Electromagne,c Waves

Transcription

1 Electromagne,c Waves

2 Energy and Power Power is energy per,me, or wa# = joules/seconds Power Companies use kilowa;- hours as unit of energy or 1 kwh = 1000 Watt-hours 1000 x (joules/sec) x hours 1 kwh = 3.6 x 10 6 joules

3 Intensity Intensity is power density. Units: W/m 2 = W m - 2 Intensity = Power Area A

4 Solar Intensity The solar power output is 4 x Watts. How much of that hits us? When the Sun is directly over head, it delivers the equivalent of Watt light bulbs over each square meter (m 2 ) of ground!!! This amount, 1340 W m - 2, is known as the Solar Constant How is solar energy delivered from the Sun to the Earth? As Light!!!!

5 Is the intensity of light reaching the surface always 1340 W/m 2? 1. Yes 2. No What would change the value?

6 Electromagne,c Wave Electromagne,c Wave: propaga,ng wave of electric and magne,c fields that oscillate perpendicular to each other and the direc,on of propaga,on In a vacuum, wave propagates with speed = 3 x 10 8 m/s (cosmic speed limit) Electric field Magne,c field

7 Wave Proper,es speed (v): how much distance the wave moves per unit,me (for an EM wave v = c = 3 x 10 8 m/s) frequency (f): number of peaks that pass a loca,on is a given,me (units: Hertz (Hz) = 1/s = s - 1 )

8 Wave Proper,es speed (v): how much distance the wave moves per unit,me (for an EM wave v = c = 3 x 10 8 m/s) frequency (f): number of peaks that pass a loca,on is a given,me (units: Hertz (Hz) = 1/s = s - 1 ) wavelength (λ): distance between two consecu,ve peaks (units: km, m, cm, mm, µm, nm )

9 Wave Proper,es speed (v): how much distance the wave moves per unit,me (for an EM wave v = c = 3 x 10 8 m/s) frequency (f): number of peaks that pass a loca,on is a given,me (units: Hertz (Hz) = 1/s = s - 1 ) wavelength (λ): distance between two consecu,ve peaks (units: km, m, cm, mm, µm, nm ) These three proper,es are related: f = v λ

10 If wavelength is 10 m and frequency is 100 Hz (oscilla,ons / seconds), what would be the speed of the wave? m/s 2. 1 m/s m/s m/s

11 If wavelength is 10 m and frequency is 100 Hz (oscilla,ons / second), what would be the speed of the wave? f v = = = v λ fλ ( 100 s -1 ) ( 10 m) = 1000 m/s

12 The Photon Light behaves like both a par,cle and a wave! Photon: smallest bundle of light energy a par,cle of light) (i.e. Photons carry light energy: 1. A photon s energy is propor,onal to frequency (E ph f). 2. A photon s energy is inversely propor,onal to wavelength (E ph λ - 1 ). E ph hf = hc λ = Plank s constant (h) = x J s

13 The Visible Spectrum: How is a difference in the frequency or wavelength of light observed? As Color of Light 13

14 Ma;er actually a wave too! All ma;er exhibits par,cle and wave proper,es For ordinary objects, the wave nature of ma;er is much too small to measure The wavelength of a baseball moving at 80 mph would be about meters But for small par,cles, this is wave nature of ma;er is measurable The wavelength of an electron is about meters Electron diffrac,on pa;ern showing its wave nature

15 How does a prism work? Dispersion: Speed of light in the prism (glass or plas,c) depends on the frequency (color) Refrac,on: Change in speed of light causes a change in its direc,on Result: Blue changes direc,on most since its speed is the lowest inside the prism. And red changes direc,on least since its speed is highest inside the prism. 15

16 Visible light is just a small part of the electromagnetic spectrum 16

17 Why are we spending so much,me discussing the electromagne,c spectrum? Not easy to visit astrophysical objects (the Sun, planets, other stars) and make direct in situ measurements We rely on remote sensing of EM radia,on. Tells us the temperature and composi/on This gives us important clues to the origins of these objects. 17

18 The Solar Spectrum: When we look at the Spectrum of the Sun, we see a distinct distribution of colors. Other stars have similar patterns as do most hot objects. The main difference is where the peak color is. Gustav Kirchhoff (1862) called this kind of emitter a Blackbody. 18

19 Ideal Black Bodies Black body: an object that radiates energy into space in a manner that is characteris,c only of the temperature of the radiator. Characteris,cs of Black Bodies 1. Characteris,c I vs. λ curve (shown lef) (all BBs radiate in more than one color) 2. Curve is related to T only 3. Increasing T, increases total intensity 4. As T increases, peak moves to lower λ 5. Wavelength of peak intensity related only to T (Wien s Law): Wavelength (nm) 0.29 cm K λ = max T K 19

20 Ideal Black Bodies Amount of light thermally radiated: j = σt 4 Where j = total intensity, T is temperature, and σ = 5.67 x 10-8 W m - 2 K - 4 (constant) Wavelength (nm) 20

21 Calcula,ng The Sun s Temperature So how well does this work? Pre;y well!!!! T Sun 6000 K 21

22 UV Visible Infrared 22

23 UV Visible Infrared 23