Light
Light – Wave or particle?
• For many years scientists argued over the nature of light, "Is light
a wave or a stream of particles?"
• In some experiments light exhibits wave like properties, the
Doppler effect, interference, refraction, diffraction
• and in other experiments, like the photo electric effect, it exhibits
particle like properties
• The fact is that light exhibits behaviors which are characteristic of
both waves and particles.
Early Concepts of Light
• Early Greek philosophers:
tiny particles entering the eye
to create vision
• Socrates & Plato:
streamers/filaments from the
eye contacting the object.
“How else can you explain
why you don’t see an object
until you look at it?”
• Newton: light made of
particles
The Speed of Light
• Does light take time to travel, or does it arrive
at the same instant it departed?
• Ancients: speed of light is infinite
• 1880: Michelson measured speed to be about
300,000 km/s
300,500,000
300,000,000
Measured speed of light, m/s
299,500,000
299,000,000
298,500,000
298,000,000
297,500,000
1840
1860
1880
1900
1920
Year of Measurement
1940
1960
1980
2000
Electromagnetic Waves
• Visible light is a small part of the
electromagnetic spectrum
• EM waves with higher frequency than visible
light are ultraviolet
• EM waves with lower frequency than visible
light are infrared
Light and Transparent Materials
a
3 of many atoms
b
LIGHT
GLASS
c
e
d
f
g
LIGHT
Chain of absorptions/re-emissions
Opaque Materials
• Reflect light instead of transmitting
• Opacity in one part of the EM spectrum,
transparent in another part
– EX: People are opaque in visible light, more
transparent in X-rays
Also called M31, this is
closest neighboring galaxy
in the wider universe and is
just about visible from a
good site.
Radio, microwave,
infrared, visible, ultraviolet
and x-ray.
Opaque Materials
• Can only reflect a color if that color is present
in the incident light
http://phet.colorado.edu/simulations/sims.php?sim=Color_Vision
Shadows: http://www.schulphysik.de/ntnujava/shadow/shadow.html
RGB Color Mixing
Polarization
BUT . . . There’s a controversy!
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“DST” – Dark Sucker Theory
Electric bulbs = “Dark Suckers”
Candles are primitive DSs
Portable DSs need a Dark Storage Unit
Dark has mass
Dark is heavier than light
Dark is faster than light
BUT . . . There’s a controversy!
• Finally, we must prove that dark is faster than
light. If you were to stand in a lit room in front
of a closed, dark closet, and slowly opened
the closet door, you would see the light slowly
enter the closet. But since dark is so fast, you
would not be able to see the dark leave the
closet. Next time you see an electric bulb,
remember that it is a Dark Sucker.
The Color Spectrum
• Homework for next time:
• Read Chapter 27, Hewitt: Do RQ 1-23, T&E1,410
• Chapter 28 – Will pull extra credit questions
from here! 