1. Chapter 5 Light: The Cosmic Messenger

2. 5.1Basic Properties of Light and Matter Light: electromagnetic waves 1. Velocity (c = speed of light), wavelength and frequency (colors), energy. 2. Electromagnetic spectrum, visible spectrum, atmospheric windows Matter: Atoms. How do light and matter interact? 5.2Learning from Light: Origin of Starlight (some not in book) 1. How photons are produced 2. Relation temperature  motion of atoms 3. Blackbody Radiation (hot iron example). Wien’s Law: hotter  brighter, cooler  dimmer hotter  bluer, cooler  redder ( max ~1/T) 4. Colors of Stars: redder are cooler, bluer are hotter 5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission 6. Radial Velocity: Doppler effect 5.3Telescopes: reflecting and refracting, ground, airborne, space. Outline Ch 5 Light: The Cosmic Messenger

3. 5.1 Basic Properties of Light and Matter Our goals for learning What is light? What is matter? How do light and matter interact?

4. What is light?

5. Light is an electromagnetic wave Light is also a particle Photons: “pieces” of light, each with precise wavelength, frequency, and energy. Speed of light “c” is a constant in a vacuum = 300,000 km/sec

6. The Electromagnetic Spectrum

9. Atmospheric “Windows” 1.Visible Window (plus some UV and some infrared) 2.Radio Window

10. Question The higher the photon energy… a)the longer its wavelength. b)the shorter its wavelength. c)energy is independent of wavelength.

11. The higher the photon energy… a)the longer its wavelength. b)the shorter its wavelength. c)energy is independent of wavelength.

12. What is matter? Atomic structure:

13. Atomic Terminology Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + neutrons

14. Atomic Terminology Isotope: same # of protons but different # of neutrons ( 4 He, 3 He) Molecules: consist of two or more atoms (H 2 O, CO 2 )

15. How do light and matter interact? Emission Absorption Transmission Reflection or Scattering Terminology: Transparent: transmits light Opaque: blocks (absorbs) light

16. Interactions of light and matter

17. Question Why is the rose red? a)The rose absorbs red light. b)The rose transmits red light. c)The rose emits red light. d)The rose reflects red light.

18. Why is the rose red? a)The rose absorbs red light. b)The rose transmits red light. c)The rose emits red light. d)The rose reflects red light.

19. What have learned? What is light? Light is an electromagnetic wave that also comes in individual “pieces” called photons. Each photon has a precise wavelength, frequency and energy. Forms of light are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays

20. What have we learned? What is matter? Ordinary matter is made of atoms, which are made of protons, neutrons and electrons. How do light and matter interact? Matter can emit light, absorb light, transmit light or reflect light

21. 5.2. Learning from Light Our goals for learning What types of light spectra can we observe? How does light tell us what things are made of? How does light tell the temperatures of planets and stars? How does light tell us the speed of a distant object?

22. 5.2Learning from Light: Origin of Starlight (much of 5.2 not in book) 1. How photons are produced 2. Relation temperature  motion of atoms 3. Blackbody Radiation (hot iron example). Wien’s Law: hotter  brighter, cooler  dimmer hotter  bluer, cooler  redder ( max ~1/T) 4. Colors of Stars: redder are cooler, bluer are hotter 5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission a.Model of atoms: energy levels b.Continuous spectrum c.Emission lines and absorption lines 6. Radial Velocity: Doppler effect

23. 5.2.1 How photons are produced? When the motion of an electron is disturbed 5.2.2 Relation temperature  motion of atoms (from Ch.4) The higher the temperature the faster the atoms in a substance will be moving As atoms collide the electrons collide and their motion is disturbed When the motion of electrons gets disturbed they produce photons The higher the temperature, the more collisions, the more photons

24. Temperature Scales (from Ch.4)

25. 5.2Learning from Light: Origin of Starlight 1. How photons are produced 2. Relation temperature  motion of atoms 3. Blackbody Radiation (hot iron example). Wien’s Law: hotter  brighter, cooler  dimmer hotter  bluer, cooler  redder ( max ~1/T)

26. How does light tell us the temperatures of planets and stars? Cooler Hotter

27. Properties of Blackbody Radiation: 1.Hotter objects emit more light (per unit area) at all wavelengths. i.e. hotter  brighter, cooler  dimmer 2.Hotter objects emit photons with a higher average energy. i.e. hotter  bluer, cooler  redder

28. 3.Wein’s Law ( max ~1/T) i.e., if we can measure the maximum radiation emitted by an object we can determine its temperature Maximum emission max2 max1 max3 Properties of Blackbody Radiation:

29. 5.2Learning from Light: Origin of Starlight 1. How photons are produced 2. Relation temperature  motion of atoms 3. Blackbody Radiation (hot iron example). Wien’s Law: hotter  brighter, cooler  dimmer hotter  bluer, cooler  redder ( max ~1/T) 4. Colors of Stars: redder are cooler, bluer are hotter Stars behave like “blackbodies” so we can use their colors to determine their temperatures

30. 5.2Learning from Light: Origin of Starlight 1. How photons are produced 2. Relation temperature  motion of atoms 3. Blackbody Radiation (hot iron example). Wien’s Law: hotter  brighter, cooler  dimmer hotter  bluer, cooler  redder ( max ~1/T) 4. Colors of Stars: redder are cooler, bluer are hotter 5. Types of spectra( Kirchhoff’s 3 laws ): Continuous, Absorption and Emission (page 117-119 of book) a.Model of atoms: energy levels b.Continuous spectrum c.Emission lines and absorption lines

31. What types of light spectra can we observe?

33. This process produces an emission spectrum

35. This process produces an absorption spectrum

36. 1.Continuous Spectrum (thermal radiation spectrum) 2.Emission Spectrum 3.Absorption spectrum Kirchhoff’s Laws (p117-119 in book) 13 2

38. Continuous Spectrum

40. Emission Spectrum

43. Absorption Spectrum

46. Solar Spectrum

48. How does light tell us what things are made of? Electrons in atoms have distinct energy levels. Each chemical element, ion, molecule, has a unique set of energy levels. We can identify the chemicals in gas by their fingerprints in the spectrum. Distinct energy levels lead to distinct emission or absorption lines.

50. If the temperature of a star goes from 6000 K to 5000 K, what happens to its light? A.1. It becomes brighter B.2. It becomes bluer C.3. It becomes fainter D.4. It becomes redder E.5. It remains constant The correct answer is: A.A. 3 only B.B. 4 only C.C. 5 only D.D. 1 and 2 E.E. 3 and 4 Question 1

51. Can one use the visible color of the Moon to determine its temperature? A.Yes, because the Moon is similar to stars B.Yes, because the Moon does not reflect light C.Yes, because the Moon orbits Earth D.None of the above are correct Question 2

52. Which is hotter? a)A blue star. b)A red star. c)A planet that emits only infrared light.

53. Question Why don’t we glow in the dark? a)People do not emit any kind of light. b)People only emit light that is invisible to our eyes. c)People are too small to emit enough light for us to see. d)People do not contain enough radioactive material.

54. Radial Velocity Approaching stars: more energy, Receding stars: less energy, 5.2.6 Doppler Effect

55. Remember Exam 1 (Ch. 1-5) on Thursday Feb. 10 HW due next Monday: Ch. 4 and 5.