1. What are the 3 ways heat can be transferred? Radiation: transfer by electromagnetic waves. Conduction: transfer by molecular collisions. Convection: transfer by circulation of a fluid.

2. Electromagnetic waves Everyday observation: A hot piece of metal glow (that means it radiates) whose colour varies with temperature. A precise observation: To radiate em wave an object need not be so hot. All objects radiate such energy continuously whatever be their temperatures, though which frequencies predominate depends on temperature. At room temperature most of the radiation is in infrared region so they are invisible. How can we explain the origin of the radiation emitted by bodies of matter?

3. An object at any temp emits radiation sometimes referred to as thermal radiation. Characteristic of this radiation depends on the temp and the properties of the object. At low temp the wavelengths of thermal radiation are in the infrared region, as the temp increases the objects begins to glow red means enough visible radiation is emitted so that object appears to glow. As temp increases the thermal radiation it emits consists of a continuous distribution of wavelengths from the infrared, visible and ultraviolet portions of the spectrum.

4. From classical viewpoint thermal radiation from accelerated charged particles in the atoms near the surface of the object. This classical theory was inadequate. The basic problem was in understanding the observed distribution of wavelengths in the radiation emitted by a black boy. As the temp increases two distinct behaviour are observed. Peak of the distribution shifts to shorter wavelengths.

5. Blackbody radiation Blackbody: An ideal body which absorbs all the incident radiation regardless of the frequency. Idealization of Black body: Hollow object with a very small hole leading to its interior. Any radiation striking the hole enters the cavity, where it is trapped by reflection back and forth until it is absorbed. The cavity walls are constantly emitting and absorbing radiation and we are interested about the properties of this radiation.

6. Blackbody radiation Continuous spectra Examples - Star spectra are nearly Black Body Radiation out of an oven or a blast furnace Radiation coming out of human mouths, noses and other orifices The Universe Anything which is "hot" will absorb and emit electromagnetic radiation.

8. Blackbody Radiation : Wien’s displacement Law Spectral Distribution depends ONLY on Temperature. Sunlight Lower Temps

9. The second effect is that the total amount of energy the objects emits increases with temperature. This is descrobed by stefen’s law.

10. Blackbody Radiation : Stefan-Boltzmann Relation R = Radiation intensity, T = Temp. in Kelvin,  = 5.67×10 -8 W/m 2 K 4 For non-ideal black body, R =  T 4 E where E = emissivity < 1. Short Long Experimental Spectral Distribution

11. Rayleigh-Jeans law To describe the distribution of energy from a black body it is useful to define I(,T)d to be the power per unit area emitted in the wavelength interval d. The result of a calculation based on a classical model of black body radiation known as the Rayleigh-Jeans law. kBkB Is the boltzman,s conatant

12. In this classical model of black body radiation, the atoms in the cavity walls are treated as a set of oscillators that emit em waves at all wavelengths. This model leads to an average energy per oscillator that is proportional to T.

13. Rayleigh-Jeans experiment Raleigh-Jeans equation behaves well at long (low energy). BUT, explodes to infinity for short (high energy).  UV catastrophe! An experimental plot of the blackbody radiation spectrum is shown in figure together with the theoretical prediction by Rayleigh-Jeans law. I(,T)

14. As approaches to zero the function I(,T) given by the equation approaches to infinity. Not only should short wavelengths predominate in blackbody spectrum but also the energy emitted by any black body should become infinite in the limit of zero wavelength. this mismatch of theory and experiment was so disconcerting that scientists called it the ‘ ultraviolet catasstrophe’. another discrepancy between theory and experiment concerns the total power emitted by the black body.

15. Experimentally the total power per unit area given by Remains finite even though the Rayleigh-Jeans law says it should diverge to infinity. in 1900 Planck derived a formula for black body radiation that was in complete agrement with experiment at all wavelengths.

16. Planck’s analysis led to the experimental curve. The function I(,T) proposed by him is which explains the curve exactly is This function includes a parameter h, which planck Adjusted so that his curve matched the experimental Data at all wavelengths. The value of this parameter is found to be independent of the material of which The black body is made and independent of the temp.

17. Rather than a variable parameter, it is fundamental constant of nature. The value of h, planck’s constant is h = 6.626 x 10 -34 J.s. At long wavelengths the plancks equation reduces to Reyleigh Jeans expression, and at short wavelength it predicts an exponential decrease in I with decreasing wavelength.

18. In his theory planck made two bold and controversial assumptions concerning the nature of the oscillating molecules at the surface of the black body. 1) the molecules can have only discrete values of energy E n given by E n = nh  Where n is the positive integer called quantum number and  is the natural frequency of oscillation of the molecules. This is quite different from the classical model of the harmonic oscillator where energy of identical oscillator is related to the amplitude of the motion and unrelated to the frequency. Because the energy of a molecule can have only discrete values given by the above equation we may say the energy is quantized.

19. Each discrete energy value represents a different quantum state for the molecule. With each value of n represents a specific quantum state. When the molecule is in n=1 state its energy is h . The molecules emit or absorb energy in discrete packets that later came to be called photons. The molecule emit or absorb these photons by jumping from one quantum state to another. If the jump is downward from one state to adjuacent lower state this equ shows that the amount of energy radiated by the molecule in a single photon equals h . Hence the energy of one photon corresponds to the energy difference between two adjacent quantum state is E = h 