Lecture 1 Phys 322 Optics Lecture 1 Historical introduction Optics in Ancient History A mirror was discovered in workers' quarters near the tomb of Pharaoh Sesostris II (1900 BCE). Pyramid of Sesostris II (also known as Senusret II) Ancient Greeks (500-300 BCE) Burning glass mentioned by Aristophanes (424 BCE) Law of reflection: “Catoptrics” by Euclid (300 BCE) Refraction in water mentioned by Plato in “The Republic” But they thought that the eye emits rays that reflect off objects. Ancient Greeks: Ancient light weapons Early Greek and Roman historians report that Archimedes equipped several hundred people with metal mirrors to focus sunlight onto Roman warships in the battle of Syracuse (213 - 211 BCE). This story is probably apocryphal. Optics in the Middle Ages: Alhazen Arab scientist Alhazen (~1000 AD) studied spherical and parabolic mirrors. Alhazen correctly proposed that the eyes passively receive light reflected from objects, rather than emanating light rays themselves. He also explained the laws of reflection and refraction by the slower movement of light through denser substances. Optics in early 17th-century Europe Hans Lippershey applied for a patent on the Galilean telescope in 1608. Galileo (1564-1642) used one to look at our moon, Jupiter and its moons, and the sun. Two of Galileo’s telescopes Galileo’s drawings of the moon 17th century: optics takes off Lecture 1 Progress in optical instrumentation: refracting telescope, compound microscope • 1621: Law of Refraction, Willebrord Snell • 1664: interference: color in thin films, Robert Hooke light is a rapid vibratory motion in media • 1665: diffraction, Francesco Grimaldi • 1677: wave theory, Christiaan Huygens • 1704: particles, Isaac Newton Newton about Newton’s rings: "I forbore to treat of these Colors, because they seemed of a more difficult Consideration, and were not necessary for establishing the Properties of Light there discoursed of." Willibrord Snell Willibrord Snell discovered the Law of Refraction, now named after him. 1 n1 Willibrord Snell (1591-1626) n2 2 nn1122sin( ) sin( ) ni is the refractive index of each medium. 17th-century Optics Descartes reasoned that light must be like sound. So he modeled light as pressure variations in a medium (aether). Rene Descartes (1596-1659) Robert Hooke (1635-1703) studied colored interference between thin films and developed the first wave theory of light. Christiaan Huygens Huygens extended the wave theory of optics. He realized that light slowed down on entering dense media. Christiaan Huygens He explained polarization and (1629-1695) double refraction. Huygens‘ principle says that a wave propagates as if the wave-front were composed of an ar- ray of point sources each emitting a Double refraction spherical wave. Isaac Newton "I procured me a triangular glass prism to try therewith the celebrated phenomena of colours." (Newton, 1665) Isaac Newton (1642-1727) After remaining ambivalent for many years, he eventually concluded that it was evidence for a particle theory of light. Particles or waves? Isaac Newton Christiaan Huygens 1643 – 1727 1629 – 1695 …18th century… Corpuscular theory prevails Wave theory is forgotten… 19th century • 1801: interference, Thomas Young famous double-slit experiment color in thin films diffraction of light diffraction grating: "These colors may be easily seen, in an irregular form, by looking at any metal, coarsely polished, in the sunshine; but they become more distinct and conspicuous, when a number of fine lines of equal strength are drawn parallel to each other, so as to conspire in their effects." • 1814, Frensel ‘rediscovers’ interference and diffraction James Clerk Maxwell Maxwell unified electricity and magnetism with his now famous equations and showed that light is an electromagnetic wave. B EE 0 t James Clerk 1 E Maxwell (1831- BB 0 ct2 1879) where E is the electric field, B is the magnetic field, and c is the velocity of light. Maxwell’s equations simplify to the wave equation for the electric field. 1 2 E 2 E 0 ct22 which has a simple sine-wave solution: E(,)rt cos( t k r ) where ck / The same is true for the magnetic field. 19th century: Maxwell Maxwell’s equations ~1864: q E E A Maxwell introduced four 0 equations that described all known B B A 0 electro-magnetic phenomena and showed theoretically that B E E||l electromagnetic pulse or wave t moving in space could exist. E B||l 0 I 0 t Surprisingly, he found that this EM- wave must move at a speed of 300,000 km/s - i.e. speed of light! Maxwell suggested that light is electromagnetic wave Light is an electromagnetic wave. The electric (E) and magnetic (B) fields are in phase. The electric field, the magnetic field, and the propagation direction are all perpendicular. 19th century: Hertz In 1886 Heinrich Hertz experimentally proved the electromagnetic wave nature of light "It's of no use whatsoever,“ "This is just an experiment that proves Maestro Maxwell was right - we just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there.“ 1857 - 1894 EM wave transmitter EM wave receiver Michelson & Morley Michelson and Morley then attempted to measure the earth's velocity with respect to the aether and found it to be zero, effectively disproving the existence of the aether. Albert Edward Michelson Morley (1852-1931) (1838-1923) 20th century Difficulties: Wave theory cannot explain: - black body radiation spectrum - photoelectric effect - speed of light measured by two detectors moving in respect to each other is exactly the same… 20th century: The birth of quantum theory 1900: To explain black body radiation, Planck suggested that energy of light consists of quanta Planck’s constant E = h photon frequency Max Karl Ernst Ludwig Planck 1905: Albert Einstein proposed that light consists of particles (photons) that have energy and momentum depending on frequency Special theory of relativity: speed of light in vacuum is the same in all inertial reference systems 20th century: quantum theory Quantum nature of light is pronounced at low light intensities Light: dual nature, both wave and particle In this course, we will mostly work in the frame of classical EM wave theory of light Albert Einstein Einstein showed that light: is a phenomenon of empty space; has a velocity that’s constant, independent of observer velocity; is both a wave and a particle; Albert Einstein (1879-1955) Excited medium and undergoes stimulated emission, the basis of the laser..