Chapter 27 Light

1. Chapter 27 Light Conceptual Physics Chapter 27

2. Early Concepts of Light • Ancient Greek philosophers thought that light consisted of tiny particles which could enter the eye to create the sensation of vision. • Socrates and Plato thought that vision resulted from streamers emitted from the eye making contact with an object. • Up until the end of the 18th century most scientists, including Newton, thought that light consisted of particles. • In the 19th century, a wave theory of light was accepted by most after Dutch scientist Christian Huygens found clear evidence that light undergoes diffraction – a wave property. • In 1905, Einstein’s discovery of the photoelectric effect was further evidence that light consists of particles. • Scientists now agree that light has a dual nature – part particle and part wave. Conceptual Physics Chapter 27

3. Speed of Light • Since light travels so fast, many early attempts to measure this speed failed. • Galileo and others tried to measure the time it takes for a light beam to travel to a distant mirror and back, but only discovered their own reaction time was far greater than the light’s transit time. • Danish astronomer Olaf Roemer was the first to provide clear evidence that light traveled at a finite speed. Conceptual Physics Chapter 27

4. Roemer’s Approximation • Roemer made careful observations of Jupiter’s moon Io and found the period of revolution of Io to be 42.5 hours. • Roemer discovered that the measured periods of Io were all somewhat longer when Earth receded from Jupiter and somewhat shorter when Earth approached Jupiter. • Roemer concluded that the 22 minute discrepancy in measured values was due to the extra distance the light had to travel across the diameter of the Earth’s orbit. • With the time discrepancy and known diameter of Earth’s orbit, Roemer was able to determine the speed of light. Conceptual Physics Chapter 27

5. Michelson’s Approximation • In 1880 American physicist Albert Michelson used an octagonal-shaped mirror to direct an intense beam of light to a stationary mirror located atop a tall mountain 35 km away. • The octagonal mirror was spun creating short bursts of light which could be seen by an observer positioned nearby the mirror. • When the mirror is spun at just the right speed, the observer will see a continuous beam of light. • Michelson became the first American to win the Nobel Prize in physics for his work. Run Simulation Conceptual Physics Chapter 27

6. Speed of Light • Roemer’s approximation led to a value of 200,000,000 m/s with most of the error resulting from an inaccurate value of the diameter of Earth’s orbit. • Michelson’s approximation led to a value of 299, 920,000 m/s. • We now use a value of 299,792,458 m/s. For our purposes, we will round the value to 3.0 x 108 m/s. • This value does not only indicate the speed at which visible light travels, but the speed of all electromagnetic waves. Conceptual Physics Chapter 27

7. Speed of Light The speed of light in a vacuum is a universal constant. Light takes 8 minutes to travel from the sun to Earth and 4 years from the next nearest star, Alpha Centauri. Light is so fast that if a beam of light could travel around Earth, it would make 7.5 trips in one second. The distance light travels in one year is called a light-year. Conceptual Physics Chapter 27 7

8. Electromagnetic Waves • Light energy is emitted by accelerating or vibrating electric charges. • The energy travels in a complex wave form that is partly electric and partly magnetic. Conceptual Physics Chapter 27

9. Electromagnetic Spectrum • All waves that are able to propagate through empty space are included on the electromagnetic spectrum. • The spectrum shown below is arranged from the lowest frequency waves (about 104 Hz) to highest frequency waves (about 1018 Hz). Wavelengths range from 104 m to 10-10 m. • The low frequency waves have the longest wavelengths while the high frequency waves have the shortest wavelengths, but all these waves travel at the same speed. Conceptual Physics Chapter 27

10. Absorption of Light • Atoms and molecules contain electrons. • We can imagine these electrons to be connected to the atomic nucleus by springs. • Electrons of different atoms have a tendency to vibrate at different natural frequencies depending on their different “spring strengths”. • When a light wave with this same frequency encounters these electrons, the electrons absorb the energy and begin to undergo large amplitude vibrations – resonance occurs! • These large amplitude vibrations lead to many collisions with neighboring atoms and the energy is distributed across the material as heat - the light has been absorbed. Conceptual Physics Chapter 27

11. Transmission of Light • If the frequency of the incident light does not match the natural frequency of the electrons in the material, no resonance will occur. • Instead the electrons will undergo much smaller vibrations for very brief periods of time. • The time delay, however, between the absorption and the reemission of light reduces the average speed of the light in the medium. The instantaneous speed of light is a constant – 3.0 x 108 m/s. Conceptual Physics Chapter 27

