Transcript 4 Light

Physics
Mrs. 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 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
700x10-9 - 400x10-9
Ultraviolet
X-rays
Gamma rays
10-7 - 10-10
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 (700nm)
550 x 10-9
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 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 / (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