Experiment 5: Spark Gap Microwave Generator Dipole Radiation, Polarization, Interference W14D2

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1 Experiment 5: Spark Gap Microwave Generator Dipole Radiation, Polarization, Interference W14D2 1

2 Announcements Week 14 Prepset due Fri at 8:30 am PS 11 due Week 14 Friday at 9 pm in boxes outside Sunday Tutoring May pm in Final Exam Monday May 21 9 am-12 noon. Johnson Athletic Center. 2

3 Outline Generating EM Waves Polarization Experiment 5: Spark Gap Microwave Interference 3

4 Generating Electric Dipole Electromagnetic Waves 4

5 Types of Radiation: 1. Sinusoidal oscillations of charges in a metal wire lead to antenna radiation. 2. Impinging and stopping of a beam of electrons in a metal target gives rise to x-rays or bremsstrahlung radiation. 3. Centripetal acceleration of electrons in a circular orbit leads to synchrotron radiation. 5

6 Group Problem: Radiation From Accelerating Charge (Simulation) Open up PhET Radiating Charge. Explore the radiation pattern from a charge undergoing different types of motion. 6

7 CQ: Accelerating Charge The point charge below got a kick a little before the moment shown. The direction of the kick was: 1. Up or down 2. Left or right 3. Cannot tell, depends on past history 7

8 Radiation From Oscillating Charge The radiation pattern from an oscillating charge: 8

9 CQ: Electromagnetic Radiation Which of the following statements is true? 1. A neutral object moving in a circle at constant speed radiates electromagnetic energy. 2. A charged particle moving at constant speed in a straight line radiates electromagnetic energy. 3. A neutral object moving at constant speed in a straight line radiates electromagnetic energy. 4. A charged particle object moving in a circle at constant speed radiates electromagnetic energy. 5. None of the above. 9

10 State of Polarization: Describes how the direction of the electric field in an EM wave changes at a point in space. 1. Linear polarization 2. Circular polarization 3. Elliptical polarization 10

11 Demo: Polarization of Microwaves K3 Some materials can absorb waves with the electric field aligned in a particular direction (for example, sunglasses) 3&show=0 11

12 Demo: Polarization of Radio Waves Dipole Antenna K4 4&show=0 12

13 Spark Gap Generator: An LC Oscillator Hertz s 450 MHz transmitter and receiver demonstrated the fundamentals of high-frequency technology in

14 Clothespin Spark Gap Antenna 1) Charge up gap (RC): time scale for RC circuit τ = RC = ( Ω)( F) = s 2) Breakdown! (LC): The oscillations damp out as energy is dissipated and some of the energy is radiated with the period T = 4l/c where l = 31 mm is the length of 1/2 of the dipole and c is the speed of light.! T = 4l / c = 4( m) / ( m s 1 ) T = s 1 f = 1/ T = Hz 14

15 Experiment 5 Spark Gap Generator: Find the Angular Distribution of Radiation, and its Polarization 15

16 Interference 16

17 Demo Ripple Tank C31 17

18 Huygens-Fresnel Principle All points on a wave front act as point sources of spherical secondary wavelets which propagate outward with speeds characteristic of waves in that medium. At some later time, the new wave front is the surface tangent to all the wavelets 18

19 Superposition Principle: Interference When two or more beams of radiation are superimposed, the distribution of the intensity in the region of the intersection of the two beams varies from point to point between maxima which exceed the sum of the individual intensities and minima which may have zero intensity. 19

20 Young s Experiment: Coherent Light of Fixed Wavelength Bright Fringes: Constructive interference Dark Fringes: Destructive interference 20

21 Interference: Waves and Particles No Interference: if light were made up of particles Interference: If light is a wave we see spreading and addition and subtraction 21

22 Lecture Demonstration: Double Slit Interference for Light 10&show=0 22

23 Interference Interference: Combination of two or more waves to form composite wave use superposition principle. Conditions for interference: 1. Coherence: the sources must maintain a constant phase with respect to each other 2. Monochromaticity: the sources consist of waves of a single wavelength 23

24 Interference Phase Shift Consider two traveling waves, moving through space: In phase: Look here as function of time Constructive Interference (zero phase shift) 180 degree phase shift: Look here as function of time Destructive Interference (180 degree phase shift) 24

25 Extra Path Length: Constructive Interference Extra path length: integer multiples of wavelength ΔL = mλ, m = 0, ±1, ± 2, Constructive Interference 25

26 Extra Path Length: Destructive Interference Extra path length: half integer multiples of wavelength ΔL = (m +1/ 2)λ, m = 0, ±1, ± 2, Destructive Interference 26

27 Interference: Phase Shift and Path Length What can introduce a phase shift between two waves each of wavelength λ? 1. From sources that are out of phase 2. Sources in phase, but travel different path lengths because they come from different locations φ ΔL ΔL λ = φ 2π constructive destructive 27

28 Summary: Two in-phase Sources and Screen Assuming: L >> d >> λ : y = Ltanθ Lsinθ Constructive interference: δ = d sinθ = mλ y max = mλl / d; m = 0, ±1, Destructive interference: δ = d sinθ = (m )λ y min = (m +1/ 2)λL / d; m = 0, ±1, 28

29 Demonstration: Microwave Interference Two Transmitters 4&show=0 29

30 Microwave Interference 30

31 Lecture Demo The distance to the interference minima are given by y min = (m +1/ 2)λL / d; m = 0, ±1, When L = 1.16 m and d = 0.24 m, suppose the distance to the first minimum is measured to be 7.25 cm. What is the wavelength and frequency of the microwaves?! λ = 2y min d / L = (2)(7.25 cm)(0.24 m) / (1.16 m) = 3.0 cm f = c / λ = ( cm s 1 ) / (3.0 cm) = Hz 31

32 Coherent monochromatic plane waves impinge on two apertures separated by a distance d. An approximate formula for the path length difference between the two rays shown is 1. d sin θ 2. L sin θ 3. d cos θ 4. L cos θ CQ: Double Slit 32