Light travels at 300,000 km/s. It has properties of both waves and particles. Spectral lines identify elements in stars - each element produces a unique set of lines. Astronomers use spectral lines and Wien's and Stefan-Boltzmann laws to determine surface temperatures of stars and planets from their emitted light.

1. Junhel C. Dalanon, DMD, MAT

8. Although Isaac Newton suggested that light was made of tiny particles called P HOTONS 130 years earlier, Thomas Young demonstrated in 1801 that light has wave-like properties. He passed a beam of light through two narrow slits which resulted in a pattern of bright and dark bands on a stream. This is the pattern one would expect if light had wave-like properties.

9. Imagine water passing through two narrow openings as shown below. As the water moves out, the resulting waves alternatively cancel and reinforce each other, much like what was observed in Young’s Double Slit Experiment . This is the pattern one would expect if light had wave-like properties.

10. Today, we understand that light has characteristics of both particles and waves. Light behaves according to the same equations that govern electric and magnetic fields that move at 300,000 km/s so light is also called electromagnetic radiation . Electromagnetic radiation consists of oscillating electric and magnetic fields. The distance between two successive wave crests is called the wavelength and is designated by the letter  .

11. Electromagnetic radiation is produced by stars at a wide variety of wavelengths in addition to visible light. Astronomers sometimes describe EM radiation in terms of frequency,  , instead of wavelength,  . The relationship is: c =  x  Where c is the speed of light, 3 x 10 8 m/s

14. B LACKBODY C URVES : Each of these curves shows the intensity of light emitted at every wavelength for an idealized object (called a “blackbody”) for several different temperatures. These are called blackbody curves . Note that for the objects at the highest temperature, the maximum intensity is at the shorter wavelengths and that the total amount of energy emitted is greatest.

15. Astronomers most often use the Kelvin or Celsius temperature scales. In the Kelvin scale, the 0 K point is the temperature at which there is essentially no atomic motion is called absolute zero. In the Celsius scale, this point is –273 º C and on the Fahrenheit scale, this point is -460 ºF.

19. Each chemical element produces its own unique set of spectral lines.

22. The brightness of spectral lines depend on conditions in the spectrum’s source.

26. Kirchhoff’s Laws

27. Features of the Sun’s spectrum created by passing sunlight through a prism.

28. Emission Line Spectra of A Few Common Elements

29. The Electromagnetic Spectrum

30. Electromagnetic Radiation: Radio Waves (TV,  ~ 1m) Antenna size ~1m

31. But, where does light actually come from? Light comes from the movement of electrons in atoms.

32. Rutherford’s Experiment (1915) Showed that Atoms Are Largely Empty Space! Alpha particles from a radioactive source are channeled through a very thin sheet of gold foil. Most pass through showing that atoms are mostly empty space, but a few are rejected showing the tiny nucleus is very massive.

33. An atom consists of a small, dense nucleus surrounded by electrons ( Note: Nucleus actually much smaller)

35. Orbits of electrons

36. Spectral lines are produced when an electron jumps from one energy level to another within an atom.

39. The wavelength of a spectral line is affected by the relative motion between the source and the observer.

40. Doppler Effect: Caused by Motion