Physics 2D Lecture Slides Jan 29 Vivek Sharma UCSD Physics - - PowerPoint PPT Presentation

▶
physics 2d lecture slides jan 29
SMART_READER_LITE
LIVE PREVIEW

Physics 2D Lecture Slides Jan 29 Vivek Sharma UCSD Physics - - PowerPoint PPT Presentation

Dec 15, 2022 346 likes •587 views

Physics 2D Lecture Slides Jan 29 Vivek Sharma UCSD Physics Disasters in Classical Physics (1899-1922) Disaster Experimental observation that could not be explained by Classical theory (Phys 2A, 2B, 2C) Disaster # 1 : Nature of

slide-1
SLIDE 1

Physics 2D Lecture Slides Jan 29

Vivek Sharma UCSD Physics

slide-2
SLIDE 2

Disasters in Classical Physics (1899-1922)

  • 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
slide-3
SLIDE 3

Photo Electric Effect

slide-4
SLIDE 4

Einstein’s Explanation of Photoelectric Effect

  • Energy associated with EM waves in not uniformly

distributed over wave-front, rather is contained in packets

  • f “stuff”⇒ PHOTON
  • E= hf = hc/λ [ but is it the same h as in Planck’s th.?]
  • Light shining on metal emitter/cathode is a stream of

photons of energy which depends on frequency f

  • Photons knock off electron from metal instantaneously

– Transfer all energy to electron – Energy gets used up to pay for Work Function Φ (Binding Energy)

  • Rest of the energy shows up as KE of electron KE = hf- Φ
  • Cutoff Frequency hf0 = Φ (pops an electron, KE = 0)
  • Larger intensity I more photons incident
  • Low frequency light f not energetic enough to
  • vercome work function of electron in atom
slide-5
SLIDE 5

Einstein’s Explanation of PhotoElectric Effect

slide-6
SLIDE 6
slide-7
SLIDE 7

Is “h” same in Photoelectric Effect as BB Radiation?

Slope h = 6.626 x 10-34 JS Einstein Nobel Prize!

No matter where you travel in the galaxy and beyond… ..no matter what experiment You do h : Planck’s constant is same NOBEL PRIZE FOR PLANCK

slide-8
SLIDE 8

Work Function (Binding Energy) In Metals

slide-9
SLIDE 9

2 2

Light of Intensity I = 1.0 W/cm incident on 1.0cm surface of Assume Fe reflects 96% Photoelectric Effect on some Iron Surface: further only 3% of incident

  • f

l F ight is e light µ

2

(a) Intensity available for Ph. El effect I = Violet region ( = 250nm) barely above thres (b) ho hold frequency for Ph. El e w many photo-electrons emit 3% 4% (1.0 W ted per sec ff

  • n

/cm ) d ? ect # λ µ × ×

8

2 9 9 34

Power = h f hc (250 10 )(1.2 10 / ) = (6.6 10

  • f p

)(3.0 10 / ) hotoelectro ns 3% 4% (1.0 W/c m ) m J s J s m s µ λ

− − −

= × × × × × × i

9 15 10

  • 15
  • 1

1 9 9

= (c) Current in Ammeter : i = (1.6 10 )(1.5 10 ) (d) Work Function = ( )( ) 1.5 10 f = 2.4 10 h 4.14 1.1 1 4.5 eV 1 C A eV s s

− −

× × × = Φ = × × × i

slide-10
SLIDE 10

Photon & Relativity: Wave or a Particle ?

  • Photon associated with EM waves, travel with speed =c
  • For light (m =0) : Relativity says E2 = (pc)2 + (mc2)2
  • ⇒E = pc
  • But Planck tells us : E = hf = h (c/λ)
  • Put them together : hc /λ = pc

– ⇒

p = h/λ – Momentum of the photon (light) is inversely proportional to λ

  • But we associate λ with waves & p with

particles ….what is going on??

