2. Chapter 5 Light: The Cosmic Messenger

3. Light in Everyday Life Power – describes the rate of energy use. 1 Watt = 1 Joule/sec. Spectrum – The component colors of a light source when viewed through something that creates a spectrum. Ex: Prism, diffraction grating. Emission – The transfer of light energy out of the object. Absorption – The transfer of light energy into the object.

4. A mirror reflects light along a path determined by the angle of reflection. Notice that the angle of reflection equals the angle of incidence. Light Scattering: The movie screen scatters a narrow beam of light into many that reach all members of the audience

5. Transmission – The passing of light through a body. If light passes through a body, it is said to be transparent to that light. Reflection – The return of light off of a surface. Note: A body may be reflective and/or transparent to only part of the incident light.

6. Properties of Light Particle or wave? Light is electromagnetic energy in which the electric and magnetic fields vibrate. Light behaves as both a particle and a wave. Light as a Particle: Photon- discrete bundle of light energy. Light as a Wave: Electromagnetic Waves

7. Wave-Particle Duality Electron Interference Pattern Stages of film exposure show photon by photon formation of photograph. Diffraction Fringes LightElectron beam Ref: P.G. Hewitt, Conceptual Physics, 9 th ed.©2002 Pearson Education

9. Some definitions for waves Wavelength : the distance between adjacent peaks of a wave. –We measure electromagnetic wavelengths in nanometers or angstroms. Frequency: the number of peaks per second, that pass by a certain reference point. –We measure frequency in Hertz (Hz)

10. Many Forms of Light

11. Light and Matter Matter may absorb light or emit light. We can measure the amount of absorption or emission by specifying the Intensity of the light (I) in Watts/m 2.

12. Proto-typical Spectrum

13. Absorption and Emission by Thin Gases Various Energy Level transitions in Hydrogen Emission line spectrum Absorption line spectrum

14. Atomic Emission Line Spectra

15. Molecular Emission Line Spectrum Molecular Hydrogen (H 2 )

16. Thermal Radiation Hotter objects emit more total radiation per unit surface area. Hotter objects emit photons with higher average energy.

17. Thermal Radiation

18. Thermal Radiators emit Radiation according to the Planck Curve Max Planck gives the first successful explanation for the characteristic curves of thermal radiators. This ushers in the Quantum Theory. (1858- 1947)

19. Representative Planck Curves

20. Kirkhhoff’s Laws Any “Complex” object produces thermal radiation over a broad range of wavelengths. When thermal radiation passes through a thin cloud of gas, the cloud leaves “fingerprints” that may be either absorption lines or emission lines, depending on its temperature

21. Kirkhhoff’s Laws If the background source is colder than the cloud, or if there is no background source at all, the spectrum is dominated by bright emission lines produced by the cloud’s atoms and molecules Summary of Kirkhhoff’s Laws – Next Slide

23. Reflected Light The color of an object that does not act as a source of light depends on the light that it reflects. A planet may reflect certain colors from the sun and absorb others. The reflected light gives the planet its characteristic color, while the absorbed light heats the surface and influences its surface temperature.

24. Putting It All Together- a typical spectrum

25. The Doppler Effect If an object that is emitting a wave moves toward us (or we toward it), then the wavelengths are shortened (frequency is increased). If an object that is emitting a wave moves away from us (or we move away from it), then the wavelengths are lengthened (frequency is decreased). For light, the lengthening of the wavelength due to this Doppler shift is called a Doppler Red Shift. The shortening is called a Doppler Blue Shift.

26. This observer sees the light red-shifted This observer sees the light blue-shifted

27. Doppler Shift The Doppler shift also appears in the observed spectrum. (See next slide)

29. Collecting Light With A Telescope Light- Collecting Area: The cross-sectional area of the primary mirror or objective lens. The Light collecting area determines how much light the telescope can focus. Angular Resolution: The smallest angular size a telescope can measure. –Ex: The Hubble Space telescope has an angular resolution of 0.05 arcseconds.

30. In general, larger diameter telescopes have both a greater light-collecting area and a better angular resolution. Other factors that limit resolution include: –The engineering and construction of the telescope. –Effects of the atmosphere for ground based telescopes.

31. Basic Telescope Design 1m refractor at Yerkes Observatory

32. 5m Reflector at Mt. Palomar Observatory

33. Telescope Designs

34. 10-m Keck Telescope

35. The Hubble Space Telescope

36. Use of Telescopes Imaging: provides pictures of astronomical objects. Spectroscopy: Involves dispersing the light into a spectrum for analysis of composition, temperature, relative motion, etc. Timing: Monitors how the light intensity hitting a detector varies with time. This can provide information about the objects rotational motion.

37. Atmospheric Effects on Observations Light Pollution: The scattering of bright artificial light from cities and towns at night. This causes glare which reduces “seeing” conditions. Turbulence: The changing direction of air motion near the telescope bends the light in shifting patterns. This causes the “twinkling” of stars but also blurs astronomical images.

38. Some Ways to Fix the Problems Due to The Atmosphere Adaptive Optics: The controlled deformation of the primary mirror in order to counteract the effects from atmospheric distortion. Adaptive Optics can eliminate most atmospheric distortion.

39. Ground based observation of a double star Same object now using adaptive optics system

40. Telescopes Across The Spectrum

41. X-Ray Telescopes

42. Radio Telescopes – 305m radio telescope at Arecibo, Puerto Rico.

43. The Trajectory of Voyager 2

44. Images of Jupiter from the Voyager Mission

45. Saturn (from Voyager Mission)

47. What lies ahead? Artist’s Conception of Lunar Observatory