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 speedof 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