Prepared by Lisa M. Will, San Diego City College

1. Lecture Slides CHAPTER 4: Light and Telescopes Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College

2. Light and Telescopes Most knowledge of the universe beyond Earth comes from light. We want to understand: • The properties of light. • How telescopes are used to collect light.

3. Properties of Light • Light moves at 300,000 km/s in the relative vacuum of space. • The speed was first measured by Rømer when observing Jupiter’s moons.

4. Properties of Light: Electromagnetic Wave • Light is a combination of electricity and magnetism, an electromagnetic wave. • It is a transverse wave, similar to a drop of water creating ripples. • Unlike sound, Light does not require a material to travel through.

5. Properties of Light: Electromagnetic Wave (Cont.)

6. Properties of Light: Electromagnetic Wave (Cont.)

7. Properties of Light: Wavelength, Amplitude, and Speed Wavelength (): length between crests. Amplitude: height. Speed (v): how fast the wave travels.

8. Properties of Light: Frequency • The speed of light, c, is constant. • Frequency (f): number of waves that pass by each second. • Speed of any wave = wavelength * frequency

9. Class Question Which of the following statements about light waves is correct? • As the frequency increases, the wavelength decreases. • As the frequency increases, the wavelength increases. • Changing the frequency has no effect on wavelength.

10. Properties of Light: Spectrum Spectrum: light sorted by frequency and wavelength. Visible spectrum is seen in this rainbow. Red is the longest wavelength. Violet is the shortest wavelength.

11. Properties of Light: Photons Light behaves as both a wave and a particle. Photon: particle of light. Photons carry energy and can have different amounts of energy. The energy of a photon is proportional to its frequency (E = hf)

12. Properties of Light: Frequency Vs. Energy Energy is directly proportional to frequency => higher frequency = higher energy Therefore a photon of blue light carries more energy than a photon of red light.

13. Class Question For red light and blue light to have the same total amount of energy, which of the following must be true? More blue photons are present. More red photons are present. Both colors are present in equal amounts.

14. Properties of Light: Visible Spectrum Visible spectrum is only a small part of the full electromagneticspectrum. Gamma rays have the shortest wavelengths Radio waves have the longest wavelengths. Learn the order! Shortest to longest wavelength: Gamma ray, X-ray, UV, Visible, IR, microwave, radio

15. Spectrographs or spectrometers break up incoming light into different wavelengths. Allow astronomers to analyze those different wavelengths. Properties of Light: Spectrographs

16. Class Question Which of the following portions of the electromagnetic spectrum has the longest wavelengths? Gamma Rays Infrared Visible All are the same.

17. Class Question Which of the following portions of the electromagnetic spectrum has the highest energies? Gamma Rays Infrared Visible All are the same.

18. Class Question Which of the following portions of the electromagnetic spectrum has the fastest speeds? Gamma Rays Infrared Visible All are the same.

19. Telescopes The eye sees wavelengths between 350 nm (deep violet) and 700 nm (far red). Photons are collected by your retina. Your brain interprets the image.

20. Telescopes: Modern Astronomy Photography opened the door to modern astronomy. Astronomers could create permanent images and spectra of celestial objects. Astronomers could see objects previously too faint to observe.

21. Telescopes: Digital Detectors Digital detectors, such as charged-coupleddevices, are more efficient than film. Digital detectors record the photons as pixels. Photons create a signal in the array to create an image.

22. Telescopes: Digital Detectors (Cont.)

23. Telescopes: Digital Detectors (Cont.)

24. Telescopes: Unique Spectra Each element and molecule interacts with light in a unique way => unique spectra! Spectrometers allow astronomers to learn composition, temperature, density, etc… for celestial objects.

25. Telescopes: Unique Spectra (Cont.)

26. Telescopes: Unique Spectra (Cont.)

27. Telescopes: Reflection and Refraction Reflection occurs as light hits a mirror and bounces off. Refraction occurs when light enters/leaves a material. Light bends when moving between air and glass, forcing us to shape lenses to get a single focus point.

28. Telescopes: Reflection and Refraction (Cont.)

29. Telescopes: Reflection and Refraction (Cont.)

30. Telescopes: Lenses Refracting telescopes use lenses. Primary lens: refracts (bends) the light. Aperture: size of the primary lens.

31. Telescopes: Focal Length Focal length: distance between lens and the image. (Longer = larger image) Aperture sets the light-gathering power. (Larger aperture = more light)

32. Telescopes: Focal Length (Cont.)

33. Telescopes: Focal Length (Cont.)

34. Telescopes: Mirrors Reflecting telescopes use mirrors. There are primary and secondary mirrors.

35. Telescopes: Effects of Atmosphere The atmosphere does not allow all light through. Nearly all Gamma ray, X-ray, ultraviolet, and infrared wavelengths are blocked. A large range of radio waves is unblocked.

36. Telescopes: Radio Waves and Radio Telescopes Radio waves have wavelengths of a centimeter to about 10 meters, so a large collecting area is needed. Radio telescopes are typically tens of meters in diameter.

37. Telescopes: Radio Waves and Radio Telescopes (Cont.)

38. Telescopes: Radio Waves and Radio Telescopes (Cont.)

39. Telescopes: Interferometric Arrays Interferometricarrays combine the signals from many telescopes. This combination improves the quality of the astronomical data. An array like this can act like a giant telescope the size of the entire array!

40. Telescopes: Resolution Resolution = smallest details that can be separated. For a given wavelength, the larger the telescope, the better the resolution. Diffraction changes some light from its path, blurring the image.

41. Telescopes: Adaptive Optics Adaptive optics can help correct for this atmospheric distortion. Earth-based image quality can compete with the Hubble Space Telescope in the visible.

42. Telescopes: Earth’s Atmosphere Earth’s atmosphere distorts images because air bubbles refract incoming light waves. Astronomicalseeing = limit on resolution due to the atmosphere. Space-based telescopes do not suffer from atmospheric blurring.

43. Telescopes: Space Telescopes Space telescopes also can detect wavelengths that the atmosphere blocks. Examples: Hubble Space Telescope (UV, visible, IR) Chandra (X-rays) Spitzer Space Telescope (infrared)

44. Chapter Summary Light behaves both like a particle and a wave. Astronomers study light to understand the temperatures and compositions of celestial objects. Telescopes are used to gather this light. Some portions of the electromagnetic spectrum are best studied from space.

45. AstroTour Geometric Optics and Lenses Click the image to launch the AstroTour Animation (Requires an active Internet connection)

46. AstroTour Light as a Wave, Light as a Photon Click the image to launch the AstroTour Animation (Requires an active Internet connection)

47. Nebraska Applet EM Spectrum Module Click the image to launch the Nebraska Applet (Requires an active Internet connection)

48. Nebraska Applet Hydrogen Atom Simulator Click the image to launch the Nebraska Applet(Requires an active Internet connection)

49. Nebraska Applet Three Views Spectrum Demonstrator Click the image to launch the Nebraska Applet(Requires an active Internet connection)

50. Nebraska Applet Doppler Shift Demonstrator Click the image to launch the Nebraska Applet(Requires an active Internet connection)