–A new paradigm of conversation with the subatomic particles : Quantum Physics

slide-11
SLIDE 11

X Rays : “Bremsstrahlung”: Braking Radiation

Produced by bombarding a metal target with energetic electrons Produced in general by ALL accelerating charged particles X rays : very short λ ≅ 60-100 pm (10-12m), large frequency f Very penetrating because energetic Useful for probing structure of sub-atomic Particles

slide-12
SLIDE 12

An X-ray Tube from 20th Century

The “High Energy Accelerator” of 1900s: produced energetic light : X –Ray , gave new optic to subatomic phenomena

Xray e

slide-13
SLIDE 13

X Ray Spectrum in Molybdenum (Mo)

slide-14
SLIDE 14

Bragg Scattering: Probing Atoms With X-Rays

Constructive Interference: nλ=2dsinϑ

X-ray detector

slide-15
SLIDE 15

Compton Scattering : Quantum Pool !

  • 1922: Arthur Compton (USA) proves that X-rays (EM Waves) have particle like

properties (acts like photons)

– Showed that classical theory failed to explain the scattering effect of

  • X rays on to free (not bound, barely bound electrons)
  • Experiment : shine X ray EM waves on to a surface with “almost” free electrons

– Watch the scattering of light off electron : measure time + wavelength of scattered X-ray

slide-16
SLIDE 16

Compton Effect: what should Happen Classically?

  • Plane wave [f,λ] incident on

a surface with loosely bound electrons interaction of E field of EM wave with electron: F = eE

  • Electron oscillates with

f = fincident

  • Eventually radiates spherical

waves with fradiated= fincident

– At all scattering angles, ∆f & ∆λ must be zero

  • Time delay while the

electron gets a “tan” : soaks in radiation

slide-17
SLIDE 17

Compton Scattering : Setup & Results

( ' ) (1 cos ) Scattered ' larger than incident λ λ λ θ λ ∆ = − ∝ −

(1 cos )

e

h m c θ λ    −  ∆   =

How does one explain this startling anisotropy?

slide-18
SLIDE 18

Compton Effect : Quantum (Relativistic) Pool

slide-19
SLIDE 19

Compton Scattering: Quantum Picture

2 e e e

E+m ' p = p'cos +p cos p'sin -p sin Use these to e Energy Conservation: Momentum Conserv liminate electron deflection angle (n

  • t measured

: )

e

c E E θ φ θ φ = + =

e e e 2 2 2 2 4 2 e 2 2 e e 2

p 2 'cos p cos 'cos p sin 'sin Square and add Eliminate p & using E & E ( ') '

e e e e

p c m c E E m p p p E p pp p c φ θ φ θ θ = = − + + = = + − = − ⇒

( )

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

( ) E For light p= c ' ( ') 'cos E-E' ( ') ' 2 ' 2( ') ( ' 2 'cos ' ' ' 2 cos 1 ( ) ( )(1 1 cos ) EE' cos )

e e e e

p pp p c E EE E c c c c m c EE E E m E E m c E E E c EE m c E E E mc h m c θ θ θ θ λ λ θ ⇒ = ⇒ − + − = − ⇒ = − + = + − +   − + +     − + − − = −  − ⇒  −  

slide-20
SLIDE 20

Checking for h in Compton Scattering

Plot scattered photon data, calculate slope and measure h

∆λ

1-cos ϑ

( ' ) ( )(1 cos )

e

h m c λ λ θ − = −

It’s the same h !!

C

  • m

p t

  • n

w a v e l e n g t h λC = h / me c

Energy Quantization is a UNIVERSAL characteristic

  • f light (EM Waves)
slide-21
SLIDE 21

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 Gentlemen, all five of you have touched only one part of the elephant ..elephant is all of above LIKEWISE WITH LIGHT ! Blindmen & an Elephant

slide-22
SLIDE 22

Next Question : What is the Nature of Matter

  • Fundamental Characteristics of

different forms of matter

– Mass – Charge – Measure them

( ) F q E v B = + ×