Chapter 27 Light. Early Concepts Greek philosophers thought that light consisted of tiny particles...
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Transcript of Chapter 27 Light. Early Concepts Greek philosophers thought that light consisted of tiny particles...
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Chapter 27Light
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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
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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
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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.
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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
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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:
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• 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
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• 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
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•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.”
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• ray – a thin beam of light
• shadow – formed where light rays cannot reach
• umbra – total shadow
• penumbra – partial shadow
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Crepuscular rays – columns of sunlit air separated by darker cloud-shadowed regions
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Polarization
When wave motion is confined to one plane it is said to be polarized.