Light Physics Mr Berman Electromagnetic Waves oscillating electric

Light Physics Mr. Berman

Electromagnetic Waves: oscillating electric and magnetic fields. http: //www. physchem. co. za/OB 12 -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 Waves Type of radiation Wavelengths(m) Radio TV Microwave Infrared radiation 570 - 2. 8 5. 6 - 0. 34 0. 1 - 0. 001 10 -3 - 10 -7 Visible Light 700 x 10 -9 - 400 x 10 -9 Ultraviolet X-rays Gamma rays 10 -7 - 10 -10 -10 -12 shorter than 10 -12

Visible Light is detected by the human eye. red orange yellow green blue indigo violet 700 x 10 -9 (700 nm) 550 x 10 -9 400 x 10 -9 (400 nm)

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 x 10 14 Hz, what is its wavelength? • What color light is it? • Answer: 550 x 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 x 108 m/s

Albert Michelson, American (1880’s) • Measured time for light to make a round trip in California mountains 35 km apart. • Measured speed of light: 2. 997996 x 108 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 Objects emit light. Illuminated Objects reflect 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 / (4πd 2 ) d : distance from source • Unit: Lumens/ m 2=lux=lx

Problem • A lamp is moved from 20 cm to 40 cm 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 1 m 2 of a sphere of 1 m radius. • cd = Luminous flux/π • One of the fundamental SI units