02ch2LightMatter1

# 02ch2LightMatter1 - Chapter 2 LIGHT MATTER AND ENERGY...

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Chapter 2: LIGHT, MATTER AND ENERGY Topics (aims are similar to those given in small box upper right, p21) Properties of Light Waves and particle properties, wavelength, frequency, speed Units, Measure and Units Conversion electromagnetic spectrum Interaction of Light with Matter Continuous spectra Black Body (thermal) radiation measuring temperature with Wien’s Law and the total energy emitted Atoms Atomic energy levels Emission and absorption spectra The Doppler effect gives velocities

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Light travels like a Wave Light is a moving electromagnetic wave . The electric and magnetic fields oscillate at right angles to each other as the wave passes. The electric field (E) shows how an electron would move – up and down as the wave passes. Question: what is varying. .?
Properties of a Wave The distance between crests or troughs is the wavelength λ (lambda). The crests pass with a frequency ν waves per second (ν = Greek “nu” pronounced “new”, or f in some books). Frequency = complete waves passing per second. Measured in Hertz. 10 Hz = 10 waves per second Electromagnetic waves travel at speed c=300,000 km/s through a vacuum, and slower in all other media, e.g. air, water. From the definitions, we have: λ ν = c. Q: What is the frequency. .? What is the wavelength. .?

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Wavelength The picture below shows 4.5 waves = complete cycles, returning to the starting point. The horizontal lines each show one wavelength. The horizontal axis is distance (position in space) or time.
Physical constants have units . The speed of light, c, is one of many physical constants that we use in many equations. They usually have precisely measured values. The speed of light has units that are the same as the product of the units in its definition: c = λν, λ has units of length: meters, or km or feet or miles ν has units of frequency: cycles per second, or s -1 , since cycles or “number of waves” has no units. c has units of ms -1 or m/s. 8

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Changing Units Units can be changed into other measures of the same type . We can change a unit of length into other units of length: meters, km, cm, mm, feet, miles etc. We can change units of time into other units of time: seconds, minutes, hours, days, years. When the units change, the numerical value also changes, to preserve the same measurement. Hence, c=3x10 8 m/s = 3x10 5 km/s, since k=10 3 This can be written: c=3x10 8 m/s = 3x10 8 (m/s) (1km/1000m), where (1km/1000m)=1 = 3 x 10 8 x 10 -3 (m/s)(km/m), set up so that the m will cancel, = 3 x 10 8-3 (km/s) 5
Changing the units on c We can convert c from (km/s) to (miles/s), by changing km to miles. 1 mile = 1.6093km, approximately.

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## This note was uploaded on 02/17/2009 for the course PHYS 7 taught by Professor Staff during the Winter '08 term at UCSD.

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02ch2LightMatter1 - Chapter 2 LIGHT MATTER AND ENERGY...

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