1 Electromagnetic Radiation. 2 3 4 c=  How many wavelengths pass through point P in one second? Frequency! P.

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Presentation transcript:

1 Electromagnetic Radiation

2

3

4 c=  How many wavelengths pass through point P in one second? Frequency! P

5 Electromagnetic Radiation

6 A radio operator broadcasts at a frequency of 14.2 MHz (megahertz). What is the wavelength of the radio waves put out by the transmitter? - Solve example Calculate the wavelengths of the electromagnetic radiation presented in previous slide.

7 Atomic Spectra Now, replace white light source with a hydrogen lamp.

8 H 2 → 2H H → H* (excited) H* (excited) → H (Ground state ) + light cathode anode Discharge tube

9 Continuous spectrum Line spectrum Problem: No explanation provided by classical physics. Scientists (such as Lyman, Balmer and Paschen) analyzed the observed lines with respect to their wavelengths. Rydberg summarized their efforts in the so-called Rydberg’s equation: R= ×10 -2 nm -1 Rydberg’s constant n: positive integer.

10 Calculate the wavelength, in nanometers, in the line of spectrum of hydrogen Corresponding to n 1 =2 and to n 2 =4!

11 Energy of Light - Energy of electromagnetic waves h h h h h h h h h h -However, Planck (Black body radiation) Einstein (photoelectric effect) Light composed of tiny particles, called quanta (photons) Energy of each photon (quantum) = h × E photon = h × Number of photons determines light intensity. h=6.6 × J.s Planck’s constant

12 Bohr’s Theory Bohr’s Postulates: Electron moves in circular orbits around the nucleus. Electron can possess only certain energy values corresponding to the orbit. Electron can “jump” from one orbit to another, the energy difference will be emitted or absorbed in the form of light quanta.

13 A=2.18 ×10-18 J=13.6 eV Z : atomic number n : positive integer = 1, 2, 3, …

14 The larger n - the larger is the orbit size, the farther is the electron from nucleus - the larger is the electron energy Comparison to throwing stone upwards. Negative sign means that the electron is under the influence of the nucleus. Electron free from nucleus attracting force when n= ∞.

15 Explanation of line spectrum n low n high

16 Lyman series Balmer SeriesPaschen Series ends at n=1 ends at n=2 ends at n=3 UV visible Infra-red

17 Example 7.5 Calculate the energy, frequency and wavelength of the photon emitted when an electron in the hydrogen atom drops from the fifth to the second energy level.

18 De Broglie Hypothesis Light behaves: - as waves (electromagnetic waves) Hertz experiment - as particles (photons) Photoelectric effect, Compton effect Why should light be special??!!!!! Generalization of dual nature (wave nature & particle nature) to all matter. Any moving object can be considered to be a wave!!! Energy of that object is E=mc 2 If object considered to be a wave, E=h De Broglie suggested:

19 De Broglie relationDe Broglie wavelength Experimental evidence: electron diffraction Diffraction is a phenomenon that only waves can undergo Includes waves interference

20 Example: Calculate the wavelength of a football player weighing 60 kg moving in the yard with 10 km/h velocity. too small to be observed experimentally Example: Calculate the wavelength of an electron moving with a velocity of 1000 km/h. X-ray: diffraction on crystals

21 Wave mechanics A version of quantum mechanics (modern concepts in physics) Electron in atoms are considered to be standing waves. Each electron in atom is described by a set of numbers called Quantum numbers.