Light and Color
Chapter 27 and 28
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.
Models of Light - Waves
• Electromagnetic waves (light) originate from vibrating or
accelerating electric charges
• Electromagnetic waves are made up of an electric field
and a magnetic field oscillating at right angles relative to
one another
• An electromagnetic wave
(light) is a transverse wave
Unlike other waves, light waves can travel
through a vacuum
Models of Light - Particles
• Particle of light are called photons
• Photons have zero rest mass and travel at the speed of
light through a vacuum.
Light and Energy
• For waves, the amplitude or intensity is usually
related to the energy of the wave
• For light, this is not true. The energy of light waves
was found to be directly related it is frequency.
• An experiment demonstrating the photoelectric
effect demonstrated the particle nature of light and
that E = hf, where E is energy, h is Planks constant,
and f is frequency.
http://phet.colorado.edu/new/simul
ations/sims.php?sim=Photoelectric
_Effect
The Electromagnetic Spectrum
The electromagnetic spectrum is the range of electromagnetic waves
extending from radio waves to gamma rays
Increasing frequency
ROYGBIV
The Visible Spectrum
• We can only see a small part of the
electromagnetic spectrum
• The visible spectrum is a range of light waves
extending in wavelength from about 400 to
700 nanometers.
Increasing wavelength
Increasing frequency
Increasing energy
Speed of Light - c
• In the early 17th century, many scientists believed that
there was no such thing as the "speed of light"; they
thought light could travel any distance in no time at all.
• In the 1670's Roemer was able to calculate a value for
the speed of light by carefully studying the orbit of one
of Jupiter’s moons, Io. He noticed that the time between
the eclipses of the moons of Jupiter was less as the
distance away from Earth is decreasing than when it is
increasing.
• In 1926 scientist Albert Michelson used the reflection
from a rotating mirror on a distant mountain and
measured the speed of light at 299,796 km/second
• The current accepted value is 300,000,000 meters per
second (3 x 108 m/s) or 186,000 miles per second. Light waves
obey the wave
equation, c = lf
Transparent vs. Opaque
• Transparent - the term applied to materials through which
light can pass in straight lines. If the object is transparent,
then the vibrations of the electrons are passed on to
neighboring atoms through the bulk of the material and
reemitted on the opposite side of the object.
• Opaque - the term applied to materials that absorb light. If
the object is opaque, then the vibrations of the electrons are
not passed from atom to atom through the bulk of the
material; rather the electrons vibrate for short periods of time
and then reemit the energy as a reflected light wave.
Light Questions
1)
Rank the following in terms of energy, frequency and wavelength:
microwaves, visible light, IR, UV, x-rays, gamma rays, and radio waves.
2) Rank the following in terms of energy, frequency and wavelength: Red,
orange, yellow, blue, and green.
3) Why is it possible to get a sunburn with the window down in the car, but
not when the window is up?
4) Find the frequency of 80 m radio waves.
5) What did the photoelectric effect show about light?
6) Is light a wave or a particle?
7) What is the name of a particle of light?
8) What is the origin of light?
9) Why is light also called electromagnetic radiation?
10) Why is it possible to see a cell phone ringing in a bell jar (vacuum jar) but
not possible to hear it ringing?
11) Do radio waves travel at the speed of light or the speed of sound?
Color
• The color of an opaque object is the result of the color of the
light which strikes the object and the color of the light that it
reflects.
• The color of a transparent object is the result of the color of
the light which strikes the object and the color of the light
that it transmits.
• Red filters transmit red light and absorb the other colors, etc.
• Red objects reflect red light and absorb the rest, etc.
white (
)
green
red
green
red
Visible Colors
• Different wavelengths of light are perceived as
different colors.
• Pure Colors: ROYGBIV
• White light contains equal amounts of these colors.
(ROYGBIV)
• At an interface, light can be...
