Physics 2 - Lecture 6: Quantum Optics - Huynh Quang Linh

Nội dung text: Physics 2 - Lecture 6: Quantum Optics - Huynh Quang Linh

1. QUANTUM OPTICS Tran Thi Ngoc Dung – Huynh Quang Linh – Physics A2 HCMUT 2016
2. THERMAL RADIATION 1. The fundamental sources of all electromagnetic radiation are electric charges in accelerated motion. 2. All bodies emit electromagnetic radiation as a result of thermal motion of their molecules; this radiation, called thermal radiation, is a mixture of different wavelengths.
3. b) Total Irradiance RT the energy emitted by a unit area of the surface per unit time and transmitted by electromagnetic waves of all frequencies: R(T) r(,T)d r(,T)d [W/m2] 0 0 Total Irradiance is dependent on : - Absolute temperature T - The nature of the object ( glass, metal, balckbody )
4. Blackbody (Vật đen tuyệt đối) Definition: a(,T) =1 for all , at all T Blackbody A black body is an ideal system that absorbs all radiation incident on it. The electromagnetic radiation emitted by the black body is called blackbody radiation. A good approximation of a black body is a small hole leading to the inside of a hollow object as shown in Figure 40.1. Any radiation incident on the hole from outside the cavity enters the hole and is reflected a number of times on the interior walls of the cavity; hence, the hole acts as a perfect absorber. The nature of the radiation leaving the cavity through the hole depends only on the temperature of the cavity walls and not on the material of which the walls are made.
5. The meaning of the universal function : • Applying the Kirchhoff’s law for a blackbody r ( ,T) r ( ,T) Blackbody Blackbody f ( ,T) aBlackbody ( ,T) 1 • Universal function f(,T) is the spectral irradiance of the blackbody Consequences of Kirchhoff’s law of thermal radiation r(,T) a(,T)f (,T) a(,T)rBlackbody(,T) a(,T) 1 r(,T) rB.B(,T) r(,T) 0 if a(,T) 0 and rBb (,T) 0 a) Spectral irradiance of a real object is smaller than Spectral irradiance of a blackbody b) A real object of temperature T radiates electromagnetic wave of frequency  if this object of temp. T can absorp the EM wave of frequency  and the blackbody of temperature T radiates electromagnetic wave of frequency 
6. PLANCK‘s QUANTUM THEORY 1. Ultraviolet catastrophe 2. PLANCK‘s QUANTUM THEORY 3. Planck‘s formula 4. Stefan-Boltzman‘s law 5. Wien‘s law
7. 2. Planck‘s quantum theory The concept of quantization of radiation was discovered in 1900 by Max Planck, who had been trying to understand the emission of radiation from heated objects, known as black-body radiation. Planck assumed: a) Energy can only be absorbed or released in tiny discrete packets, which are an integer multiple of a quantum energy E n n 1,2,3 b) Quantum energy of an electromagnetic wave of frequency , (wavelength  ) is c  h h  Planck derived the spectral irradiance of the black body , called Planck’s formula: 8  2 h f ( ,T) c2 h e kBT 1
8. Wien‘s Displacement Law The peak in the spectral irradiance occurs at a wavelength m which is inversely proportional to the absolute temperature of the blackbody. b  m T Wien constant b=2.898x10-3 (m.K)
9. An FM radio transmitter has a power output of 150 kW and operates at a frequency of 99.7 MHz. How many photons per second does the transmitter emit? P hN P 150 103 J / s N 2.27 1030 photons / s h 6.625 10 34 J.s 99.7 106 s 1
10. Einstein’s Photon Theory Einstein suggested that: - Light is composed of tiny particles called photons - Each photon has energy E=h=hc/ - The speed of photon is c=3x10^8m/s - An object emits or absorps electromagnetic waves An object emits or absorps photons hc Photon Energy : ε hν λ ε h Photon mass : m c2 λc h Photon Momentum : p mc λ m relativisticmass : m o v2 1 c2 for a photon:v c mo 0 : rest massof photon
11. EXPLAINATION • The shifted peak at ’ is caused by the scattering of x-rays from free electron, weakly bound to the target atoms. • The unshifted peak at  is caused by x-rays scattered from electrons tightly bound to the target atoms.  ’
12. Compton Scattering in radiography
13. Energy Momentum particle Before After Before After collision collision collision collision photon hc/ hc/’ p=h/ p’=h/’ 2 m v 2 mec e electron E =m c E 0 pe o e 2 v2 v 1 1 c2 c2
14. The shift of the wavelength conservatio n of momentum 2   '  2 sin ( ) p 0 p' pe o 2 h h p ;p'  ' Conservation of energy : hc hc E E λ o λ' hc hc d E Eo λ λ'  Kineticenergy Energy of Energy of of the electron incident photon scattered photon hc hc hc hc E ’:wavelength of incident X-ray KE _ electron   '   2 sin2( ) ’: wavelength of scattered X-ray o 2 o: Compton wavelength of 2  E if sin ( ) 1or  electron KE _ electron _ max 2 : scattering angle , the angle between the incident and scattered x rays