The Nature of Light

1. 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) • Electric fields • Magnetic fields • Oscillating charges produce electric and magnetic fields • Famous 4 equations (outside the scope of this course) Lecture 6

2. Light as a Wave • Wave • Diffraction • Interference • Can describe waves in terms of wavelength and frequency Lecture 6

3. 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 Lecture 6

4. EM Radiation Spectrum • The frequency/wavelength varies dramatically • Most EM radiation cannot penetrate the Earth’s atmosphere Lecture 6

5. 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: Lecture 6

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 • Gamma rays • Low frequency EM radiation has low energy photons • Long-wave radio • Described by Quantum Mechanics Lecture 6

7. 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 maxT = 2.9 x 10-3 mK Lecture 6

8. 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 • E = T4 where E is the emitted energy, T is the temperature, and  is a constant Lecture 6

9. Spectroscopy in Astronomy • EM radiation carries information about the nature of astronomical object • Visible light is the most used • Light can be • Reflected • From a mirror • Refracted • Through a lens • Dispersed • Separated by wavelength • Prism • Spectrometer Lecture 6

10. 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) Lecture 6

11. Discrete Emission Spectra • When atoms are heated, they emit light at specific wavelengths characteristic of those atoms Lecture 6

12. 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 Lecture 6

13. Probing the Atom • The electron was discovered by J.J. Thomson in 1897 • Related to electricity, lightning • In 1911, Ernest Rutherford bombarded a thin foil of gold with alpha particles from naturally occurring radioactive radium Lecture 6

14. 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 Lecture 6

15. The Hydrogen Atom • The simplest atom is the hydrogen atom • Composed of 1 electron and 1 proton • Electron has charge -1 • Proton has charge +1 • 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 • Heisenberg Uncertainty Principle • We cannot simultaneously know the position and energy of a particle to arbitrary precision Lecture 6

16. 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 • The neutron and proton have almost the same mass but the neutron has not charge • 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 Lecture 6

17. 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 • Isotopes • Hydrogen has 3 naturally occurring isotopes • Hydrogen, 1H • Stable • Deuterium, 2H • Stable • Tritium, 3H • Radioactive Lecture 6

18. The Bohr Atom • Rutherford’s model of the atom had some tragic flaws • Orbiting electrons are accelerating and should radiate energy • 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 Lecture 6

19. 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 = |Ef - Ei| • Ef and Ei are the final and initial energies respectively. • If Ei > Ef, then radiation occurs while if Ef > Ei, 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. Lecture 6

20. 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 • 700 nm • E = 6.62 x 10-34 * 3 x 108 / 700 x 10-9 = 2.8 x 10-19 J = 1.8 eV • 400 nm • E = 6.62 x 10-34 * 3 x 108 / 400 x 10-9 = 5.0 x 10-19 J = 3.1 eV Lecture 6

21. Three Kinds of Spectra • We will consider three kinds of spectra • Continuous • Light bulb or other source • Emission • Heated cloud of gas • Absorption • Continuous spectra passing through a cold cloud of gas Lecture 6

22. 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 Lecture 6

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