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Lecture

October 20.docx

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Department
Astronomy
Course
AS101
Professor
Shohini Ghose
Semester
Fall

Description
October 20, 2011 The nature of light:  The warmth of sunlight tells us that light is a form of energy: radiative energy  Units of energy: joules  We can measure the flow of energy in light in unit of watts: 1 watt=1joule/s  Newton showed white light is made up of many different colours  Quantum Mechanics: Light behaves as both, particle and wave  A wave is a periodic motion that can carry energy without carrying matter along with it  Light is an electromagnetic field o Light wave is a vibration of electric and magnetic fields o Light interacts with charged particles  Wavelength and frequency o Wavelength x frequency = speed of light = constant o Speed of light is 300,000,000 m/s o Thus lower wavelength means higher frequency and vice versa o Wavelengths/frequencies are related to colour o Longer wavelengths/lower frequencies: redder light o Short wavelengths/higher frequencies: bluer light  The electromagnetic spectrum o Visible 400-700 nanometers o Highest frequency waves – gamma rays, x-rays o Lowest frequency waves – radio waves o Energy and wavelengths are inversely related  Interactions of light with matter: o Emission – energy in matter can be converted into light that is
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