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Chapter 24 - Light.docx

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McGill University
PHYS 101
Kenneth Ragan

Chapter 24 – Light and its Phenomena 24.1 – Waves Vs. Particles; Huygens’ principle and Diffraction: Light clearly carries energy, two ways this is done: particles (that is, physically carrying mass) and waves (the medium doesn’t carry, but energy is carried by the medium) Newton favored “particle/corpuscular” theory of light, taking as evidence the sharpness of shadows (as compared to the way sound waves bend around corners).  But experiments, concerning interference seemed to indicate that light was a wave.  It is said to be an electromagnetic wave, therefore it does not require a medium at all  In the 1900s, it became apparent that it had particle properties too, photos as “particles of light” Huygen’s Principle: a wave theory, which states that each point on a wave front can be considered as a new source of wavelets (starting point of new waves). The new wave front is the envelope of all the wavelets.  Useful for analyzing what happens when waves impinge an obstacle and the wave front is interrupted, also predicts that waves bend in behind the obstacle  Diffraction: the bending of waves around obstacles (occurs for waves, but not particles) The law of diffraction says that an “edge” the (angular) bending of the wave will be: 𝜃𝑑𝑖𝑓𝑓 = 𝜆/𝐷 This means a wave only casts a “shadow” if its size D is greater than the wavelength of the wave.  Diffraction, like refraction is a wave phenomenon. o It does occur for light, but the angles are so small that we do not normally perceive it: shadows appear sharp unless we look very closely o So Newton was wrong, and light appears to be a wave o The wave model of light accounts for diffraction, how ray models do not When light travels from one medium to another, its frequency does not change, but the wavelength does. 𝜆 𝜆𝑛 = 𝑛 Chapter 24 – Light and its Phenomena 24.3 – Interference: Young’s Double-Slit Experiment 1. Light from a single source falls on a screen containing two closely space slits 1 and S2 2. If light consisted of particles, one would see two bring lines on the screen behind the slit. 3. But instead a series of lines are observed 4. This is due to diffraction, the waves leaving the two small slits spread out  Constructive Interference: (bright area at the center of the screen) the waves from the two slits travel the same distance, so they are in phase: a crest of one wave arrives at the same time as a crest of the other wave. Hence the amplitudes of the two waves add to form a larger amplitude o Also occurs when the paths of the two rays differ by one wavelength (or any whole number of wavelengths) 𝑑sin𝜃 = 𝑚𝜆  Destructive Interference: (screen is dark) if one ray travels and extra distance of ½ wavelength, the two waves are exactly out of phase when they reach the screen: the crests of one wave arrive at the same time as the troughs of the other wave and so they add to produce a zero amplitude 1 𝑑sin𝜃 = 𝑚 + 𝜆 2 The bright fringes are beaks or maxima of light intensity, while the dark fringes are minima. The intensity of the bright fringes is greatest for the central fringe (m=0) and decreases for higher orders. Fringe spacing depends on wavelength, with longer wavelengths having larger spacing. So what happens if you put a mix of wavelengths onto the slit?  The zeroth order fringe (n=0) has no wavelength dependence, so it will be white light  But all higher order fringes (n>0) will be
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