Chapter 27Light

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.