Light

1. Light

2. Review Question What are Newton’s laws of motion?

3. Review Question How did Newton know that there had to be a force acting on the Moon to keep it in its orbit about the Earth?

4. Review Question What is Newton’s law of gravity?

5. Review Question How did Newton know that the force of gravity gets weaker with distance?

6. Review Question Why are there two high tides each day on Earth?

7. Review Question What is tidal friction?

8. Review Question Explain how tidal friction is responsible for the synchronous rotation of the Moon?

9. Newton and Kepler Newton’s laws of motion combined with Kepler’s laws led directly to the formulation of the universal law of gravity. Using Newton’s laws and the law a gravity you can derive Kepler’s laws of planetary motion. Planetary motion is thus just a special case of ordinary Newtonian motion.

10. Kepler’s 1st law The planets move in ellipses with the Sun at one focus. Conic section orbits are a natural outcome of the 1/r2 nature of the gravitational force.

11. Kepler’s 2nd law A line drawn from the planet to the Sun sweeps out equal areas in equal intervals of time. Just another way of saying angular momentum is conserved which comes from Newton’s 2nd law of motion.

13. Kepler’s 3rd law The square of the sidereal period is proportional to the cube of the semimajor axis of the orbit: p2 a3 Newton’s laws require:

14. The Scientific method at work • Use the simplest model to explain the observations • Refine the model, make it more complicated, only if new observations require • Explain why the model works

15. Explanation vs. usefulness Kepler’s laws were adopted because they were useful for predicting planetary motions. Newton’s laws explained why Kepler’s laws worked in a broader context.

16. Gravity That one body may act upon another at a distance through a vacuum, … and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has … a competent faculty in thinking, can ever fall into it. Isaac Newton 1692

17. Discovery of Neptune Saturn and Uranus did not follow Kepler’s laws exactly Le Verrier and Adams use these deviations to predict the existence and position of Neptune which was observed in 1846

18. Light The primary source of information we have about the solar system comes from the light received from space.

19. Does light travel instantaneously from one place to another? Galileo unsuccessfully tried to measure speed of light between two mountains using lanterns.

20. Speed of Light Olaus Romer – in 1676 noticed that the timing of eclipses of the moons of Jupiter was dependant on the relative positions of Jupiter and the Earth. The eclipse of a moon of Jupiter was 16.6 minutes later when Jupiter was near conjunction than when Jupiter was near opposition.

21. Discussion Why would the eclipses happen later when Jupiter is at conjunction, then they did when Jupiter was at opposition?

22. Romer’s Speed of Light Determination

23. What is the Nature of Light? Light is a form of electromagnetic radiation, i.e. light is oscillating electric and magnetic fields. An oscillating electric field produces an oscillating magnetic field and an oscillating magnetic field produces an oscillating electric field.

24. Electric and Magnetic Waves

25. Water Waves

26. Frequency Wavelength Relation

27. Electric Field Waves

28. Frequency and wavelength Because the speed of light is constant, higher frequency electromagnetic waves have shorter wavelengths and lower frequency electromagnetic waves of longer wavelengths.

29. We see variations in wavelength or frequency as color White light contains all colors of the rainbow

30. The energy of electromagnetic waves. E = h f = h  c/ h is a constant, f is the frequency • is the wavelength, and c is the speed of light The higher the frequency, or the smaller the wavelength, the higher the energy.

31. The Electromagnetic Spectrum

32. Light interacts with matter in four general ways • Emission • Absorption • Transmission • Reflection

33. Discussion In the movie Superman, the caped wonder claims to be able to tell the color of Lois Lane’s underwear with his X-ray vision. Is this possible, or did Superman rifle though her dresser drawers when she wasn’t home?

34. There are two types of emission • Blackbody or thermal emission which produces a continuous spectrum, i.e. all colors of the rainbow. • Line emission which produces only certain wavelengths of light.

35. Continuous, Absorption and Emission Spectra

36. Discussion All objects emit thermal (blackbody) radiation. Why? Things to consider: Of what is matter composed? What is electromagnetic radiation? What does it mean to say an object is hot?

37. Atom

38. Temperature

39. Properties of thermal radiation • Blackbodies emit radiation at all frequencies. Thus, they emit a continuous spectrum, all the colors of the rainbow.

40. Properties of thermal radiation 2) Wien’s law – the temperature of the object is directly proportional to the frequency of maximum emission.

42. Discussion Why do you think the frequency of maximum emission increases with the temperature of the object?

43. Properties of thermal radiation 3) Stefan-Boltzmann law – the intensity of the emission is proportional to the 4th power of the temperature.

44. Thermal Spectra

46. Infrared Image