Sci Phy 119a Lecture 4 Eclipses; Radiation from atoms 10/02/006 Opening music: Felix Mendelssohn (1809-1847): "Wedding March" from The Midsummer Night's Dream Overture. First performance: 1827. This piece was written when Mendelssohn was the age of the students in this course (17), yet it permeates the life of virtually everyone married in a conventional ceremony today, in preparation for the richer world of independence and development represented by marriage. Likewise in astronomy, the measurement of the first stellar parallax (Bessell 1838, and the centerpiece of the lecture today), formally separates the science of astronomy before and after the event, as it confirmed the theories of Copernicus and Galileo and proved conclusively that we live in a very large, extended Universe, not a shell on which stars are painted. The recording is by the Chicago Symphony Orchestra, Frederick Stock conducting, made in 1916, a year after Einstein published his theory of general relativity, a theory that again opened a wider world in science, removing one of the more shaky principles of Newtonian physics, action-at-a-distance, and leading to a mathematical toolkit for dealing with the Universe as a whole. The recording played is the first symphony orchestra recording made in the USA. Opening sculpture: Versailles, 1682, designed by Louis the XIV. Louis was trying to identify his new Kingdom with classical elements. He designed the gardens, then wrote a guide book, so that others could guide royalty around in case he was not present. The fountains required so much water when operating that he only had on those fountains that could be seen from a particular perspective at one time, so the tour was carefully orchestrated. Opening art: View from His Window at Le Gras (1822). Gernsheim Collection, Harry Ransom Research Center, University of Texas at Austin. This photograph by Joseph Nicephore Niepce (1765-1833), the Frenchman who invented the photographic process is the earliest surviving photograph. Niepce' process led to the Daguerrotype (Neipce was dead when the process was named). Willian Henry Talbot announced his paper negative process as soon as the Daguerrotype was
announced in 1839. Photography, "nature's painting" as Talbot called it, was an idea whose time was come, invented independently by several individuals. The Niepce photo was made in the year that Olber's described his paradox that the sky was dark, a dilemma that threw the question of stars extended in infinite space into question. In a set of independent discoveries reminiscent of the photograph, three different individuals discovered parallax, between 1836 and 1838. Closing music: Symphony No. 9 in D Minor, by Ludwig von Beethoven (1770-1827). First performed in 1824, based on a text by Schiller, "Ode to Joy". No symphony before this had employed the human voice as an integral part of the work. Beethoven was deaf by the time he wrote this symphony. His assistants had to turn him around to receive the acclaim of the audience after he conducted the first performance because he could not hear the thunderous applause. The music and art today commemorates the discovery of the first parallax. Closing art: The Death of Sardanapalus (1827) by Eugene Delacroix (1798-1863). Currently at the Louvre in Paris. This painting was finished the year Mendelssohn's "Wedding March" was first performed. In France, history painting, in which the artist depicts a famous historic event, was the ultimate test of an artist. In the ecole des Beaux Arts, the government-run painting academy, the final exam required the student to paint a scene of the academy's choosing. The painter had to be well versed in a wide range of biblical and secular stories, know previous paintings on the same theme, and incorporate a repertoire of classical forms and figures. Delacroix won instant notoriety for history paintings in which the figures were convulsed in an intoxicating mix of violence and sensuality unknown to painting before, and certainly a reflection of the violence of his times.
Literature: Oliver Twist by Charles Dickens was published in 1838. The Hunchback of Notre Dame by Victor Hugo was published in 1831.
Derivation of the Universal Gravitational Constant, G I meant to perform the Cavendish experiment, but had to leave this for
a lab. Eclipses and the scale of the solar system. I discussed the scale of the Solar system and how this could be determined in ancient times. The full presentation is in the previous lecture. See next homework. Radiation As a prelude to understanding how we learn about the physics of stars, I explained interference of audio and of light waves and explained the operation of a grating that disperses light into the colors of the rainbow. Wave motion has the property that if a small screen is set up in the path of waves, the waves seem to regenerate themselves as they pass through each hole in the screen. This can be easily demonstrated using a picket fence of popsicle sticks closely space in a sink, for instance. Waves caused by a disturbance break up into new waves at each hole in the screen. A fine pattern of interference ripples is seen on the other side of the screen, opposite the side impacted by waves from the disturbance. A grating consists of fine slits, or grooves, like the openings between the sticks in the above example. The more grooves there are, the more cancellations of crests and troughs of the regenerated waves occur. The positive interference of waves of different wavelength occurs at slightly different positions and a spectrum is formed. In the case of light waves, blue light is always closest to the line perpendicular the to grating, red light is further away. Multiple spectra, called orders, are actually formed. (You saw several orders by looking further and further to one side with the handheld gratings we used in class.) I used three gratings with different numbers of grooves per millimeter and a green laser to demonstrate how the multiple spectra that one sees when using a grating depend on the construction of the grating. I then pointed out that if one spreads the light enough, one can measure small shifts of stellar absorption lines and measure motions of stars, in space as free bodies, and around each other in binary stars, bound to each other by gravity. The relative velocity for a given shift in wavelength is v/c=/ where c is the speed of light, 3x1010 cm/sec.
Emission Lines from Gases I explained that all objects emit continuous radiation that has a fixed shape, called a black body distribution (of photons). The distribution falls off in intensity to the blue and to the red, around a wavelength at which the emission of photons per second reaches a peak. Later, this was demonstrated by looked at the spectrum of the lamps in the ceiling of the lecture hall. Stars emit as almost perfect black bodies. The spectra are slightly modified by effects that will take up much of the discussion time in this course. However, pure gases, excited by an electric charge in the laboratory, produce single, narrow emission lines. I used various cells filled with pure gases (H, He, N, Ne) to demonstrate that each element has a fingerprint of emission lines. With a grating, one can recognize a gas. I also showed the converging series of the hydrogen energy levels. Setting the Bohr levels above to a lower state of 2 and upper states of 3,4,5, one finds the wavelengths are 6563A (red), 4861A (green) and 4340A (blue) and we looked at these lines. Balmer had derived the equation for wavelengths of light of the lines emitted by the hydrogen atom at visible wavelengths empirically, in 1885. He found close agreement if he assigned the values of the integers given above, but did not know what the origin of the discrete integers was. The existence of sharp lines seen in the lamps I used shows that the energy that comes from individual atoms is discrete, not continuous. The energy changes that occur internally in the atoms must then also be discrete (energy is conserved, so for positive energy to be released by an atom, energy must be lost from the atom itself). Bohr, in 1913, finally showed that if the H atom is assumed to consist of an electron in orbit around a charged nucleus, the energy of light emitted corresponds to a change in the radius of the electron orbit. By analogy with planetary motion, the electron must move closer to the nucleus, since such orbits have lower total energy than more distant orbits. (Recall that the potential energy becomes more negative, i.e., decreases, and that the velocity of more distant planets is faster than inner planets, so the kinetic energy is also smaller in the closer orbits.) The analogy holds since the force that binds electron (-) and nucleus (+) is a force proportional to 1/r2, with the masses being replaced by the charges of
the electron and proton. Atomic Spectra Finally, I explained the concept of energy levels in atoms, based on the Bohr assumption that electrons in orbits around atomic nuclei can only exist in particular orbits of particular energy...