Electromagnetic Radiation PDF

Electromagnetic Radiation Wavelength (λ) (Lambda) : It is defined as the distance between two nearest crest or trough. It is measured in terms of Å (Angstrom), pm (picometre), nm (nanometer), cm(centimetre), m (metre) 1Å = 10–10 m, 1 Pm = 10–12 m, 1nm = 10–9 m, 1cm = 10–2m Time period (T) : Time t...

Electromagnetic Radiation Wavelength (λ) (Lambda) : It is defined as the distance between two nearest crest or trough. It is measured in terms of Å (Angstrom), pm (picometre), nm (nanometer), cm(centimetre), m (metre) 1Å = 10–10 m, 1 Pm = 10–12 m, 1nm = 10–9 m, 1cm = 10–2m Time period (T) : Time taken by a wave to pass through one point. Wave number ( ) ( nu bar) : It is the reciprocal of the wavelength that is number of waves present Frequency ( ) (nu) : Frequency of a wave is defined as in unit length. the number of waves which pass through a point in1 s. It is measured in terms of Hertz (Hz ), –s1 , or cycle/s(cps) It is measured in terms of cm–1, m–1 etc, (1 Hertz = 1 s–1 ) Pr M. AZOUZ 3 Electromagnetic Radiation (3) All waves have: frequency and wavelength symbol: (Greek letter “nu”) (Greek “lambda”) units: “cycles per sec” = Hertz “distance” (nm) All radiation: λ ν =c where c = velocity of light = 3.00 x 10 8 m/sec Note: Long wavelength → small frequency Short wavelength → high frequency increasing increasing Pr M. AZOUZ frequency wavelength 4  The Electromagnetic Spectrum Pr M. AZOUZ 10 Emission Spectra Electric discharge in a gas at low pressure: The gas emits characteristic light A series of discrete bright lines This series of lines is called an emission spectrum Pr M. AZOUZ 23 Continuous vs. Line Spectra For atoms and molecules, one does not observe a continuous spectrum (the “rainbow”), as one gets from a white light source. Only a line spectrum of discrete wavelengths is observed. Each element has a unique line spectrum. Pr M. AZOUZ 24 ATOMIC SPECTRUM OF HYDROGEN  The emission line spectrum of hydrogen can be obtained by passing electric discharge through the gas contained in a discharge tube at low pressure.  The light radiation emitted is then examined with the help of a spectroscope.  The bright lines recorded on the photographic plate constitute the atomic spectrum of hydrogen Pr M. AZOUZ 25 A. The Energy Levels of Hydrogen Atomic states  Excited state– atom with excess energy  Ground state– atom in the lowest possible state When an H atom absorbs energy from an outside source, it enters an excited state. Pr M. AZOUZ 26 Pr M. AZOUZ 27 Maximum number of spectral lines Pr M. AZOUZ 30 6 5 4 3 2 1 31 6 5 4 656 nm 3 2 1 32 6 5 4 656 nm 3 486 nm 2 1 34 6 5 4 656 nm 3 486 nm 2 1 35 6 5 4 434 nm 656 nm 3 486 nm 2 1 36 6 5 4 434 nm 656 nm 3 486 nm 2 1 37 6 5 4 434 nm 656 nm 3 410 nm 486 nm 2 1 38   In 1885, J.J. Balmer developed a simple relationshipamong the different wavelengths of the series of visible lines in the hydrogen spectrum.  The relationshipis : As the other series of hydrogen spectral lines were discovered, a more general expression was found as : The complete spectrum of hydrogen atom is shownin : 39 Hydrogen spectrum (also has other spectral lines) The line spectrum at the bottom of the previous slide is not the entire spectrum of hydrogen ; it is just the visible- light portion. Hydrogen also has series of spectral lines in the infrared and the ultraviolet. 52