ch27_jnlr

Chapter 27- Light **
 * toc

27.1 Early Concepts of Light

 * Photons**- In the particle model of electromagnetic radiation, a particle that travels only at the speed of light and whose energy is related to the frequency of the radiation in the wave model.


 * Photo-electric effect**- Einsteins theory that light consists of particles-- massless bundles of concentrated electromagnetic energy--later called photons.

Most philosophers and scientists believed that light consisted of particles. However, some argued with this and taught that it traveled as a wave. Huygens provided evidence that under some circumstances light does spread out backing up the wave theory. Scientists now agree that light has a dual nature, part particle and part wave.

27.2 The Speed of Light
math Speed of Light = \frac{extra distance traveled}{extra time measured} math math = \frac{300 000 000 km}{1000 s} = 300 000 km/s math


 * light-year**- the distance light travels through a vacuum during one year.

The most famous experiment measuring the speed of light was performed by the American Physicist Albert Michelson. Light from an intense source was directed by a lens to an octagonal mirror initially at rest. The mirror was carefully adjusted so that a beam of light was reflected to a stationary mirror located on a mountain 35 km away and then reflected back to the octagonal mirror and into the eye of the observer.

27.3 Electromagnetic Waves

 * Electromagnetic wave**- A wave that is partly electric and partly magnetic and carries energy. Emitted by vibrating electric charges.


 * Electromagnetic spectrum**- The range of electromagnetic waves extending from radio wave to gamma rays


 * infrared**- Electromagnetic waves of frequencies lower than the red of visible light


 * Ultraviolet**- Electromagnetic waves of frequencies higher thn those of violet light.

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.

27.4 Light and Transparent Materials

 * Transparent**- Term applied to materials that allow light to pass through them in straight lines.

Exactly how a recieving material responds when light is incident upon it depends on the frequency of the light and the natural frequency of electrons in the material. All materials that are springy (elastic) respond more to vibrations at some frequencies than others. Bells ring at a particular frequency. the natural vibration frequencies of an electron depend on how strongly it is attached to a nearby nucleus.

When the electromagnetic wave has a lower frequency than ultraviolet, as visible light does, the electrons are force into vibrations with smaller amplitudes. The atom holds the energy for less time, with less chance of collision with neighboring atoms, and less energy is transferred as heat.

27.5 Opaque Materials

 * Opaque**- Term applied to materials that absorb light without reemission and thus allow no light through them.

In opaque materials, any coordinated vibrations given by light to the atoms and molecules are turned into random kinetic energy. These materials will get slightly warmer.

When light shines on metal and set these free electrons into vibration, their energy does not "spring" from atom to atom in the material, but is reemitted as visible light. This light is seen as a reflection, and why we see metals as shiny.

27.6 Shadows

 * Ray**- A thin beam of light


 * Shadow**- A shaded region that results when light falls on an object and thus cannot each into the region on the far side of the object.


 * Umbra**- The darker part of a shadow where all the light is blocked.


 * Penumbra**- A partial shadow that appears where light from part of the soure is blocked and light from another part of the source is not blocked.

Sharp shadows are produced by a small light source nearby or by a larger source farther away. A large light source produces a softer shadow.

27.7 Polarization

 * Polarization**- the aligning of vibrations in a transverse wave, usually by filtering out waves of other directions.


 * Polarization axis**- is the direction of the vibrations of the polarized light wave.

Vertically vibrating electrons emits light that is vertically polarized, while horizontal vibrating electrons emit light that is horizontally polarized.

[]

27.8 Polarized Light and 3-D Viewing
Seeing in three dimensions depend on the fact that both eyes see different images. We see the same scene from different angles.

One can make a 3-D movie by using polarization. The lens' you wear, allow only the right eye to see the image at times, while other times it only allows the left eye to see the image. Your brain takes this in as one image, creating a three dimensional effect.

Works Cited: Conceptual Physics- Third Edition with Expanded Technology by Paul G. Hewitt