Chapter. 3 Wave & Particles I

Size: px

Start display at page:

Download "Chapter. 3 Wave & Particles I"

Ann Gilbert
5 years ago
Views:

Announcmnt Cours wbpag http://highnrgy.phys.ttu.

3 Wav & Particls I Planck s Law ( = nhf) Photolctric ffct (Thrshold frquncy) M-

Photolctric ffct (instin; 1905; 1921*) Th Production of X-Rays (Rontgn;1901;

1 Announcmnt Cours wbpag Txtbook PHYS-2402 Lctur 8 Quiz 1 Class avrag: 14.2 (out of 20) ~ 70% Fb. 10, 2015 HW2 (du 2/19) 13, 17, 23, 25, 28, 31, 37, 38, 41, 44 Chaptr. 3 Wav & Particls I Planck s Law ( = nhf) Photolctric ffct (Thrshold frquncy) M- Wavs bhaving lik Particls Outlin: Blackbody Radiation (Plank; 1900; 1918*) Th Photolctric ffct (instin; 1905; 1921*) Th Production of X-Rays (Rontgn;1901; 1901*) Th Compton ffct (Compton; 1927; 1927*) Pair Production (Andrson; 1932; 1936*) Is It a Wav or a Particl?! Duality? Albrt instin proposd: Th light is bhaving as a collction of particls calld s ach of thm having nrgy

2 Th Photolctric ffct (Albrt instin 1905) xampl (1): Vry intnsiv light bam, low frquncy light vn With Vry-Vry wak light intnsity, bam but of high nough frquncy = nhf lctrons jctd What happns is that 1 PHOTON jcts 1 LCTRON bam = nhf SMALL (blow th thrshold) LARG (n is larg) Thr is no PHOTON capabl of jcting an LCTRON NO lctrons jctd xampl (2): SINGL PHOTON Vry wak light bam of high frquncy nrgy Consrvation: bam = = nhf LARG (abov th thrshold) 1 lctron jctd = hf K max "! Th PHOTON jcts 1 LCTRON!

3 Also known at that tim: Rpls lctrons! = 380nm To fr an lctron from th mtal, on has to pay a crtain amount of nrgy th Work Function " =?! f max min =? =? U < 2Volts

4 Problms 1. Th work function of tungstn surfac is 5.4V. Whn th surfac is illuminatd by light of wavlngth 175nm, th maximum photolctron nrgy is 1.7V. Find Planck s constant from ths data. c K W = h W λ Problms 1. Th work function of tungstn surfac is 5.4V. Whn th surfac is illuminatd by light of wavlngth 175nm, th maximum photolctron nrgy is 1.7V. Find Planck s constant from ths data. 7 ( K + W) λ ( 1.7V + 5.4V) m 15 c h= = = V s 8 K W = h W c 3 10 m/ s λ = V s J / V = J s 2. Th thrshold wavlngth for mission of lctrons from a givn mtal surfac is 380nm. (a) what will b th max kintic nrgy of jctd lctrons whn λis changd to 240nm? (b) what is th maximum lctron spd? (b) (a) c h W c c c " 1 1 # K = h W = h h = hc% & = 1.9V ' ( λ = 0 λ1 λ1 λ0 λ1 λ0 2K 2 5 K = mv /2 v= = m/ s m Th Production of X-Rays (Wilhlm Rontgn 1901) (Th opposit of th Photolctric ffct) W us th nam X-rays for M radiation whos wavlngths ar in th 10-2 nm to 10 nm rgion of spctrum Th Production of X-Rays (Wilhlm Rontgn 1901) (Th rvrs of th Photolctric ffct) X-rays can b producd by smashing highspd lctrons into a mtal targt. Whn thy hit, ths dclrating charg produc much radiation, calld Brmsstrahlung CLASSICAL physics: Radiation covrs ntir spctrum Brmsstrahlung

5 SURPRIS: xprimnts indicat a cutoff wavlngth: SURPRIS: xprimnts indicat a cutoff wavlngth: Frquncy f, nrgy =hf Thr is no classical xplanation for so sharp a trmination of th spctrum 1 -> 1 lctron (?) 1 lctron -> 1 (?) Frquncy f INDD: If th radiation is quantizd, th minimum allowd at f is hf (singl ). W can t produc half a, so if multipl lctrons don t combin thir s into a singl, no could b producd of > K of a singl lctrons. Stting th K of an incoming lctron = of on Th Compton ffct (Arthur Compton 1927) Frquncy f INDD: 1 lctron -> 1 Hypothsis: Is this tru? Compton providd th 1 st xprimntal vidnc!! xprimnt?

6 Momntum & nrgy whn a strik a fr lctron Momntum & nrgy whn a strik a fr lctron momntum nrgy Bfor Collision: A approachs an lctron at rst Aftr Collision: Th lctron scattrs at spd u, angl φ. A of wavlngth λ scattrs at angl θ Momntum & nrgy whn a strik a fr lctron nrgy and Momntum Consrvation

Problm An X-ray with λ=60nm is scattrd ovr 150 0 by a targt lctron. (a) Find th chang of its wavlngth. (b) Find th angl btwn th dirctions of motion of th rcoil lctron and th incidnt.

of th rcoil lctron. Th Compton ffct X-ray tub targt

- (a) (c) (b) p= h/ λ ( 1 cos ) λ' λ = λ = λc θ cos150 = = 0.866 2 12 12 Δ λ= λc ( 1 cosθ) = ( 2.43 10 m)( 1+ 0.866) = 4.

7 Problm An X-ray with λ=60nm is scattrd ovr by a targt lctron. (a) Find th chang of its wavlngth. (b) Find th angl btwn th dirctions of motion of th rcoil lctron and th incidnt. (c) Find th nrgy of th rcoil lctron. Th Compton ffct X-ray tub targt (light atoms,.g. graphit) Photons carry momntum lik particls and scattr individually with othr particls X-ray dtctor indd, th wavlngth shift is indpndnt of th targt matrial and th initial wavlngth. - (a) (c) (b) p= h/ λ ( 1 cos ) λ' λ = λ = λc θ cos150 = = Δ λ= λc ( 1 cosθ) = ( m)( ) = m p' = h/ λ ' θ ϕ p x 0 3 momntum consrvation: along y axis: along x axis: p λ = 60 nm λ' = 64.53nm sinθ = 0.5 sin ϕ = p'sinθ p= p cos ϕ+ p'cosθ ( J s)( 3 10 m/ s)( m) ( m)( m) hc hc Δλ λ λ' λλ' K = = hc = = J = 1.44kV 12 λ ' = m p cos ϕ = p p'cosθ hc sinθ p 'sinθ ' λsinθ tanϕ = = λ = p p'cos θ hc hc cosθ λ' λcosθ λ λ' tanϕ = ϕ = Th photolctric ffct and th Compton ffct ar two important ways in which M radiation intracts as a particl with mattr. W now discuss a third!

Physics 2D Lecture Slides Lecture 12: Jan 28 th 2004

Physics 2D Lecture Slides Lecture 12: Jan 28 th 2004 Brian Wcht, th TA, is away this wk. I will substitut for his offic hours (in my offic 3314 Mayr Hall, discussion and PS sssion. Pl. giv all rgrad rqusts to m this wk (only) Quiz 3 Will Covr Sctions.1-.5