# IB Physics HL Topic 11

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 11.1.1 - Describe the nature of standing (stationary waves) A standing wave is the product of the propagation of 1 wave against a wall and its reflected wave with the same speed, same wavelength, same amplitude, opposite direction. Because the wave is reflected, the energy that is propagated returns to the same point of origin. Velocity=displacement / time, and since there is no displacement, the wave has no velocity. As well, we say that no energy is propagated. 11.1.2 - Explain the formation of one-dimensional standing waves As one wave is propagated at a certain fundamental frequency by the vibration of a source and at point B, it hits point A and is reflected back to point B at pi out of phase (i.e. as a particle on the forward wave reaches its amplitude, a particle in the same position on the reflected wave is half way to its amplitude) forming a standing wave with a greater amplitude 11.1.3 - Discuss the modes of vibration of strings and air in open and in closed pipes See image. 11.1.4 - Compare standing waves and travelling waves See image. 11.2.1 - Describe what is meant by the Doppler effect 1. Doppler Effect - The change of frequency of a wave due to the movement of the source or the observer relative to the medium of wave transmission. 11.2.2 - Explain the Doppler effect by the reference to wavefront diagrams for moving-detector and moving-source The source moves towards observer B and away from observer A. The wavecrests are piling in front of the source and thus the crests reach B at time intervals which are shorter than those on emission. 11.2.3 - Apply the Doppler effect equations for sound ATTA 11.2.6 - Outline an example in which the Doppler effect is used to measure speed See image. 11.3.1: Sketch the variation with angle of diffraction of the relative intensity of light diffracted at a single slit. See image. 11.3.2: Derive the formula θ=λ/b for the position of the first minimum of the diffraction pattern produced at a single slit. See image. 11.4.1: Sketch the variation with angle of diffraction of the relative intensity of light emitted by two point sources that has been diffracted at a single slit. Students should sketch the variation where the diffraction patterns are well resolved, just resolved and not resolved. 11.4.2:State the Rayleigh criterion for images of two sources to be just resolved CMOFM Students should know that the criterion for a circular aperture is θ=1.22*λ/b 11.4.3 - Describe the significance of resolution in the development of devices such as CDs and DVDs, the electron microscope and radio telescopes Radio Telescopes – the wavelength of radio waves is large, so the telescope needs to be large so that the resolution is good CDs – the information on the drive is read by lasers reflecting from the surface. A laser with a higher resolution can read more information from the surface of a disk. Electron Microscope – electrons are propagated with a very short wavelength through a species. Hence, the image produced is very clear due to the microscope’s high resolution power 11.5.1 - Describe what is meant by Polarized light 1. Polarized Light – light in which the electric field vector vibrates in one plane only 11.5.2 - Describe polarization by reflection When unpolarized light falls on a material the reflected light is usually partially polarized. At one particular angle, called the polarization angle, the partially reflected light is completely polarized. At this angle the partially reflected ray and the refracted ray are at right angles to each other. The vibrations in the partially reflected ray are parallel to the surface. This may be illustrated using light or microwaves. The use of polarized sunglasses should be included. 11.5.3 - State and apply Brewster’s law 1. Brewster’s Law – When light is incident on a surface at such an angle that the reflected and transmitted rays are perpendicular, the reflected ray is completely plane polarized. Then the index of refraction of the substance is equal to the tangent of the angle of incidence. (n = tan theta) 11.5.4 - Explain the terms polarizer and analyzer 1. Polarizer – device that produces plane polarized light from an unpolarized beam 2. *Analyzer – polarizer used to detect polarized light 11.5.5: Calculate the intensity of a transmitted beam of polarized light using Malus’ law. Malus’ Law – The intensity of transmitted polarized light is equal to the product of the incident intensity and the square of the cosine of the angle between the transmission axes of the polarizer and the analyzer. (I = Io cos2 θ ) 11.5.6 - Describe what is meant by an optically active substance Certain crystals (e.g. quartz) and liquids (e.g. sugar solutions) rotate the plane of vibration of polarized light passing through them. They’re called ‘optically active’.Rotate plane of polarization of light. 11.5.7: Describe the use of polarization in the determination of the concentration of certain solutions. For a solution the angle through which the plane of vibration has been rotated depends on the concentration of the solution. A polarimeter can be used to measure the concentration 11.5.8 - Outline qualitatively how polarization may be used in stress analysis When glass, perspex, polythene and some other plastics are under stress they become doubly refractive. If they are viewed in white light between two crossed Polaroids, coloured fringes are seen around the regions of strain. This effect is called ‘photoelasticity’ and is used to analyse stresses in plastic models of various structures. 11.5.9: Outline qualitatively the action of liquid-crystal displays (LCDs). In a LCD display a thin liquid crystal cell is used in which the crystals are arranged so that their alignment changes through 90 degrees from one face of the cell to the other. If the cell is place between two crossed polarizing films, light can pass through because the 90 degree twist in the liquid crystals rotates the light through 90 degrees. The glass faces of the cell have a conducting coating through which a pd can be applied. An applied pd across the liquid crystal cell creates an electric field. The liquid crystals all align with the electric field. The liquid crystals can no longer rotate the polarized light.