see in the figure that we can make a right triangle using, L, and y, which is the distance a fringe is from the bright central. We will use an angle, q, from the point in the middle of the two slits.

Schematic Representation

Singleslit

Lightsource

DISPERSIONChromatic dispersion is the change ofindex of refractionwith wavelength. Generally the index decreases as wavelength increases, blue light traveling more slowly in the material than red light. Dispersion is the phenomenon which gives you the separation of colorsin aprism.

Usually the dispersion of a material is characterized by measuring the index at the blue F line of hydrogen (486.1 nm), the yellow sodium D lines (589.3 nm), and the red hydrogen C line (656.3 nm). The dispersion is measured by a standard parameter known as Abbe's number, or the v value or V number, all of which refer to the same parameter:

Abbes NumberThe Abbe number VD, of a material is defined aswherenD,nFandnCare the refractive indices of the material at the wavelengths of theFraunhoferD-, F- and C-spectral lines(589.3nm, 486.1nm and 656.3nm respectively).

Effect of DispersionThe effect of dispersion on the focal length of a lens can be examined by calculating the change in the focal length with wavelength.

SCATTERINGIf a beam of light illuminates a rough surface, or a cloud of small particles, some of the light is removed from the beam and redistributed in all directions. This angular redistribution is calledscattering.The scattered rays go off in many directions different from that of the incident light.On the other hand, light falling on a polished surface is reflected in a particular direction. The typical example of such a surface is a mirror; so this reflection is calledspecular(as opposed todiffuse, for scattered light).

Aerosol ScatteringLight scattered by the atmosphere makes air visible. This air light is mostly scattered by small particles suspended in the air. The absence is scattered light makes cloud shadows visible, producingcrepuscular rays.The small scattering particles suspended in air are called aerosol, so this kind of scattering is calledaerosol scattering. The aerosol may be small mineral grains (dust) or droplets of salt water. Smog contains aerosol particles produced by photochemical reactions, a result of sunlight shining on hydrocarbon vapors. Smoke and diesel-engine exhaust contain small carbon particles that scatter as well as absorb light.

Most of the aerosol particles are just a few wavelengths of light across, so the wave nature of light must be used to calculate aerosol scattering. As a small obstruction can diffract light much like a pinhole of the same size, much of the scattered light is scattered bydiffraction.

However, because the aerosol particles are usually a little bigger than the wavelength of light, they scatter all wavelengths about equally well. The scattered light is, on the average, white. That's why clouds are white, and a smoggy sky is whitish or grayish.

Sometimes there's a slight preference for the shorter wavelengths. If the particles are comparable to the wavelength in size, a very fine aerosol can look bluish. A familiar example is cigarette smoke. As clear air contains a mixture of aerosol particle sizes, the scattered light usually has a slightly bluish tinge.

But even perfectly clean air still scatters some light. The molecules themselves, though much smaller than the wavelengths of light, can still scatter a little. But, because the molecules are smaller than the wavelength, they scatter short wavelengths (which are more nearly comparable to the size of the molecules) better than long wavelengths.

Rayleigh scatteringis the cause of the blue sky, the shortest wavelengths of sunlight (blue and violet) are scattered better than the longer ones, and the average color of the scattered light is the blue of the sky. This color is much more intense than the pale blue of aerosol scattering.

Because the short wavelengths are selectively scattered, the remaining direct sunlight contains mostly the longest wavelengths: red and orange. That's why the setting Sun looks reddish. This removal of light by scattering is the main component of atmosphericextinction.

Rayleigh ScatteringRayleigh scattering refers to the scattering of light off of the molecules of the air, and can be extended to scattering from particles up to about a tenth of the wavelength of the light. It is Rayleigh scattering off the molecules of the air which gives us theblue sky. Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons is not changed. Scattering in which the scattered photons have either a higher or lower photon energy is calledRaman scattering.