DIFFRACTION OF LIGHT WAVES

Diffraction effect becomes significant only when wavelength of wave comparable or bigger than aperture size/ aperture size comparable or smaller than wavelength of wave ß know how to state as suitable condition for observable diffraction/interference pattern Single slit: Diffraction pattern: Pattern of light and dark bands due to different points along slit creating wavelets that interfere with one anotherGraph of Intensity against angle (for narrow and wide slit):For wider slit, higher intensity, narrower central maximum (see equation to find angular width of central maximum).Imagine the single slit divided into two halves, each ray coming from each half of the slit and interfering, hence effective slit width is .For destructive interference, the two light rays must have path difference of .Path difference across slit = (use radians!)KNOW HOW TO DRAW.When narrower slit used, b decreased, increases and central maximum becomes wider.KNOW HOW TO DRAW graph of intensity against .Hence:For circular aperture, first minimum occurs at .is the angular position of the first minimum.is the angular width of the central maximum. Resolution: Minimum resolvable distance between distinguishable objects in an image(Significance: In CD, DVD and Electron microscope, allow more data to be stored per unit area. , smaller wavelengths used e.g. HD blu-ray)Angular resolution: Minimum angular distance between two light sources that can be resolvedRayleigh criterion: Two sources are just resolvable when first minimum of diffraction pattern of one source falls on central maximum of diffraction pattern of other sourceDistance that can be resolved, , where r is the distance from the source (camera, eye, radio telescope).Outline how Rayleigh criterion affects the design of radio telescopes: Since wavelength of radio waves is relatively larger, in order to get a good resolution (small angle) the aperture must be large. This is achieved by having large dish-like receivers AND separating the receivers by large distances.A car, with its two headlights switched on, is approaching an observer who has good eyesight. Outline why, at a long distance from the observer, the images of the headlights of the car are not resolved by the observer:Diffraction of light occurs in the iris of the eye. Each lamp gives rise to a separate diffraction pattern on the back of the eye, the retina. For distant lamps the two diffraction patterns overlap, and hence the patterns of the two headlights cannot be distinguished. Polarization (LIGHT WAVES): Polarized light: Electric field vector oscillate in a single planeUnpolarized light: Plane of vibration of the electric vector is continually changing.Polarized light is not transmitted through a polarized film whose axis is perpendicular to the polarization direction.When light goes through a polarizer, only the component parallel to the polarization axis is transmitted, liquid crystal changes the plane in which the electric field vector rotates.Optically active substances rotate the plane of polarization of a beam of polarized light. Rotation results from interaction between molecules in material and incident light, as molecular structure is not symmetrical. Degree of rotation depends on the depth of liquid, hence different colours seen at different depths when viewed through a stationary polarizing filter.Barber Pole Demo: Polarized white light passes through the optically active liquid, plane of polarization of different colours changes by different amounts, different colours scattered in different directions, hence different colours seen at different heights.IMPORTANT: When unpolarized light first passes through a polarizer of any angle, its intensity is always reduced by half.Malus’ Law:Brewster’s angle: The angle of incidence at which a reflected beam of light is purely/100% polarized parallel to the surface (occurs when light strikes a non-metallic surface at any angle other than the perpendicular/normal).(no polarization occurs at normal incidence) for light passing through medium1 and reflected from medium2.Polaroid sunglasses: Vertical axes eliminate the more strongly reflected horizontal component (polarized light is not transmitted through a polarizing film whose axis is perpendicular to the polarization direction), hence reducing the glare.Saccharimeter: Polarized light used to determine angle of rotation hence measure concentration of sugars in a solution.Photoelastic stress analysis: Polarized light passed through isotropic substances. When substance is subjected to stress coloured fringes are displayed. Different colours of light are refracted through different angles depending on stress imposed on substance. Studying the stress pattern allows analysis of the design of the model.Liquid Crystal Displays (in calculators and digital watches): Liquid crystal placed between two electrodes, a reflector is placed at the back (use ambient/surrounding light). When external voltage is applied across different regions of the liquid crystal, plane of polarization of the crystal changes, reducing the amount of transmitted light in those regions, hence some parts of the display appear darker and some lighter, producing an image.

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