1. Lecture 9: Light the Messenger Astronomy 1143 Spring 2014

2. Key Ideas Electromagnetic spectrum Energy, wavelength, frequency Spectrum - amount of light at each wavelength Nature of light – wave – particle (photon) Speed of light is a constant for all observers Surprising! Postulate of Special Relativity But confirmed by observations We see distant objects as they were A light-year is distance light travels in 1 year

3. Light the Messenger Almost all information we receive about the Universe comes in the form of light Before discussing the size of the Milky Way Galaxy, the distances to nearby galaxies and the expansion of the Universe, we need to get some facts under our belts How to tell if a star is like the Sun Temperature Absorption lines Light has a wavelength, which can be stretched or squeezed How knowing brightness and luminosity = distance Light is special – start of Special Relativity

4. Electromagnetic Radiation Light is Electromagnetic Radiation. Self-propagating Electromagnetic disturbance that carries energy at the speed of light. Two ways to describe light: Electromagnetic Waves Photons (particles of light)

5. Light is a Wave Wave = a periodic fluctuation traveling through a medium.

6. Ocean wave = fluctuation in height of water. Sound wave = fluctuation in pressure. Electromagnetic wave = fluctuation in electric and magnetic fields.

7. Oscillating Electric & Magnetic Fields

8. Wavelength ( ) Frequency (f) (# waves/second) Speed (c) c =  f

9. Wavelength ( ) Frequency (f) (# waves/second) Speed (c) c =  f

10. Different than water waves If you run into the waves, the wavelength stays the same, but the frequency will increase – speed increases Most waves need a medium But light doesn’t need a medium

11. The Electromagnetic Spectrum

12. ←medical X rays ←microwave oven ←tanning rays ←cell phones ←WOSU FM ←WOSU AM

13. Waves show diffraction

14. Example of sea waves & diffraction

15. Waves vs. Particles

16. Proof that light is a wave

17. Radio Waves Diffract!

18. Photons: Particles of Light Can also treat light as particles or Photons. Photon: Massless particle that carries energy at the speed of light. Photon Energy: E = hf f = frequency of the light h = Planck’s Constant

19. Light is made of particles. Photons kick electrons out of atoms, if they have enough energy.

20. Proof that light is a particle Individual X-ray photons

21. Wave or particle? Both Light has properties of both waves and a stream of particles. It follows the rules of quantum mechanics Other quantum mechanical particles include the electron, proton and neutron Electrons are not just particles! They also show diffraction.

22. Why describe light by both? Important to think about light as a wave: Stretching of wavelengths Important to think about light as a particle: Knocking electrons off of atoms Force carrier – part of the Standard Model of Particle Physics

23. The Electromagnetic Spectrum Sequence of photon energies from low to high is called the Electromagnetic Spectrum low energy = low frequency= long wavelength Examples: Radio Waves, Microwaves, Infrared high energy=high frequency=short wavelength Examples: Ultraviolet, X-rays, Gamma Rays

24. Photons as Dogs Low energy Long wavelength Low frequency High energy Short wavelength High frequency

25. Most objects emit more than one wavelength of light Our eyes and brain report one color of light, the dominant color Primary colors for mixing light – red, green and blue If all colors of visible light are about equally represented – white light

26. What is a Spectrum? A spectrum is the distribution of photon energies emitted by a light source: Asks: how many photons of each energy are emitted? Spectra are observed by passing light through a spectrograph: Breaks light into its component colors. Uses either prisms or diffraction gratings.

27. Spectrum Prism White Light Prisms disperse light into its component colors

28. In Class Demo Gratings and white light Lasers

29. The Beginnings of Relativity Speed of light is a constant for all light waves or photons: c = 299,792.458 km/sec Independent of wavelength or frequency. It also doesn’t matter how fast the light source is moving or how fast the observer is moving. Light very different than other waves Light is special

30. Common Sense, But Incorrect, Ideas Time is absolute: The time between two events is independent of the observer. Events are always in the same order. Space is absolute: The distance between two events is independent of the observer. Speeds are relative: How fast something is moving depends on how fast you are moving

31. Test: Michelson-Morley experiment Michelson-Morley experiment showed that light is different They attempted to measure the change in the speed of light as the Earth moved around the Sun through “the ether” They found that the speed of light did not change Experimental results helped lead to a new theory

32. Relative Velocities - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 50 mph

33. Relative Velocities Person standing on the edge of the road: Two cars going 50 mph to the left One car going 50 mph to the right Cars in the top lanes One car going 0 mph One car going 100 mph to the right One person going 50 mph to the right Car in bottom lane Two cars going 100 mph to the left 1 person going 50 mph to the left

34. Einstein’s Revolution 1905: Proposed his Theory of Special Relativity Accepted the experimental work on the speed of light and the nature of electromagnetic waves Now the speed of light was absolute and time and space were relative

35. Consequence: A Real Speed Limit Imagine that you start running at the same time your friend turns on a flashlight. You are racing a beam of light. Can you win?

36. 0.99 c Galactica sees the photons arriving at speed c - - - - - - - - - - -

37. Consequence: A Real Speed Limit You cannot win! Both you and your friend will measure the speed of light to be the same. No matter how fast you go, the light will always be traveling at 2.9979x10 8 m/s relative to you It will beat you to the end every time Nothing can travel faster than light

38. If the Sun stopped shining, when would we know about it?

39. Lookback Time The fact that there is a cosmic speed limit is both good & bad  can’t visit galaxies  Have a “time machine” because we see distant things as they were a long time ago.

40. Looking Back in Time If we know how many lightyears away something is, we know how far back in time we are looking