12. Transmission of Light • The actual value for the average speed of light in a medium is dependant on the optical density of the material, which determines how frequently the light interacts with the atoms of the material. • The energy is then reemitted as transmitted light. The transmitted light will have all of the same characteristics (frequency, wavelength, energy) that it started with. Conceptual Physics Chapter 27

13. Light in Glass • Electrons in glass have a natural frequency in the ultraviolet range. • Infrared waves, which are lower in frequency than visible light, vibrate not only the electrons, but the entire glass structure. • Glass is transparent to visible light, but not to infrared and ultraviolet light. Conceptual Physics Chapter 27

14. Translucent Materials • Translucent materials allow partial transmission of light with some distortion or diffusion. Conceptual Physics Chapter 27

15. Opaque Materials • When light falls on an opaque body or material, none of the light is transmitted through the body. • Instead, the light is either absorbed or reflected. • If the light is absorbed, energy given by the light to the atoms of the material is turned into thermal energy. • In metals, the same property that allows these materials to be good conductors leads to the surface atoms in these materials vibrating briefly and then reemitting the energy as reflected light. This leads to the shiny appearance of metals. Conceptual Physics Chapter 27

16. Shadows • When an opaque body blocks light, a shadow is formed where light rays cannot reach. • The shadow will have very sharp edges if the light source is small relative to the opaque object or if the light source is far away from the opaque object. • In many cases, the shadow will be blurry with a darkened interior region called the umbra and a softer shadow around the edges referred to as the penumbra. Conceptual Physics Chapter 27

17. Shadows • The umbra is created when light from a source is completely blocked. • A penumbra is created when a portion of the light source is blocked and other light fills in. This can occur with multiple light sources or with a single source that is relatively large. Conceptual Physics Chapter 27

18. Eclipses • A lunar eclipse is generated when the Earth orbits directly between the sun and the moon, casting Earth’s shadow onto the moon. • A solar eclipse is generated when the moon casts its shadow onto the Earth. Because of the relative sizes of the Earth and moon, solar eclipses are seen by far fewer people than lunar eclipses. Conceptual Physics Chapter 27

19. Polarization • When transverse waves oscillate in a single plane, they are said to be polarized. • If a rope is shaken up and down, a vertically polarized wave is produced. • If the rope is shaken side to side, a horizontally polarized wave is produced. Conceptual Physics Chapter 27

20. Polarization of Light • A single vibrating electron emits an electromagnetic wave that is polarized. • Most light sources emit light that is not polarized – the vibrating electrons that emit the light vibrate in many different directions. Conceptual Physics Chapter 27

21. Polarization of Light • When non-polarized light shines on a polarizing filter, the light that is transmitted is polarized. • The filter is said to have a polarization axis that is in the direction of the vibrations of the polarized light wave. • Light waves vibrating in a plane that is perpendicular to the polarization axis will be absorbed by the filter, while those waves vibrating parallel to the polarization axis are transmitted. • Polaroid filters transmit 50% of non-polarized light. Conceptual Physics Chapter 27

22. Polarization of Light • 50% of non-polarized light is transmitted through a pair of polarizing filters when their polarization axes are aligned. • This is similar to the behavior of a vibrating rope passing through a pair of picket fences. Conceptual Physics Chapter 27

23. Polarization of Light • When a pair of filters have their polarization axes crossed at right angles, no light is transmitted. The first filter absorbs and re-radiates only vertically oscillating electric fields; the second will pass only horizontally oscillating fields. Conceptual Physics Chapter 27

24. Polarization of Light • When a third polaroid filter is sandwiched at 45° between two filters crossed at right angles, some light is transmitted. Conceptual Physics Chapter 27

25. Plane Polarization • When light reflects at an angle from a nonmetallic surface such as glass, water or roads, it vibrates mainly in the plane of the reflecting surface. • Glare from a horizontal surface such as a lake is horizontally polarized. • The polarization axis of Polaroid sunglasses are oriented vertically to reduce glare. Conceptual Physics Chapter 27

26. Plane Polarization Which pair of glasses is best suited for automobile drivers? (The polarization axes are shown by the straight lines.) Conceptual Physics Chapter 27 26