Light PhysicsMrs. Coyle
Electromagnetic Waves: oscillating electric and magnetic fields. http://www.physchem.co.za/OB12-ele/Graphics/wavetrain.gif
ElectromagneticWaves • Do not require a medium. • Can travel through space (vacuum). Ex.: Light from the sun reaches us. • Transverse
Electromagnetic Waves Type 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) orange yellow green 550 x 10-9 blue indigo violet400 x 10-9 (400nm)
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