Why Objects Have Color
Visible light is a combination of many wavelengths (colors), which
give it a white appearance. When light hits an object certain
wavelengths are reflected and others are absorbed.
In the first picture the tomato absorbs blue and green wavelengths and
reflects the red wavelength.
In the second picture red light is shone upon the tomato.
The tomato is still reflecting the red wavelength and thus still looks red.
3rd picture blue light is shone upon the tomato, and since the tomato
absorbs the blue wavelength the tomato appears to be black.
Light Transmission
Transparent: Materials, such as window glass,
through which light can travel easily and through
which other objects can clearly be seen.
Translucent: Materials, such as glass blocks,
through which light can pass through but no clear
image can be seen.
Opaque: Materials which absorb and reflect light.
Objects cannot be seen through the material. Most
objects are opaque.
Primary and Secondary Colors
The primary light colors are Red, Blue, and Green (RGB).
The secondary light colors are Yellow, Cyan, and Magenta.
Combining pigments in painting is exactly the opposite:
The primary pigments are Yellow, Cyan, Magenta.
The secondary pigments are Red, Blue and Green.
Light Colors Vs. Pigments
-Primary colors in light are red, green, and blue because when put together
in the right intensities they form white light.
-When lights these colors are combined in pairs they form the secondary
colors for light.
-Pigment colors are seen by reflected light.
-A primary pigment color is one that absorbs only one primary light color and
reflects the other two primary colors. Thus yellow, magenta, and cyan are
the primary colors for pigments. Yellow reflects red & green, cyan reflects
green & blue, and magenta reflects red & blue.
-Secondary pigments colors then are blue, green, and red because they
absorb two primary light colors and reflect their own light color back.
Polarized Light
Electric Field
Light coming directly from the sun or
other sources is unpolarized,
Orientations
meaning the electric and magnetic
fields oscillate
in many different planes.
Polarized light refers to light in which all waves have electric fields
oscillating in the same plane.
A polarizing filter is made of a material with long molecules that
allow electromagnetic waves of one orientation through.
If a wave has an electric field with any other orientation, the filter will
only allow a component to pass through, absorbing the rest. Note
that only transverse waves such as light can be polarized.
Much of the light we see is at least partially polarized. Some
sunglasses contain polarizing filters which helps to block glare (such
as the glare that is noticeable when looking out over a lake on a
sunny day).
Polarized Light
(cont.)
Unpolarized light propagates in all orientations. No particular
orientation is preferred.
When it passes through a filter that only allows vertical
components of electric fields to pass, its intensity is cut in half.
on average, the light is “half horizontal and half vertical” in
terms of electric field components. All horizontal components
are blocked, making the resulting polarized light half as bright.
Consider if another filter is perpendicular to the direction of the
first one, i.e., a filter that only allows the horizontal components
of electric fields to pass through. This would completely block
the remaining light. Thus, any two perpendicular filters will
block all incoming light.
Suppose now that the two filters are offset
by some angle . Regardless of the angle,
the first filter blocks half the light.
If  = 0, the second filter has no effect.
If  = 90, the second filter blocks the other
half of the light. In general, when polarized
light with an electric field of amplitude E
passes through the second filter, the
amplitude will drop to E cos.