Physics 2D Lecture Slides Jan 27. Vivek Sharma UCSD Physics

Transcription

1 Physics 2D Lecture Slides Jan 27 Vivek Sharma UCSD Physics

2 Ch 2 : Quantum Theory Of Light What is the nature of light? When it propagates? When it interacts with Matter? What is Nature of Matter? When it interacts with light? As it propagates? Revolution in Scientific Thought Like a firestorm of new ideas (every body goes nuts!..not like Evolution) Old concepts violently demolished, new ideas born Interplay of experimental findings & scientific reason One such revolution happened at the turn of 20 th Century Led to the birth of Quantum Theory & Modern Physics

3 Blindmen & an Elephant touched the trunk of the elephant, said elephant was like a branch of a tree. touched the tail of the elephant, said elephant was like a snake. touched an ear. He said elephant was a huge fan. felt a leg of the elephant., elephant was like a pillar. touched the side of the elephant, said the elephant was like a wall The Physicist: Gentlemen, all five of you have touched only one part of the Elephant...elephant is all of above LIKEWISE WITH LIGHT!

4 Classical Picture of Light : Maxwell s Equations Maxwell s Equations: permeability permittivity

5 Hertz & Experimental Demo of Light as EM Wave

6 Properties of EM Waves: Maxwell s Equations Power incident on an area A Intensity of Radiation I = Energy Flow in EM Waves : r 1 r r Poynting Vector S = ( E B) µ rr 1 = S. A = ( ) µ µ c E Larger the amplitude of Oscillation More intense is the radiation 0 ( 2 AE ) 0B0Sin kx ωt 2 0

7 Disasters in Classical Physics ( ) Disaster Experimental observation that could not be explained by Classical theory (Phys 2A, 2B, 2C) Disaster # 1 : Nature of Blackbody Radiation from your BBQ grill Disaster # 2: Photo Electric Effect Disaster # 3: Scattering light off electrons (Compton Effect) Resolution of Experimental Observation will require radical changes in how we think about nature QUANTUM MECHANICS The Art of Conversation with Subatomic Particles

8 Nature of Radiation: An Expt with BBQ Grill Question : Distribution of Intensity of EM radiation Vs T & λ Grill Radiator (grill) at some temp T Emits variety of wavelengths Some with more intensity than others EM waves of diff. λ bend differently within prism Eventually recorded by a detector (eye) Map out emitted Power / area Vs λ Prism separates Out different λ Intensity R(λ) Notice shape of each curve and learn from it Detector

9 Radiation from A Blackbody

10 R( λ) dλ T (a) Intensity of Radiation I = 4 I = σ T 4 (Area under curve) Stephan-Boltzmann Constant σ = W / m 2 K 4 (b) Higher the temperature of BBQ Lower is the λ of PEAK intensity λ ΜΑX 1 / Τ Wein s Law λ MAX T = const = mk As a body gets hotter it gets more RED then White Reason for different shape of R(λ) Vs λ for different temperature? Can one explain in on basis of Classical Physics (2A,2B,2C)??

11 Blackbody Radiator: An Idealization T Classical Analysis: Box is filled with EM standing waves Radiation reflected back-and-forth between walls Radiation in thermal equilibrium with walls of Box How may waves of wavelength λ can fit inside the box? Blackbody Absorbs everything Reflects nothing All light entering opening gets absorbed (ultimately) by the cavity wall Cavity in equilibrium T w.r.t. surrounding. So it radiates everything It absorbs Emerging radiation is a sample of radiation inside box at temp T Predict nature of radiation inside Box? less more Even more

12 The Beginning of The End! How BBQ Broke Physics Classical Calculati on # of standing waves between Wavelengths λ and λ+d λ are 8π V 3 N( λ)d λ= dλ ; V = Volume of box = L 4 λ Each standing wave cont ributes energy E= kt to radiation in Box Energy density u( λ) = [# of standing waves/volume] Energy/Standing Wave 8πV 1 8π = kt = kt 4 4 λ V λ c c8π 2πc Radiancy R( λ) = u( λ) = kt = kt λ λ Radiancy is Radiation intensity per unit λ interval: Lets plot it Prediction : as λ 0 (high frequency) R(λ) Infinity! Oops!

13 Ultra Violet (Frequency) Catastrophe OOPS! Radiancy R(λ) Experimental Data Classical Theory Disaster # 1

14 Disaster # 2 : Photo-Electric Effect Light of intensity I, wavelength λ and frequency ν incident on a photo-cathode Can tune I, f, λ i Measure characteristics of current in the circuit as a fn of I, f, λ

15 Photo Electric Effect: Measurable Properties Rate of electron emission from cathode From current i seen in ammeter Maximum kinetic energy of emitted electron By applying retarding potential on electron moving towards Collector»K MAX = ev S (V S = Stopping voltage)»stopping voltage no current flows Effect of different types of photo-cathode metal Time between shining light and first sign of photocurrent in the circuit

16 Observations : Current Vs Frequency of Incident Light I 3 = 3I 1 I 2 = 2I 1 I 1 = intensity -V S

17 Stopping Voltage Vs Incident Light Frequency Stopping Voltage ev S Different Metal Photocathode surfaces f

18 Retarding Potential Vs Light Frequency Shining Light With Constant Intensity f 1 > f 2 >f 3

19 Time Elapsed between Shining Light & Current Time between Light shining on photo-cathode And first photo-electons ejected current in circuit Depends on distance between light source & cathode surface Seems instantaneous ( < 10-9 Seconds by the experimenter s watch)

20 Conclusions from the Experimental Observation Max Kinetic energy K MAX independent of Intensity I for light of same frequency No photoelectric effect occurs if light frequency f is below a threshold For a particular metal, light with f > f 0 causes photoelectric effect IRRESPECTIVE of light intensity. f 0 is characteristic of that metal Photoelectric effect is instantaneous!...not time delay Explain all this Mr Classical Physicist