1. Lecture Outline Light ELECTROMAGNETIC SPECTRUM

2. What Is Light? Take out a piece of paper Answer the question

3. Light and Matter Almost all knowledge of the Universe beyond Earth comes from light. Light can tell us about objects in space: temperature, composition, speeds, and more.

4. The Speed of Light is Finite Light moves at 300,000 km/s in a vacuum. First measured by Rømer when observing Jupiter’s moons. Speed is slower in other materials, e.g., glass.

5. But, what is light? In the 17th Century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves. In the 19 th and 20 th Centuries Maxwell, Young, Einstein and others were able to show that Light behaves both like a particle and a wave depending on how you observe it.

6. Thomas Young’s interference experiment

7. Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields.

8. Light is an Electromagnetic Wave Light is a wave of combined electricity and magnetism, called an electromagnetic wave. Changing electric and magnetic fields create a self-sustaining electromagnetic wave.

9. In 1905 Einstein calculated the energy of a particle of light ( photon) and proposed the photoelectric effect. E photon = hc/  hf h = Planck’s constant c = speed of light = wavelength f = frequency e- photo n Which means: Long = Low f = Low E Short = High f = High E

10. Waves Wavelength ( ): length between crests. Amplitude: height Frequency (f): number of waves that pass by each second Period (P): Time to complete one cycle

11. © 2007 W.W. Norton & Company, Inc. 21st CENTURY ASTRONOMY, 2/e 11 Wavelength and Frequency

12. Wavelength, Frequency, and Speed A long wavelength means low frequency. A short wavelength means high frequency. The speed of light, c, is constant.

13. Electromagnetic Spectrum ← Visible Light There are other kinds of electromagnetic waves that we can’t see. Radio Microwave Infrared Visible Ultraviolet X-ray Gamma Ray

14. The Electromagnetic Spectrum Visible light is just one small part of the electromagnetic spectrum: Gamma rays X-rays UV Visible IR Microwave Radio increasing f decreasing

15. Where Does Light Come From? Light comes from atoms in an excited state. Energy is added to it, an electron jumps up to a higher energy level (or shell) When it comes back down out of the excited state, it emits light. Exists first in ground state.

16. Emission/Absorption Different elements emit different wavelengths (i.e. different colors) You can identify an element based on this spectrum.

17. Electron Configuration Quantum Number = Shell Number = Energy Level Notice in the last column: E n = n 2 E 1

18. Hydrogen’s First Four Principal Energy Levels

19. These Do Not Go In Order

20. Examples

21. Which of the following would be true about comparing gamma rays and radio waves? A.The radio waves would have a lower energy and would travel slower than gamma rays. B. The gamma rays would have a shorter wavelength and a lower energy than radio waves. C. The radio waves would have a longer wavelength and travel the same speed as gamma rays. D. The gamma rays would have a higher energy and would travel faster than radio waves. E. The radio waves would have a shorter wavelength and higher energy than gamma rays.

22. On the FM dial, the frequencies run from 88 – 107 MHz (million cycles per second). On the AM dial, the range is 570 – 1600 kHz (thousand cycles per second). Which statements are true? A.FM photons have shorter wavelengths than AM. B.FM photons have higher energies than AM. C.FM photons travel at the same speed as AM. D.All the above statements are true.

23. Lecture Tutorial – Electromagnetic Spectrum of Light (p. 45) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

24. Light and Matter Almost all knowledge of the Universe beyond Earth comes from light. Light can tell us about objects in space: temperature, composition, speeds, and more.

25. Light and Matter Almost all knowledge of the Universe beyond Earth comes from light. Light can tell us about objects in space: temperature, composition, speeds, and more. Today we will discuss how light can give us speed of an object.

26. The Doppler Effect Definition: “The change in wavelength of radiation (light) due to the relative motion between the source and the observer along the line of sight.”

27. Astronomers use the Doppler Effect to learn about the radial (along the line of sight) motions of stars, and other astronomical objects.

28. Real Life Examples of Doppler Effect Doppler Radar (for weather) Airplane radar system Submarine radar system – Ok, anything with radar Radar gun, used by the police

29. The Doppler Effect Definition: “The change in wavelength of radiation (light) due to the relative motion between the source and the observer along the line of sight.”

30. Doppler Effect When something which is giving off light moves towards or away from you, the wavelength of the emitted light is changed or shifted V=0

31. Doppler Effect When the source of light is moving away from the observer the wavelength of the emitted light will appear to increase. We call this a “redshift”.

32. Doppler Effect When the source of light is moving towards the observer the wavelength of the emitted light will appear to decrease. We call this a “blueshift”.

33. The Doppler Effect Definition: “The change in wavelength of radiation due to relative motion between the source and the observer along the line of sight.”

34. Doppler Effect “Along the line of sight” means the Doppler Effect happens only if the object which is emitting light is moving towards you or away from you. An object moving “side to side” or perpendicular, relative to your line of sight, will not experience a Doppler Effect.

35. Astronomy Application V=0

36. The Doppler Effect 1. Light emitted from an object moving towards you will have its wavelength shortened. 2. Light emitted from an object moving away from you will have its wavelength lengthened. 3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength. BLUESHIFT REDSHIFT

37. Measuring Radial Velocity The amount of shift of the lines in the spectrum (measure from where they should be to where they are) tells us how fast the object is moving along our line of sight (radial velocity) If the lines are shifted to smaller wavelengths = blueshift = moving towards us If the lines are shifted to larger wavelengths = redshift = moving away from us  = v r 0 c  = redshift or blueshift change in wavelength 0 = source wavelength v r = radial velocity of source c = speed of light

38. A bright star is moving toward Earth. If you were to look at the spectrum of this star, what would it look like? A.an absorption spectrum that is redshifted relative to an unmoving star B.an emission spectrum that is redshifted relative to an unmoving star C.an absorption spectrum that is blueshifted relative to an unmoving star D.an emission spectrum that is blueshifted relative to an unmoving star

39. Lecture Tutorial – Doppler Shift Parts I & II (p. 73) Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.