Chapter 26 Properties of Light Electromagnetic Waves Traveling, oscillating, electric and magnetic...
Transcript of Chapter 26 Properties of Light Electromagnetic Waves Traveling, oscillating, electric and magnetic...
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Chapter 26Chapter 26
Properties of LightProperties of Light
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Electromagnetic WavesElectromagnetic Waves• Traveling, oscillating, electric and
magnetic fields which are emitted by vibrating charges.
• The electric and magnetic fields carry energy.
• All electromagnetic waves travel at the speed of light in vacuum. (i.e.the wave speed = the speed of light)
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Typical Electromagnetic Wave
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Michael Faraday & James Clerk Maxwell
• c 1831: Michael Faraday discovers electromagnetic Induction. Faraday also introduces the field-line concept for electricity and magnetism.
•c 1865: James Clerk Maxwell develops mathematical relationships between electric and magnetic fields. Shows that all of electricity and magnetism are wave phenomena.
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• When Maxwell calculated the speed of these “Electro-magnetic” waves, he came up with the speed of light.
The speed of light = constant in vacuum
c 3.00 x 108 m/s = 300,000,000m/s
• electromagnetic wave in vacuum c = f
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• Note that everything you have learned about mechanical waves, with the exception of the need for a medium, also applies here to electromagnetic waves
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Electromagnetic Spectrum
Radio Micro I.R. Visible U.V. X-ray -ray
104 106 1010 1013 1023Hz
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Visible Part of EM Spectrum
red =700nm
fred =4.29 x 1014Hz
violet=400nm
f violet= 7.50 x 1014Hz
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Electromagnetic Spectrum
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Transparent Materials
• When light is incident on a particular material, the electrons and atoms of the material begin to vibrate while the light propagates through the material.
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• If the frequency of the light matches the:
–natural frequency of the electrons of the atoms in the material.
then a resonant condition is established, and the light is absorbed.
the vibrational frequencies of the atoms and molecules as a whole.
OR
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• For example:
Most glass is made up of atoms (Si and other elements).
They possess electrons that have natural frequencies of vibration that closely match the frequency of ultra-violet (UV)light.
(1015- 1017) Hz
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The larger atoms and molecules will vibrate at lower IR frequencies. So if IR is present, it will also be absorbed with the energy being dissipated as heat.
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• The visible radiation is sequentially absorbed, and then re-emitted from atom to atom. Eventually, the visible light passes through the entire piece of glass.
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• A state of resonance occurs for UV light.
•We say that the glass is transparent to visible light, but not to UV or IR.
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Glass
Visible light
UV radiationUV & IR get
absorbed
Visible light is transmitted
IR radiation
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Diffraction
Any bending of light by means other than reflection and refraction is called diffraction.
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Light Source
Wide Window
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Light Source
Narrow Slit
Interference Pattern
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Two or more slits – Diffraction Grating
Light Source
Interference PatternTwo or
more slits
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Thin Film Phenomena – Constructive and destructive Interference
Water
OIL
Air
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Polarization
The alignment of the transverse waves.
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Time delay between absorption and re-emission of visible light
Medium Average speed
Vacuum c = 3 x 108 m/s
Water v = 0.75c
Glass v = 0.67c
Diamond v = 0.41c
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Opaque Materials
• In Opaque materials, light is absorbed without re-emission. The energy of the light including visible, goes into the internal energy of the material – heating.
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• Metals are opaque but reflective.
•These free electrons are responsible for why metals make such good electrical and thermal conductors.
•Also, the light does not propagate from atom to atom as it does in a transparent material, but goes into reflection.
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ShadowsShadowsLight rays from
the sun are
essentially parallel
Sharp
shadowLarge far away light source
Small nearby light sourceSharp
shadow
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Large nearby light source
Penumbra(Partial shadow)
Umbra(Total shadow)
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EclipsesEclipses
Sun
Full Full MoonMoon
Not To Scale
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Lunar EclipseLunar Eclipse
Sun
Earth
Moon
Not To Scale
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Solar Eclipse
Sun
MoonEarth
Drawing Not To Scale
SUN
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Lunar Eclipse
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Solar Eclipse
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SOLAR
ECLIPSE
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The End of Chapter 25