1. ISP 205 - Astronomy Gary D. Westfall1Lecture 6 The Nature of Light Light and other forms of radiation carry information to us from distance astronomical objects Visible light is a subset of a huge spectrum of electromagnetic radiation Maxwell pioneered the theory of electromagnetic radiation (and light) n Electric fields n Magnetic fields n Oscillating charges produce electric and magnetic fields n Famous 4 equations (outside the scope of this course)

2. ISP 205 - Astronomy Gary D. Westfall2Lecture 6 Light as a Wave Wave n Diffraction n Interference Can describe waves in terms of wavelength and frequency

3. ISP 205 - Astronomy Gary D. Westfall3Lecture 6 Electromagnetic Radiation Light differs from other forms of electromagnetic radiation by its wavelength Visible light has wavelengths between 400 and 700 nanometers EM radiation with wavelengths just longer than visible light is called infrared radiation (heat) EM radiation with wavelength just shorter than visible light is called ultraviolet radiation (UV) Radio waves have long wavelengths (WKAR FM is 3 meters) Microwaves have about 3 cm wavelength

4. ISP 205 - Astronomy Gary D. Westfall4Lecture 6 EM Radiation Spectrum The frequency/wavelength varies dramatically Most EM radiation cannot penetrate the Earth’s atmosphere

5. ISP 205 - Astronomy Gary D. Westfall5Lecture 6 View of the Sky with X rays If we could “see” with X rays instead of visible light and we were above the Earth’s atmosphere the sky would look like:

6. ISP 205 - Astronomy Gary D. Westfall6Lecture 6 Light as a Particle Light (and all EM radiation) exists in quantized units called photons A photon carries a specific amount of energy High frequency EM radiation has high energy photons n Gamma rays Low frequency EM radiation has low energy photons n Long-wave radio Described by Quantum Mechanics

7. ISP 205 - Astronomy Gary D. Westfall7Lecture 6 Radiation and Temperature The temperature of an object determines what wavelength of EM radiation it will emit The wavelength of the maximum energy emission is given by Wien’s Law max T = 2.9 x 10 -3 mK

8. ISP 205 - Astronomy Gary D. Westfall8Lecture 6 Energy Emitted by Stars The higher the temperature of an object, the more energy is radiated at all wavelengths The higher the temperature, the “bluer” the star looks The total energy radiated is given by the Stefan- Boltzmann law n E =  T 4 where E is the emitted energy, T is the temperature, and  is a constant

9. ISP 205 - Astronomy Gary D. Westfall9Lecture 6 Spectroscopy in Astronomy EM radiation carries information about the nature of astronomical object Visible light is the most used Light can be n Reflected u From a mirror n Refracted u Through a lens n Dispersed u Separated by wavelength H Prism H Spectrometer

10. ISP 205 - Astronomy Gary D. Westfall10Lecture 6 Continuous Spectrum When white light (a superposition of light with all wavelengths) is dispersed with a prism or a spectrometer, all colors (wavelengths) are visible Wavelengths shorter than 400 nm are invisible (UV) Wavlengths longer than 700 nm are invisible (IR)

11. ISP 205 - Astronomy Gary D. Westfall11Lecture 6 Discrete Emission Spectra When atoms are heated, they emit light at specific wavelengths characteristic of those atoms

12. ISP 205 - Astronomy Gary D. Westfall12Lecture 6 Discrete Absorption Spectra When white light passes through atoms light is absorbed at specific wavelengths Several elements were first observed in absorption spectra from the sun

13. ISP 205 - Astronomy Gary D. Westfall13Lecture 6 Probing the Atom The electron was discovered by J.J. Thomson in 1897 n Related to electricity, lightning In 1911, Ernest Rutherford bombarded a thin foil of gold with alpha particles from naturally occurring radioactive radium

14. ISP 205 - Astronomy Gary D. Westfall14Lecture 6 Rutherford’s Model of the Atom Rutherford’s results showed that most of the mass of the atom was concentrated in the nucleus Rutherford proposed a model similar to the solar system with negative electrons orbiting a positive nucleus

