From Last Time…The electric and magnetic force and fieldsProperties of EM WavesWave effects in EM radiationInterference of light wavesDestructive interferenceInterference: secondary maximaResulting diffraction patternHertz’s measurement: the speed of electromagnetic wavesLaser pointer interferenceComplex interference patternsX-ray diffractionVisible LightWhite light is a superpositionSeeing colorsRods and conesEye sensitivityInterpreting colorsRed + Green = ?QuestionRelativity and Modern PhysicsSpecial RelativityFrames of referenceWhich reference framePowerPoint PresentationGalilean relativityNewton’s laws in moving framesExample of Galilean relativityGalilean relativity: exampleSome other examplesTurning this around…What about electromagnetism?Phy107 Fall 20061From Last Time…•Electric and magnetic fields•Light, Doppler effect, interferenceToday…Interference, the speed of lightRelativityHW#5: Chapter 10: Conceptual: # 6, 11, 17, 22 Problems: # 4, 6, 8Due: Oct 18thPhy107 Fall 20062The electric and magnetic force and fields€ F =kq1q2r2€ F = qE•Electric field is from a charge and exerts a force on other charges•Magnetic field is from a moving charge and exerts a force on other moving charges!€ F = qvB€ E =kQr2•Changing electric or magnetic fields can cause magnetic or electric fieldsPhy107 Fall 20063Properties of EM Waves•Light is a set of electric and magnetic fields where the changing electric field creates the magnetic field and the changing magnetic field creates the electric field•Only works when the fields change from up to down and back again at the speed of light•The speed of light is a special value - we’ll see this again in Einstein's relativity.•Has all properties of a wave:€ v = λf€ c =Phy107 Fall 20064Wave effects in EM radiation•Same properties as sound waves: common to all waves.•Doppler shift: change in light frequency due to motion of source or observer•Interference: superposition of light waves can result in either increase or decrease in brightness.Phy107 Fall 20065Interference of light waves•Coherent beams from two slits•Constructive interference:waves in phase at screenPhy107 Fall 20066Destructive interference€ (n +12)λPhy107 Fall 20067Interference: secondary maxima€ nλPhy107 Fall 20068Resulting diffraction patternPhy107 Fall 20069Hertz’s measurement: the speed of electromagnetic waves•Hertz measured the speed of the waves from the transmitter–He used the waves to form an interference pattern and calculated the wavelength–From v = f , v was found–v was very close to 3 x 108 m/s, the known speed of light•This provided evidence in support of Maxwell’s theory•This idea still used today measure wavelengths when studying starsPhy107 Fall 200610Laser pointer interferenceEach clear area on the slide acts as a light source.Interference with many light sources is sometimes called diffraction.QuickTime™ and aGraphics decompressorare needed to see this picture.Phy107 Fall 200611Complex interference patternsWhite spaces act as array of sources.The ‘diffraction pattern’ contains information about the original pattern.QuickTime™ and aGraphics decompressorare needed to see this picture.Phy107 Fall 200612DNA X-ray diffraction patternX-ray diffraction•X-rays are short-wavelength EM wave. Short wavelengths probe small spacings•Diffraction pattern used to determine atomic structure of complex molecules.–e.g. DNADNA molecular structurePhy107 Fall 200613 Visible Light•We see only a narrow range of the EM spectrum•400-700nm•To someone who could see the entire spectrum, our limitation to this narrow range might seem odd.Phy107 Fall 200614White light is a superposition•Prism can separate the superposition into it’s constituents.•For example, ‘white’ light is an almost equal superposition of all visible wavelengths (as well a invisible ones!)•This is a simple analyzer to ‘deconstruct’ a superposition of light waves (how much of each wavelength is present in the light).Phy107 Fall 200615Seeing colors•Rods and cones send impulses to brain when they absorb light.•Brain processes into color information.Cones, 3 typesRods (one type)Phy107 Fall 200616Rods and cones•Rods are responsible for vision at low light levels. No color sensitivity •Cones are active at higher light levels•The central fovea ispopulated only by cones. •3 types of cones –short-wavelength sensitive cones(S)–middle-wavelength sensitive cones(M) –long-wavelength sensitive cones(L)Cones, 3 typesRods(one type)Phy107 Fall 20061700.10.20.30.40.50.60.7400 440 480 520 560 600 640 680ENERGY SENSITIVITYWAVELENGTH ( nm )Eye sensitivity•Eye’s wavelength sensitivity by cone type.•Sensitivities overlap.S-conesM-conesL-conesFor instance, pure yellow (single wavelength of 570Fnm) stimulates both M and L cones.M-cone: 0.44L-cone: 0.52S-cone: 0Phy107 Fall 200618Interpreting colors•Each cone sends a signal in relation to its degree of stimulation•A triplet of information (S, M, L) is conveyed.•Brain uses only this information to assign a color•Any light generating same (S, M, L) ‘seen’ as same color0.440.00.52SMLPhy107 Fall 20061900.10.20.30.40.50.60.7400 440 480 520 560 600 640 680ENERGY SENSITIVITYWAVELENGTH ( nm )Red + Green = ?•Combined Green + RedCompare to spectrally pure yellow (S, M, L)=(0,0.44,0.52)Total M-cone stimulus = 0.55+0.02 = 0.57Total L-cone stimulus = 0.49+0.17=0.66Reducing the intensity slightly (by 1.25) gives(S, M, L)=(0,0.45,0.52)Phy107 Fall 200620QuestionSuppose an eye has only two cones with spectral sensitivities shown here. It is stimulated by equal intensities of 300 and 700 nm pure spectral light. Which single wavelength might produce a similar color perception? A. 330 nmB. 430 nmC. 500 nmD. 530 nmPhy107 Fall 200621Relativity and Modern Physics•Physics changed drastically in the early 1900’s•New discoveries — Relativity and Quantum Mechanics•Relativity–Changed the way we think about space and time•Quantum mechanics–Changed our conceptions of matter.Phy107 Fall 200622Special Relativity•From 1905 to 1908, Einstein developed the special theory of relativity.•Came up completely different idea of time and space.•Everything is relative. No absolute lengths, times, energies.Showed that our usual conceptions of space and time are misguided.Phy107 Fall 200623Frames of reference•Frame of reference:–The coordinate system in which
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