1 / 62

Dual Nature of Light

Dual Nature of Light. Is light a wave or a particle?. Wave Properties. -Diffraction -Interference -Polarization. Diffraction. Constructive & Destructive Interference. Polarization. Energy. Wave E increases with A 2 /Intensity. Studies of Wave E. Planck – color ( f , l ) vs. T.

odina
Download Presentation

Dual Nature of Light

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dual Nature of Light Is light a wave or a particle?

  2. Wave Properties • -Diffraction • -Interference • -Polarization

  3. Diffraction

  4. Constructive & Destructive Interference

  5. Polarization

  6. Energy

  7. Wave E increases with A2/Intensity.

  8. Studies of Wave E • Planck – color (f, l) vs. T. • As T inc. , f inc, (l decr)

  9. Radiation & TemperatureHot Objects Emit Waves

  10. Intensity/Brightness

  11. Classical physics could not accurately predict f vs. Temperature Problem:

  12. Max Planck related f to T. Light (EM) E, is quantized--it can only take on certain whole number values. E comes in little "chunks" of f x a constant now called Planck's constant, h: • EM radiation waves • chunks • quanta • photons

  13. Can calculate E in EM waves units quanta or photons based on frequency. E = hf. h is Plank’s constant 6.63 x 10-34 Js. E is energy in Joules f is frequency of radiation

  14. Show that if E = hf,E = hc.l • For waves, v = fl. • Rearrange f = v/l. • Vacuum/air EM v = c (3 x 108m/s). • f = c/l. • E = hff = c/l. • E = hc.l

  15. Ex 1. Each photon of a certain color light has an energy of 2.5 eV. What is the frequency of and color of the light?

  16. Solution:E = hff = E/h convert eV to Joules.(2.5 eV)(1.6 x 10-19J/eV) = 6.03 x 1014 Hz 6.626 x 10-34 J s Green Light

  17. 2. The energy of a certain photon is 2.9 eV. What type of wave is it? Be specific. • 2.9 eV x 1.6 x 10-19 J = 4.64 x 10-19 J. eV E = hf (4.64 x 10-19 J) = (6.63 x 10-34 Js) f Violet Light f = 7 x 10 14Hz

  18. Finish Ex Sheet • Hwk : Text Read 830 – 833 Do pg 833 #1-4 and 839 #2, & 856 # 2-4, 9.

  19. Do Now: A photon of light has energy = 2.072 eV. What color is it? • 2.072 eV (1.6 x 10-19 J/eV). • 3.3152 x 10-19 J • E = hf. • (3.3152 x 10-19 J) = (6.63 x 10-34 Js)f • f = 5.00 x 1014 Hz. • Orange

  20. So EM Radiation comes in chunks, maybe it’s not waves. • All objects above 0 K radiate EM waves as E. • Hotter = more E = higher freq. (different color) • E = hf (J).

  21. What is the evidence? • Photoelectric effect. http://phet.colorado.edu/en/simulation/photoelectric

  22. Photoelectric Effect When EM waves shine on a metal surface, the E in wave may be enough to kick out surface e-. Materials that emit e- in this way are called photoemissive. The ejected e- are called photoelectrons.

  23. Phet observations

  24. http://phet.colorado.edu/en/simulation/photoelectric • Use higher Amplitude/Intensity/brightness – • more e- fly off w same speed. • Current increases (A, C/s) • Increased f • e- fly off faster w higher KE.

  25. Classical Mechanics cannot explain why increasing A or exposure time does not increase photoemission. After all: • Boat would be tossed higher & faster with increased wave amplitude. • But not ejected e-.

  26. Einstein: Maybe EM waves quantized too – photons. • Increasing f, increases E of each photon. • Increasing Intensity (A) increases number of photons hitting more e- so more fly out. • Envision EM as little chucks. High f are heavier. • http://phet.colorado.edu/en/simulation/photoelectric

  27. Photoemission only works with metals with weakly bound e-.

  28. Each metal, has a threshold frequency fo = lowest f that will free an e-. Light frequencies below the fo eject no e-, no matter how intense or bright the light.

  29. Light frequencies above the fo eject e-, no matter how low the A (how dim).

  30. A metal has a threshold frequency fo in the blue light range. What will happen if very bright red light is shone upon the metal? No e- will be emitted more e- will be emitted The emitted e- will have less energy.

  31. Einstein confirmed EM waves come in quanta or photons, each has E =hf. Higher-frequency photons have more E, so they can knock out e- & make the e- fly out faster with greater KE.

  32. 2. A metal has a threshold frequency in the blue light range.What will happen if UV light is shone upon the metal? • nothing b) the emitted e- will have more energy (KE) c) more e- will be emitted with the same energy.

  33. Increasing the intensity/ampl, increases the rate of e- emission or the current; the number of e- ejected, but each e- won’t gain any extra energy. Only increased f increases E of photoelectrons.

  34. A metal has a threshold frequency in the blue light range. 3. What will happen if the blue light is made twice as bright? • nothing b) the emitted e- will have more energy (KE) c) more e- will be emitted with the same energy.

  35. All EM waves can be described as quanta or photons. The energy carried by photons is: Ephoton = hf or Ephoton = hc/l. (for photon traveling at speed of light).This energy is absorbed by photo-emissive materials.

  36. The min. frequency to free e- is fo.The min energy needed to free an e- is called work function Wo, or F.Metals have low Wo.

  37. 4. A certain metal has a work function (Wo) of 1.7 eV. If photons of energy 3.0 eV are absorbed by the metal: • a) No e- will be emitted at that energy. • b) More e- will be emitted than would be at the Wo. • c) Higher KE e- will be emitted than would be at Wo.

  38. Summary: • EM waves as chunks of energy/photons travel at c. • Calculate the Energy J E = hf, or • E = hc/l. • Evidence for photons – from Photoelectric Effect Experiment – • f not A responsible for KE of ejected e-. • High f = high E, photon. • High A = high number of photons. • Photo-emissive materials have: • fo = min f to eject e- (Hz) • Wo= min E to eject e- (J)

  39. Read Txt 830-835Intro to Modern Physics Packet #1-14

  40. Graph of Photoelectric Experiment • KE of photoelectron vs. frequency.

  41. max KE of photo e- vs. f for metal. As f of EM wave increases, KE increases, slope = h. F (work function), is minimum energy needed to eject e-. Work function

  42. State the work function & threshold frequency of this metal eV 0.4 0.0 0.4 0.8 1.0 2 3 4 5 6 7 8 x 1014 Hz

  43. Wo & fo. The threshold frequency fo, = min frequency needed to eject e- so the min energy needed, Wo in J is: Wo = hfo. Any photon E left over after the work function, goes into KE of e-.

  44. 5. A particular metal has a threshold frequency fo, of 5 x 1014 Hz. What is its work function Wo in J & eV? Wo = hfo. 3.3 x 10-19 J 2.07 eV

  45. Ephoton = hf is the total E available. • Absorbed photon E splits between Wo & KE photo e-, so total E of absorbed by e- is: • Epho = Wo + KE. • The maximum KE of ejected e- is: • KEelc = Epho – Wo. • Don’t forget Wo = hfo.

  46. 6: Photoelectric Effect:Light having f = 1 x 1015 hz shines on a sodium surface. The photoelectrons have a maximum KE of 3 x 10-19 J.Find the threshold frequency for sodium.

  47. Photon Photoelectron.Etot = Wo + KE.Etot – KE = Wo.hf – KE = hfo. fo = (hfphoton – KEmax) (h)

More Related