»absorbed
»reflected
»transmitted
Primary Colors of Light
Primary Colors
White
Red
Green
Blue
Yellow
Cyan
Magenta
Combining Colors of Light
• When two of primary colors of light are combined they form
the primary colors of pigment; yellow, magenta and cyan
• Combining colors of light is an additive process, when all
three primary colors are combined, you get white light
• Color Addition Example:
– Tiny dots called pixels on Color TV's and Computer
Monitors are colored only red, green, or blue.
Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
Complementary Colors
• Complementary Colors - any two colors that
add together to produce white
Complementary Colors of Light
Red and Cyan
Green and Magenta
Blue and Yellow
Primary Colors of Pigment (Subtractive Colors)
• Subtractive Primary Colors:
• Yellow
• Magenta
• Cyan
• One can produce any color by varying the amount of yellow, magenta and
cyan pigments
• Combining pigments is a subtractive process because each color absorbs a
color or colors of light. Combining all three primary colors produces black
(all colors are absorbed)
• Color Subtraction Example:
– Newspapers and zip-lock sandwich bags (yellow and cyan makes
green) use color subtraction.
Color Blindness
• Colorblindness - about 10% of population
• Red-green is predominant
• Yellow-blue - a few
• Total - some
Optical Illusions
Color Questions
1)
2)
3)
4)
5)
List the primary colors of light. What happens when combine the three primary
colors of light?
List the primary colors of pigment. What happens when combine the three
primary colors of pigment?
_______ + green = yellow Blue + _______ = cyan Yellow + _______ = white
Cyan + _______ = white
You have a blue opaque object (an object that appears blue under white light).
What color will it look under the following colors of light?
– Blue
blue
White
– Red
– Green
– Cyan
– Yellow
What color does a red shirt appear when the room lights are turned off and the
room is entirely dark? ____________ What about a blue shirt? ____________ ...
a green shirt? ____________
More Color Questions
6) Fill in the blanks:
7) True or False/Explain: White and black are colors of light.
8) You have a cyan opaque object (an object that appears cyan under white
light). What color will it look under the following colors of light?
– Blue
– Red
cyan
White
– Green
– Cyan
– Yellow
Things that can separate white light
•
•
•
•
Prism
Raindrops
CD’s
Diffraction Grating
Things the produce electromagnetic waves
• Radio waves
– electrons moving up and down an antenna
• Visible Light
– electrons changing energy states in an atom
The Structure of the Atom and Emission
• An atom is composed of electrons,
protons and neutrons.
• When an electron is raised to a
higher energy level, the atom is said
to be excited.
• When the electron returns to a lower
energy level, energy is released in the
form of light.
• Different transitions from high levels
to low levels result in different colors
of light.
The Kirchhoff-Bunsen Experiment
• These two scientists found that burning
chemicals over an open flame resulted in a
spectrum with bright lines.
• They found that each chemical element
produced its own characteristic pattern of
bright spectral lines.
Emission Spectra of Hydrogen
Hot gas produces a bright line emission spectrum.
Discrete Emission Spectrum
Slit
Film
Low Density
Glowing
Hydrogen Gas
Prism
Photographic Film
Every element can be “fingerprinted” by it spectra.
Hydrogen
Helium
Oxygen
Carbon
Incandescence
• Hot, dense solids produce a continuous
spectrum.
Continuous Spectrum
• The brightness and color of light emitted by a
hot object changes with its temperature.
• Glowing object colors:
•
•
•
•
•
Reddish
 coolest glowing object
Orange-ish
Yellowish
White
Bluish
 hottest glowing object
Absorption Spectra
• Cool gas in front of a continuous source of
light produces an absorption line spectrum.
• Fraunhofer lines in our Sun's spectrum
showed that cool helium gas surrounds the
Sun
Absorption Spectrum
Absorption Spectra of Hydrogen
Discrete Emission Spectrum
Discrete Absorption Spectrum
Slit
Hydrogen Gas
Film
White Light
Source
Prism
Photographic Film
Sources
• Conceptual Physics by Paul Hewitt
• www.physicsclassroom.com
• http://observe.phy.sfasu.edu/courses/phy101
/lectures101/