AS III Electromagnetic Radiation (EMR)

Transcription

1 AS III Electromagnetic Radiation (EMR) 3-1 Modern Structure of Atom concerned mostly w/ location & of e s energy 3-2 Physics Background Info Wave Nature of EMR Particulate Nature of EMR Ref 8: 1, 3 Prob 8: in-text 1, 2 end-of-chapter 1-6, 19-22, 25 Adv Rdg 8: 2, 4 experimental basis: spectroscopy = interaction between matter & EMR ( light... ) NB. EMR has wave & particle properties Background on Wave Physics HT Fig Traveling Wave Variables After 1 period (at t = 1T) wave has gone through 1 cycle, has traveled 1 wavelength (λ) 3-4 def. of frequency: general, for 1 cycle ν = # of cycles time = 1 T, velocity of wave: v = distance time = λ T = λ ν check units: m s = m s 1

2 Important Characteristics of Waves HT Fig ) Diffraction & Interference see HT Fig Diffraction at slit, edge new wave is generated in all directions slit should be of same size as wavelength edge should be sharp HT Fig Interference trains of waves interact: in-phase combination ( crest meets crest ) constructive interference amplitude out-of-phase combination ( crest meets trough ) destructive interference cancellation of wave ( darkness ) HT Fig. 3.4 HT Fig.3.5

3 Pet. Fig ) Refraction/Dispersion 2-1 Refraction light rays are bent at boundary of media (e.g., at air-to-glass boundary, rays are bent towards normal) 2-2 Dispersion different λ s are bent to diff. extent allows separation ( & isolation) of diff. λ s ( e.g., red s, green s,...) HT Fig 3.6 EMR as Wave Phenomenon EMR consists oscillating of electrical & magnetic fields which travel in wave fashion see Pet Fig. 8.2 Note: travel speed is same for all λ s of EMR c = speed of light in vacuum = m s km s value very close for travel in air

4 Pet. Fig. 8.2 Classification of EMR wavelengths (λ s) range from m m most relevant for CHEM101/3 is light ultraviolet visible infrared UV VIS IR approximately < 400 nm nm > 800 nm VIBGYOR (reverse of Roy G. Biv) for details see Pet. Fig Pet. Fig. 8.3 Particulate Nature of EMR (Evidence of Quantization of EMR Energy) (1. Blackbody Rad n : skip) 2. Photoelectric Effect Basic Experimental Set-up vary ν, observe current I can determine when electrons are released also KE of released electrons

5 photoelectric effect... Pet. Fig Pet. Fig shows more sophisticated set-up includes grid for counter voltage, V s, which can be adjusted to prevent current I in turn, V s, is used to determine KE of e s (physics) important: KE of e s is prop. to V s photoelectric... Observations ν o = min. frequency requ d to release e at higher ν, ΚΕ of e at lower ν, no e s are released, no matter how intense the radiation photoelectric... Interpretation at metal surface (at C), e s are held by binding energy, work function, Φ light ( or EMR in general) does not come as a diffuse package of energy, but rather as individual energy particles = photons each individual photon must have sufficient energy to liberate an e energy of photon: E = hν where h = Js Planck equ n

6 interpretation... photons w/ ν o ( E o ) have the min. energy to release an e Φ = E o = hν o if ν > ν o then xs energy is converted to KE of e KE = E E o = h (ν ν o ); this relationship can be used to predict KE of e Main Message EMR ( light ) energy does not come as a diffuse wave, but rather as an assembly of discrete, small energy packets = photons thus EMR has particulate character Overall, EMR has dual nature: i.e., it has wave characteristics ( e.g., interference phenomenon) & particulate characteristics ( e.g., photoelectric effect) 3-23 Summary of Lesson AS III terms & symbols: wavelength, frequency, period, velocity v = λν definition/description of diffraction/interference definition/description of refraction/dispersion electromagnetic spectrum: γ rays radio waves sequence of colors in visible light photoelectric effect: experimental setup, results and interpretation Planck equ n : E = hν EMR has wave & particle nature HMWK