Chapter 27 Light

Early Concepts Greek philosophers thought that light consisted of tiny particles Soctrates and Plato thought that vision resulted from streamers or filaments emitted by the eye making contact with an object Empedocles taught that light traveled in waves. Christian Huygens, a contemporary of Newton argued that light was a wave. He demonstrated that light seemed to travel in straight lines but in other circumstances it spreads out (diffraction). This supported the wave theory. In 1905, Einstein published a theory explaining the photoelectric effect. According to this theory, light consists of particles called photons. Photon – elementary particle and basic unit of electromagnetic radiation

The Dual Nature of Light The observation that light can behave as both a wave and a particle is called the dual nature of light

Speed of Light Danish astronomer Olaus Roemer (1675) made the first measurement that showed that light travels at a finite speed by careful observations of Jupiter’s moons. Albert Michelson (1880) performed the first experiment that measured the speed of light.

Speed of light in vacuum: 300,000 km/s 186,000 miles/s Light Year: Distance that light travels in 1 year A beam of light could make 7.5 trips around the earth in 1 second Light takes about 8 minutes to travel from the sun to the earth Light takes 4.2 years to travel from the nearest star, Alpha Centauri to Earth

Microwaves are unable to pass through the screen in the window of a microwave because the wavelength of the microwaves is greater than the width of the holes in the screen:

Light is emmitted by accelerating electric charges – often electrons in atoms electromagnetic spectrum (see above) Infrared waves – electromagnetic waves with a longer wavelength than the wavelength of red light Ultraviolet waves – electromagnetic waves with a shorter wavelength than the wavelength of violet light

Light is energy carried in an electromagnetic wave that is generated by vibrating electric charges. When light is incident upon matter electrons in the matter are forced into vibration. Exactly how a receiving material responds when light is incident upon it depends on the frequency of the light and the natural frequency of electrons in the material. Glass and water are two materials that allow light to pass through in straight lines. They are transparent to light. Materials that absorb light without reemission are opaque

All materials that are springy (elastic) respond more to vibrations at some frequencies than others. Bells rings at a particular frequency, tuning forks vibrate at a particular frequency, and so do the eletrons in matter. The natural vibration frequencies of an electron depend on how strongly it is attached to a nearby nucleus. Different materials have different electric “spring strengths.”

ray – a thin beam of light shadow – formed where light rays cannot reach umbra – total shadow penumbra – partial shadow

Crepuscular rays – columns of sunlit air separated by darker cloud- shadowed regions

Polarization When wave motion is confined to one plane it is said to be polarized.