Physics Mrs. Coyle. Electromagnetic Waves: oscillating electric and magnetic fields. .
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Transcript of Physics Mrs. Coyle. Electromagnetic Waves: oscillating electric and magnetic fields. .
PhysicsMrs. Coyle
Electromagnetic Waves: oscillating electric and magnetic
fields.
http://www.physchem.co.za/OB12-ele/Graphics/wavetrain.gif
Electromagnetic Waves
• Do not require a medium.
• Can travel through space (vacuum). Ex.: Light from the sun reaches us.
• Transverse
Electromagnetic WavesType of radiation Wavelengths(m)
Radio 570 - 2.8 TV 5.6 - 0.34 Microwave 0.1 - 0.001 Infrared radiation 10-3 - 10-7
Visible Light 700x10-9 - 400x10-9
Ultraviolet 10-7 - 10-10
X-rays 10-10 -10-12
Gamma rays shorter than 10-12
Visible Light is detected by the human eye.
red 700 x 10-9 (700nm)
orangeyellowgreen 550 x 10-9
blueindigoviolet 400 x 10-9 (400nm)
Electromagnetic Spectrum
Speed of Light, c
• All EM waves have a speed through vacuum of 3 x 108 m/sec.
c= f λ
Problem• If a light wave’s frequency is 5.45 x10 14 Hz,
what is its wavelength?
• What color light is it?
• Answer: 550x 10-9 m, green
Calculating the Speed of Light• Danish Astronomer, Ole Roemer (~1668)
found that the period of the moon of Jupiter , Io, varied depending on the location of the Earth. Distances were known.
• Estimated speed
of light:
2.2 x108 m/s
Albert Michelson, American (1880’s)
• Measured time for light to make a round trip in California mountains 35km apart.
• Measured speed of light:
2.997996 x108 m/s
• First American to win Nobel prize.
Speed of Light (1983)
Seventeenth General Congress on Weights and Measures measured:
2.99792458 x 108 m/s
We round to 3 x 10 8 m/s
Luminous Objectsemit light.
Illuminated Objectsreflect light rays.
Luminous Flux, P
• Rate at which light is emitted from a source.
• Unit: lumen, lm
Illuminance, E, measures illumination of a surface.
E = (Luminous flux P) (area around source)
E= P / (4πd2 )
d : distance
from source• Unit:
Lumens/ m2=lux=lx
Problem
• A lamp is moved from 20cm to 40cm from a book. Compare the illumination before and after.
Candela (candle power)
• Luminous intensity of a point source is the luminous flux that falls on 1m2 of a sphere of 1 m radius.
• cd = Luminous flux/π
• One of the fundamental SI units