15. ISP 205 - Astronomy Gary D. Westfall15Lecture 6 The Hydrogen Atom The simplest atom is the hydrogen atom Composed of 1 electron and 1 proton n Electron has charge -1 n Proton has charge +1 n Proton is 2000 times heavier The electron is bound to the proton in its ground state We know now that the electron does not orbit the proton like the Earth orbits the Sun n Heisenberg Uncertainty Principle u We cannot simultaneously know the position and energy of a particle to arbitrary precision

16. ISP 205 - Astronomy Gary D. Westfall16Lecture 6 Other Atoms The next most simple atom is helium A helium atom has 2 neutrons and 2 protons in its nucleus with 2 electrons orbiting the nucleus n The neutron and proton have almost the same mass but the neutron has not charge u Neutron not discovered until 1930 by Chadwick The helium atom is much more complicated than the hydrogen atom because the 2 electrons interact with each other

17. ISP 205 - Astronomy Gary D. Westfall17Lecture 6 Isotopes The chemical properties of atoms are determined by the number protons and the number of electrons Light nuclei have roughly the same number of neutrons and protons Atomic nuclei can have different number of neutrons n Isotopes Hydrogen has 3 naturally occurring isotopes n Hydrogen, 1 H u Stable n Deuterium, 2 H u Stable n Tritium, 3 H u Radioactive

18. ISP 205 - Astronomy Gary D. Westfall18Lecture 6 The Bohr Atom Rutherford’s model of the atom had some tragic flaws Orbiting electrons are accelerating and should radiate energy n Lifetime of the atom should be 10 -10 seconds! Neils Bohr proposed that the electrons in the hydrogen atom could only exist in certain quantized orbits Jumping between the orbits required the emission or absorption of photons of a specific wavelength

19. ISP 205 - Astronomy Gary D. Westfall19Lecture 6 Radiation and Absorption Whenever a hydrogen atom changes from one stationary state to another, energy is emitted or absorbed. When that energy takes the form of electromagnetic radiation then it has a frequency f (or as it is often called, ) given by h = |E f - E i | n E f and E i are the final and initial energies respectively. If E i > E f, then radiation occurs while if E f > E i, then absorption takes place. In the diagram the first six levels are shown as well as the zero energy level (n =  ). If the transition takes place from any n to n =1, it is referred to as a Lyman line. Transitions to n = 2 are called Balmer lines and so on. Four of the Balmer lines are in the visible range.

20. ISP 205 - Astronomy Gary D. Westfall20Lecture 6 Photon Energies Visible light has wavelengths between 400 and 700 nm Photons have energy E = hf = hc/ Photons from visible light then have energies between n 700 nm u E = 6.62 x 10 -34 * 3 x 10 8 / 700 x 10 -9 = 2.8 x 10 -19 J = 1.8 eV n 400 nm u E = 6.62 x 10 -34 * 3 x 10 8 / 400 x 10 -9 = 5.0 x 10 -19 J = 3.1 eV

21. ISP 205 - Astronomy Gary D. Westfall21Lecture 6 Three Kinds of Spectra We will consider three kinds of spectra n Continuous u Light bulb or other source n Emission u Heated cloud of gas n Absorption u Continuous spectra passing through a cold cloud of gas

22. ISP 205 - Astronomy Gary D. Westfall22Lecture 6 Doppler Shift Relative motion affects waves If a source of waves is moving toward you, the frequency is higher and the wavelength is shorter If a source of waves is moving away from you the frequency is lower and the wavelength is longer Toward, shorter wavelength, blue shift Away, longer wavelength, red shift A familiar example is sound

23. ISP 205 - Astronomy Gary D. Westfall23Lecture 6 Red Shift Most of the objects in the universe seem to be moving away from us n Evidence for Big Bang n Red shift n v = c  / We observe the red shift of specific emission lines from known atoms n Hydrogen or calcium have distinctive lines and are almost always present