Download Light and Color Formation

LIGHT
&
COLOR
Very Important Concepts
When light reaches the boundary between two media,
some of its energy is reflected, some is transmitted,
and some is absorbed.
The relative amounts of each depends on…
• the frequencies of light,
• the angle the light reaches the boundary,
• and the nature of the two media.
Examples:
• Light traveling through air reaches colorless glass
• Light traveling through air reaches colored glass
• Light traveling through air reaches a red sweater
We only “see” what reaches our eyes!
Based on these interactions of light
with matter, we classify materials as:
transparent - readily transmits light;
can clearly see objects through them
translucent - transmits, but diffuses, light;
cannot see objects clearly through them
opaque - transmits no light;
cannot see through them
Visible light is that portion of the
electromagnetic spectrum which stimulates the
retina of the
human eye.
Visible spectrum
wavelengths range
from about 400 nm (violet) to 760 nm (red).
Light travels at about 3 x 108 m/s through
empty space and slightly slower through air.
Remember that for all waves, v
= f.
WHITE light is composed of all colors.
Red, orange, yellow, green, blue, violet
is the order of
increasing frequency
or decreasing wavelength.
Frequencies directly above this spectrum are
ultraviolet.
Frequencies directly below this spectrum are
infrared.
Complimentary colors are two
colors that combine to form white light.
Red and cyan,
green and magenta,
blue and yellow
are pairs of complimentary colors.
Red, green, and blue are called
primary colors or secondary pigments.
Cyan, magenta, and yellow are called
primary pigments or secondary colors.
cyan
red
magenta
green
yellow
blue
These sites let you simulate mixing colors
and pigments of light: link1, link2, link3
“But I learned that the
primary colors are red,
blue, and yellow – not
red, blue, and green.”
Read about that and more here.
Activity: Color Mixing
The color of an opaque object depends on
the colors (frequencies) of light incident
upon it and on the colors (frequencies) of
light reflected. We only “see” the color(s)
that get reflected to our eyes.
An object that is “normally” red reflects red light and
absorbs any other color that shines on it. This object will
appear red if the light that hits it has red in it. It will
appear dark, or black, otherwise.
When white light (red + green + blue) shines on a red
object, the red reflects to our eyes and the green and blue
are absorbed.
When cyan light (green + blue) shines on a yellow object
(reflects red and green), the object will appear green.
The color of a transparent object depends
on the colors (frequencies) of light incident
upon it and on the colors (frequencies) of
light transmitted. We only “see” the color(s)
that get transmitted to our eyes.
A transparent object that is “normally” red transmits red
light and absorbs any other color that shines on it.
Objects viewed through this transparency will appear red
if the light they are giving off has red in it. They will
appear dark, or black, otherwise.
When white light (red + green + blue) shines on a red
transparency, the red passes through to our eyes and the
green and blue are absorbed.
When cyan light (green + blue) shines on a yellow
transparency (transmits red and green), green light is
transmitted to our eyes.
Now that we know more about color and
what colors are seen when light passes
through or reflects from various materials,
let’s look at how colors are revealed by
various natural phenomena.
The property of light responsible for the separation
of light into colors by a prism is known as refraction.
Refraction is defined as the change of direction of
a ray of light as it passes obliquely from one medium
into another of different transmission speed.
View a simulation of refraction through a prism at
http://www.geocities.com/capecanaveral/hall/6645/dswmedia/PRISM.HTM.
The properties of light responsible
for the separation of light into
colors by a rainbow are reflection
and refraction.
Reflection is defined as the turning
back of a wave when it reaches the
boundary of the medium through
which it is traveling.
Learn more about rainbow
formation here and here.
Soap Bubbles
(and other thin films)
The properties of light
responsible for the separation of light into
colors by a soap bubble (or other thin films)
are reflection, refraction, and interference.
Interference is defined as the result of the
superposition of two or more waves.
Explore thin film interference here and here.
Thin Film Interference
The color spectrum seen in a soap bubble, or
any thin film, results from the interference
of the reflections of light from the
front and back surfaces of the film.
The colors seen depend on the thickness of the
film. The light most strongly seen has a
wavelength such that the film thickness is an
odd multiple of 1/4. Other wavelengths will
suffer partial or total destructive interference.
What is your mental model of light…
reflecting from a surface?
transmitting through a material?
being absorbed by a material?
link
Spectroscopes and “Rainbow Glasses”
The properties of light responsible for the
separation of light into colors by a spectroscope
(or diffraction grating) are diffraction and
interference.
Diffraction is defined as the spreading of a wave
around the edge of a barrier or through an opening.
View simulations of diffraction here and here.
Activity: Spectroscopes