QUANTUM OPTICS Tran Thi Ngoc Dung – Huynh Quang Linh – Physics A2 HCMUT 2016 Contents • Thermal Radiation • Spectral irradiance r,T, r,T, • Total irradiance R,T, • Spectral absorbance a,T, a,T[.]
QUANTUM OPTICS Tran Thi Ngoc Dung – Huynh Quang Linh – Physics A2 HCMUT 2016 Contents • Thermal Radiation • Spectral irradiance r,T, r,T, • Total irradiance R,T, • Spectral absorbance a,T, a,T, • Kirchhoff ‘s law of equilibrium thermal radiation • Laws of Blackbody Radiation • Stefan-Boltzmann’s law • Wien’s Displacement law • Planck’s Quantum theory • Einstein’s Photon Theory • Compton Effect THERMAL RADIATION The fundamental sources of all electromagnetic radiation are electric charges in accelerated motion 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 Spectral irradiance r(,T), r(,T) • Consider an object of temperature of T • The object emits electromagnetic waves dW(,T) of many different frequencies (wavelengths) • Let dW(,T) the energy emitted per area dS per unit time, and transmitted by electromagnetic waves of frequencies in the range (,+d) • Spectral irradiance which is the energy emitted per unit area per unit time per unit frequency is defined as: dS T dW ( , T ) W r ( , T ) [ ] dS.d m Hz Spectral irradiance is dependent on: - Absolute temperature T - Frequency - and The nature of the object ( glass, metal ) 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 0 Total Irradiance is dependent on : - Absolute temperature T - The nature of the object ( glass, metal, balckbody ) [W/m2] c) Spectral absorbance a(,T), a(,T) dW(,T) • Consider an object of temperature T • Let dW(,T) the energy of electromagnetic waves of frequencies between (,+d) , sent to the area dS, per unit time • Let dW’(,T) the energy absorbed • Spectral absorbance is defined as: dS T a (, T ) dW ' (, T ) dW (, T ) a(,T) 1 Spectral absorbance is dependent on: - Absolute temperature T - Frequency - and The nature of the object ( glass, metal ) 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 KIRCHHOFF ‘S LAW OF THERMAL RADIATION IN THERMAL EQUILIBRIUM • Consider a thermal insulated cavity, containing different objects A, B, C • These objects radiate and absorp electromagnetic waves At initial state : TA TB TC At thermal equilibrium state :TA TB TC T B A C rA (, T) rB (, T) rC (, T) a A (, T) a B (, T) a C (, T) Maintaining the equilibrium state requires that an object which radiate strongly, absorps strongly rA (, T) rB (, T) rC (, T) r ( , T ) f ( , T ) a A ( , T ) a B ( , T ) a C ( , T ) a ( , T ) KIRCHHOFF ‘S LAW “ In equilibrium thermal radiation, the ratio between spectral irradiance and spectral absorbance is not dependent on the nature of the object, it depends only on the temperature T and the given frequency Function f(,T) is called universal function or common function The meaning of the universal function : • Applying the Kirchhoff’s law for a blackbody rBlackbody ( , T) a Blackbody ( , T) rBlackbody ( , T) f ( , T) • 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) r (, T) rB.B (, T) r (, T) if a (, T) and rBb (, T) a) b) Spectral irradiance of a real object is smaller than Spectral irradiance of a blackbody 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 Spectral Irradiance of Blackbody O T1 T2 Grating F Detector The universal function f(,T) - Has a peak - At higher temperature, the peak of the curve moves to a higher frequency m , or a shorter wavelenth m - The area limitted by the function f(,T) and the horizontal axis is the total irradiance of the black body ... radiation • Laws of Blackbody Radiation • Stefan-Boltzmann’s law • Wien’s Displacement law • Planck’s Quantum theory • Einstein’s Photon Theory • Compton Effect THERMAL RADIATION The fundamental sources