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LECTURE 22
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White Light: A Mixture of Colors (DEMO)
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White Light: A Mixture of Colors (DEMO)
1. Add together magenta, cyan, and
yellow. Play with intensities of each
to get white light.
2. Try to make white light out of blue,
red, green light.
3. Take look at shadows that result
from putting an object in the light
path.
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Spectral Lines
Energy states of an atom are discrete and so are the energy transitions that cause the emission of a photon (DEMO)
4/2/12 5 Emission spectra are produced by thin gases (few collisions). The emission lines correspond to photons emitted when excited atomic states in the gas make transitions back to lower-lying levels. C ontinuum spectrum : Solids, liquids, or dense gases emit light at all wavelengths when heated. A bsorption spectrum occurs when light passes through a cold, dilute gas. Atoms in the gas absorb at characteristic frequencies. Re-emitted light is unlikely to be emitted in the same direction dark lines in the spectrum. 4/2/12^6
Dependence of Intensity on Distance
power Pav
Consider spheres at different radius from the source emitting the EM radiation 4/2/12 7
Intensity I, power Pav, pressure Pr, energy
density uav, etc. of EM waves.
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Transfer of Momentum
A t U p I c c IA F c
A t U p I c c IA F c
Pressure P = F/A , so
Pr = I c Pr = 2 I c Δ p = p ( momentum ) Δ p = 2 p ( momentum )
Total Absorption Total Reflection
time
momentum
t
p
F =
= Δ U = IA Δ t
Solar Sail: http://www.lifeslittlemysteries.com/how-do-solar-sails-work--0788/ 4/2/12 14
Wave-Particle Duality
1. Isaac Newton (1642–1727): Light is a stream of
particles.
2. Christian Huygen (1629–1695): Light is a wave.
3. Thomas Young (1801): experimental proof that light
is a wave by showing that light exhibits interference
phenomena
4. Augustin Fresnel (1819): submitted a paper on the
wave theory of light to explain diffraction to the
French Academy of Sciences. If correct, there
should be a “Fresnel bright spot”. An Academy test
of this showed its existence.
5. James Clerk Maxwell (1860): wave theory of light
6. Albert Einstein (1905): particle nature of light to
explain the photoelectric effect
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Photo-Electric Effect
itisconvenienttoexpress ineVnm: 1240 eV nm SinceenergiesareoftengivenineVandwavelengthsin nm,
- 62606876 ( 52 ) 10 Js 4. 1356673 10 eV s
- 99792458 10 m/s and 34 15 8 ⋅ = ⋅
= × ⋅ = × ⋅
= ×
− − hc hc h c
- Observation:
- Current does not depend on light intensity
- Depend on the wavelength of light
- Einstein’s hypothesis:
- Photon has energy E=hf=hc/ where h is the Planck’s constant (wave-particle duality)
- To release an electron from a metal plate E(photon)> threshold energy 4/2/12 16
Wave-Particle Duality
Light & Matter can exhibit properties of both waves
and particles.
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Emission, Absorption, Scattering
4/2/12 18 Ruby is an aluminum oxide crystal in which some Al atoms have been replaced with chromium. Chromium atoms absorb green and blue light and emit or reflect only red light.
Laser (Light Amplification by stimulated emission
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Ruby Laser Energy Levels
