Lecture6 sep23-bb (1)
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- 4. Solar Spectrum The sun emits radiation at all wavelengths Most of its energy is in the UV-VIS-IR portions of the spectrum ~50% of the energy is in the visible region ~40% in the near-IR ~10% in the UV
- 5. Radiation Energy is transferred by electromagnetic waves This includes all radiant energy: X-rays Radio waves Light (sunlight) Microwaves
- 6. Properties of Waves All electromagnetic waves travel at the same speed The speed of light: 300,000 km/s crest trough
- 7. Properties of Waves Wavelength crest (length/cycle) trough Wavelength (): the length of one complete cycle
- 8. Properties of Waves Wavelength crest (length/cycle) Amplitude trough Amplitude: 1/2 height between trough and crest
- 9. Properties of Waves Wavelength crest (length/cycle) Amplitude trough Frequency (): the number of cycles/second
- 10. Speed = wavelength x frequency c= (length/second) = (length/cycle) x (cycle/second) Light speed is constant C= 300,000 km/s Long wavelength (), low frequency () short wavelength (), high frequency ()
- 11. Who is faster? Same Whose frequency is higher? And why?
- 12. Energy of a wave Energy is proportional to frequency, and inversely proportional to wavelength E=h = h (c/ ) where h = Planck’s constant In other words, waves with shorter wavelengths (or higher frequency) have higher energy
- 13. Electromagnetic Spectrum 0.01 0.1 1 10 100 1000 (m) denotes w avelength m is m icrom eter unit of w avelength
- 14. 1 kilometer (km) 10 3 103 meters = 1 kilometer 10 1 meter (m) 1 cm (centimeter) 1 mm (millimeter) 1 m (micrometer) 1 nm (nanometer) 1 10 -1 10 -2 10 -3 100 cm = 1 m 103 mm = 1 m 10 -6 106 m = 1 m 10 -9 109 nm = 1 m
- 15. Different color light has different wavelength R e d 0.65 m green 0.51 m violet 0.42 m
- 16. Electromagnetic Spectrum 0.4 to 0.7 m visible x-rays ultraviolet light 0.01 High Energy 0.1 infrared 1 (m) ( = “micro” = 10−6) 10 microwaves 100 1000 Low Energy
- 17. Radiant Energy All objects that have temperature greater than 0K emit radiation Blackbody Any object that is a perfect emitter and also a perfect absorber of radiation sun and earth’s surface behave approximately as blackbodies Q: How much radiation is being emitted by an object, and at what wavelengths?
- 18. m ax 2897 m T (K ) most objects emit radiation at many wavelengths however, there is one wavelength where an object emits the largest amount of radiation energy This wavelength is found with Wien’s Law Q: At what wavelength does the sun emits most of its radiation? (Tsun=6000K) Q: At what wavelength does the earth emits most of its radiation? (Tearth=288K) Q: At what wavelength does your body emits most of its radiation? (Tyou=310K) Blackbody radiation curve m ax
- 19. Wien’s law max = 2897/T m The Nobel Prize in Physics 1911 Increasing energy Rayleigh-Jeans Law Wavelength (m)
- 20. Planck found that he could use a formula to fit the blackbody radiation curve Increasing energy Planck’s law u ( , T ) 8 h c 3 The Nobel Prize in Physics 1918 1 h 3 e kT 1 The beginning of quantum mechanics Wavelength (m) Photon energy h
- 21. Total radiation energy Stefan-Boltzmann law: E = sT4 Boltzmann 1844-1906 (Austria) Increasing energy Q: If T1=2T2, how many times E1 is E2? E1=sT14 hotter bodies emit more and at higher energy (shorter wavelength) E2=sT24 Wavelength (m)
- 22. His very famous formula (about entropy) is inscribed on Boltzmann's tombstone in Vienna.
- 23. Blackbody Radiation Blackbody radiation—radiation emitted by a body that emits (or absorbs) equally well at all wavelengths
- 24. Visible Light (VIS) 0.4 to 0.7 m Our eyes are sensitive to this region of the spectrum Violet-Indigo-Blue-Green-Yellow-Orange-Red
- 25. Infrared Radiation (IR) We can’t see IR, but we can feel it as radiant heat Lower energy than visible light
- 26. Ultraviolet Radiation (UV) Higher energy than visible light Can burn human skin and damage cells