CHAPTER 27

CHAPTER 27

1. Photons are massless bundles of concentrated electromagnetic energy.

   • Scientists now agree that light has a dual nature, part particle and part wave.

2. Speed of light: 3.0 x 10^8

   • speed of light = d/t = extra distance travelled/extra time measured.
     • Michelson's experimental value for the speed of light was 299,920 km/s, which is usually rounded to 300,000 km/s.
       • The distance light travels in one year is called a light-year.
         • Light passes through materials whose atoms absorb the energy and immediately reemit it as light.
           • Materials that transmit light are transparent.
             • Materials that absorb light without reemission and thus allow no light through them are opaque.
               • In opaque materials, any coordinated vibrations given by light to the atoms and molecules are turned into random kinetic energy--that is, into internal energy.
                 • A thin beam of light is often called a ray.
                   • When light shines on an object, some of the rays may be stopped while others pass on in a straight-line path.
                     • A shadow is formed where light rays cannot reach.
                       • A total shadow is called an umbra.
                         • A partial shadow is called a penumbra.

3. Electromagnetic waves

   • Light is energy that is emitted by accelerating electric charges--often electrons in atoms. This energy travels in a wave that is partly electric and partly magnetic. Such a wave is an electromagnetic wave.
     • The range of electromagnetic waves is the electromagnetic spectrum.
       • The electromagnetic spectrum consists of radio waves, microwaves, infrared, light, ultraviolet rays, x-rays, and gamma rays.
         • Electromagnetic waves of frequencies lower than the red of visible light are called infrared.
           • Electromagnetic waves of frequencies higher than those of violet are called ultraviolet.

4. Polarization

   • Light travels in waves. The fact that the waves are transverse--and not longitudinal--is demonstrated by the phenomenon of polarization.
     • Light that reflects as glancing angles from nonmetallic surfaces, such as glass, water, or roads, vibrates mainly in the plane of the reflecting surface.
       • A pair of photographs or movie frames, taken a short distance apart (about average eye spacing), can be seen in 3-D when the left eye sees only the left view only the left view and the right eye sees only the right view.