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Black body radiation BBR is the radiation emitted by a non-reflecting solid body. A perfect black body is one which absorbs all the radiations falling.

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Presentation on theme: "Black body radiation BBR is the radiation emitted by a non-reflecting solid body. A perfect black body is one which absorbs all the radiations falling."— Presentation transcript:


2 Black body radiation BBR is the radiation emitted by a non-reflecting solid body. A perfect black body is one which absorbs all the radiations falling on it. A hallow body blackened by coating with platinum black inside and a small pin hole is a typical black body. A radiation entering through pin hole gets reflected repeatedly from the walls until all the energy absorbed. A black body is both good absorber and radiator of energy and radiates the same amount of energy as it absorbs. A piece of iron at 100 0 c is not to touch but no visible radiation is emitted but with increasing temperature it becomes progressively red, orange, yellow and white. As the temperature is increased the radiation emitted contains more shorter wave lengths. Observations : Nature of radiation depends on the temperature At any temperature intensity of emitted energy increases first and then decreasing with increasing wavelength. As the temperature increases the maxima shifts to lower wavelengths. Classical theory predicts that black body should radiate over the whole wave length region including visible region even at room temperature.


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5 5 Conclusion As the temperature increases, the peak wavelength emitted by the black body decreases. As temperature increases, the total energy emitted increases, because the total area under the curve increases. The curve gets infinitely close to the x-axis but never touches it.

6 6 Summary - A black body is a theoretical object that absorbs 100% of the radiation that hits it. Therefore it reflects no radiation and appears perfectly black. - Roughly we can say that the stars radiate like blackbody radiators. This is important because it means that we can use the theory for blackbody radiators to infer things about stars. - At a particular temperature the black body would emit the maximum amount of energy possible for that temperature. - Blackbody radiation does not depend on the type of object emitting it. Entire spectrum of blackbody radiation depends on only one parameter, the temperature, T.

7 Photoelectric Effect. When a beam of light of suitable frequency falls on the surface of a metal, electrons are effected. These e - are called photo e lectrons. Very few metals can eject e - when visible light falls on them but many metals eject e - when UV light falls on them. e.g. Cs show PEE under visible light as it has lowest I.P. and e - can easily be excited ejected. 1.Metals with low I.P. 2.Threshold frequency 3.Intensity is directly proportional to no. of photons ejected 4.Kinetic energy increases with increases in frequency 5.E are ejected in vacum.

8 Heisenberg’s Uncertainty Principle According to classical mechanics the position and momentum of a moving e- is a wave then it would be impossible to know the exact location of wave. “ It is impossible to determine simultaneously the position and momentum of the e - with any desired accuracy”. e - is very small particle moving with a high velocity. To locate the exact position light has to be focused on e - and reflected beam of light should be observed. For reflection to occur the wave- length of incident radiation should be smaller than the dimension of the object reflecting it. Radiations of wave-length shorter than size of e - consist of high energy photons are used. During collision with high energy photon, the e - gets deflected as it absorbs considerable amount of energy resulting an increase in it momentum, thus changing its position. Hence it is impossible to determine the exact position of e - The product of uncertainty of simultaneous determination of position and momentum of a particle is equal to or greater than the Planck’s constt.


10 Louis de Broglie TOC Light is acting as both particle and wave Matter perhaps does also E = hf = hc/ E = mc 2 mc 2 = hc/ mc = p = h/ p = momentum (p = mv) h = Planck’s constant (6.626 x 10 -34 Js) = de Broglie wavelength

11 Heisenberg realised that... In the world of very small particles, one cannot measure any property of a particle without interacting with it in some way This introduces an unavoidable uncertainty into the result One can never measure all the properties exactly Werner Heisenberg (1901-1976)

12 Measuring the position and momentum of an electron Shine light on electron and detect reflected light using a microscope Minimum uncertainty in position is given by the wavelength of the light So to determine the position accurately, it is necessary to use light with a short wavelength

13 Measuring the position and momentum of an electron (cont’d) By Planck’s law E = hc/, a photon with a short wavelength has a large energy Thus, it would impart a large ‘kick’ to the electron But to determine its momentum accurately, electron must only be given a small kick This means using light of long wavelength!

14 Fundamental Trade Off … Use light with short wavelength: – accurate measurement of position but not momentum Use light with long wavelength: – accurate measurement of momentum but not position

15 Heisenberg’s Uncertainty Principle The more accurately you know the position (i.e., the smaller x is), the less accurately you know the momentum (i.e., the larger p is) ; and vice versa applet




19 19 Compton Scattering Compton scattering is the scattering of light (photons) from free electrons

20 Starter Think like Einstein!


22 Laws of photoelectric effect 1. Number of photoelectron is proportional to light intensity

23 Laws of photoelectric effect 2. Each metal has a different threshold frequency below which no photoelectrons are produced

24 Laws of photoelectric effect 3. A range of energies are produced but the maximum value depends on colour (frequency)

25 Wave theory can’t explain why colour is the most important factor in the photoelectric effect. It predicts that intensity should be the main factor.


27 Threshold frequency f min Emission of electrons from the surface can only occur if : ≥ ≥= min The threshold frequency is the minimum frequency (of the incident radiation) required for an emission of an electron to occur. KE of the electrons = 0 when the frequency of the incident radiation is f min


29 KE max versus frequency graph Threshold frequency




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