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.
Figure 4.2

5. Properties of Light: Electromagnetic Wave (Cont.)
Figure 4.2 (a)

6. Properties of Light: Electromagnetic Wave (Cont.)
Figure 4.2 (b)

7. Properties of Light: Wavelength, Amplitude, and Speed
Wavelength (): length between crests.
Amplitude: height.
Speed (v): how fast the wave travels.
Figure 4.4 (a, b) 7

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
Figure 4.4 (c, d)

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.
Correct Answer: A

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. 10

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)
Figure 4.5 11

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.
Figure 4.5 12

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.
Correct Answer: B

14. Properties of Light: Visible Spectrum
Visible spectrum is only a small part of the full electromagnetic spectrum.
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 14

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

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

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

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

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. 19

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.
Figure 4.10 (b) 20

21. Telescopes: Digital Detectors
Digital detectors, such as charged-coupled devices, are more efficient than film.
Digital detectors record the photons as pixels.
Photons create a signal in the array to create an image.
Figure 4.11 21

22. Telescopes: Digital Detectors (Cont.)
Figure 4.11 (a) 22

23. Telescopes: Digital Detectors (Cont.)
Figure 4.11 (b) 23

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.
Figure 4.8 24

25. Telescopes: Unique Spectra (Cont.)
Figure 4.8 (a) 25

26. Telescopes: Unique Spectra (Cont.)
Figure 4.8 (b) 26

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.
Figure 4.12 27

28. Telescopes: Reflection and Refraction (Cont.)
Figure 4.12 (b) 28

29. Telescopes: Reflection and Refraction (Cont.)
Figure 4.12 (c) 29

30. Telescopes: Lenses Refracting telescopes use lenses.
Primary lens: refracts (bends) the light.
Aperture: size of the primary lens.
Figure 4.13 30

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)
Figure 4.13 31

32. Telescopes: Focal Length (Cont.)
Figure 4.13 (a) 32

33. Telescopes: Focal Length (Cont.)
Figure 4.13 (b) 33

34. Telescopes: Mirrors Reflecting telescopes use mirrors.
There are primary and secondary mirrors.
Figure 4.14 34

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. 35

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.
Figure 4.16 36

37. Telescopes: Radio Waves and Radio Telescopes (Cont.)
Figure 4.16 (a) 37

38. Telescopes: Radio Waves and Radio Telescopes (Cont.)
Figure 4.16 (b) 38

39. Telescopes: Interferometric Arrays
Interferometric arrays 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! 39

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. 40

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. 41

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

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) 43

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

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

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

48. Nebraska Applet
Hydrogen Atom Simulator
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49. Nebraska Applet
Three Views Spectrum Demonstrator
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50. Nebraska Applet
Doppler Shift Demonstrator
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51. Nebraska Applet
Blackbody Curves
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52. Nebraska Applet
Melted Nail Demonstration
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53. Nebraska Applet
Flux Simulator
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54. Nebraska Applet
Telescope Simulator
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55. Nebraska Applet
Snell’s Law Demonstrator
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56. Nebraska Applet
CCD Simulator
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57. This concludes the Lecture slides for
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will, San Diego City College
This concludes the Lecture slides for
CHAPTER 4: Light and Telescopes
wwnpag.es/uou2 57