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Chapter 12 Notes, part III
Date: _______
12.2: Light
Light is electromagnetic radiation which travels as a transverse wave but is _____________
(given off) or ____________ (taken in) as a massless particle called a _____________. This is
called the “wave-particle duality” of light.
Like sound, light is affected by its medium. Unlike sound, the speed of light decreases
when the medium gets ______________. Since light travels as an EM wave it does not
require a medium so it travels __________ in a vacuum (see table 12-2, p. 401). The speed
of light, represented as ____, is equal to __________ m/s (or _____________________m/s) or
about _______________ miles per second!! At this speed light would travel _____ times
around the Earth in one second; from the moon to the Earth in ____ seconds; and from
the Sun to Earth (93 million miles) in ______________________. The nearest star to Earth
other than our sun, is called Proxima Centauri. It is about ________________ miles. Even
traveling at 300,000 km/s light takes ______________ to reach us from this star. Realize
that most stars we see at night are 100’s, 1000’s or even million’s of times further away
than Proxima Centauri means that star light is ___________ ________! (and that we are
looking ________ ___ ________ when we do astronomy).
Production of Light
When an atom gains a specific energy, called a ____________, electrons in its electron
cloud will jump to higher energy levels (further from the nucleus) then immediately “fall”
back down to lower levels. Regardless of how the energy was originally gained by the
electron (heat, electricity, sound), the released energy is always light. Depending on the
amount of energy released different colors can be produced following the pattern:
ROY G BIV.
The pattern of light produced is called the
_____________ _____________ of the element and
might look like this for the element to the left:
Each element has its own unique fingerprint
which can be used to identify it.
The energy of light is proportional to its frequency and inversely proportional to its
wavelength. The greater the frequency (or shorter the wavelength) the greater the energy.
Light spreads out as it travels and it follows the “____________ ____________ ______” (as
does sound.) As the distance increases the ________________ (or ________________)
decreases by the _____________ of the distance increase. Thus, if a beam of light
illuminates 1 cm2 at a distance of 1 meter, it will illuminate _____ cm2 at a distance of 2
meters (22 =4) and will illuminate _____ cm2 at a distance of 3 meters. Since the same
amount of light energy is spread over a greater area each part of the surface illuminated
by the light receives less overall energy (fewer photons) so the intensity (or brightness) is
proportionally less. The same is true for sound.
Polarization
Since light travels as a transverse wave it can be ___________________. This process causes
all of the waves to travel in the same plane and can be accomplished one of two ways:
(1) when light reflects of nonmetallic objects at shallow angles only plane polarized light
will reflect, the rest is absorbed or scattered:
(2) passing through specially coated optics called polarizing film:
LCD’s (liquid crystal displays) use polarizers--if you look at an LCD display (like a
computer monitor or TV, a calculator or a watch) wearing polarized glasses the display
will look wrong and you might not be able to see anything at all! Polarized sunglasses
reduce _________ because _________ is polarized light reflecting at shallow angles off of
water or glass:
Coherent Light: the LASER
L
A
S
E
R
Laser light is ______________________ (only one wavelength), ___ ________ (all the crests
and troughs match), and _____________________ (all waves travel parallel to each other).
Illumination
Objects can be seen because they produce their own light (like a star or candle)--these
objects are called ________________, or they reflect or scatter light and are said to be
______________________.
Objects can be clear, called _____________________, such that nearly all light passes
through it (like clear glass); ____________________--some or most light passes through it;
or ______________, when no light passes through it. ______________ objects allow certain
colors to pass through and absorb all others (so a green filter allows the color
___________ to pass but all other colors are absorbed). _____________ objects absorb
certain colors but reflect the color they appear (so a blue sheet of paper absorbs all colors
but _______ which it reflects).
Reflection of Light
When light passes through a medium is gets progressively weaker due to two effects:
___________________--energy is gained by the particles of the medium and
___________________--light bounces off the particles of the medium and travels in all
directions except the original one. In addition, at the boundary light can _____________
(bounce off) or be ____________________ (travel through). The diagram below shows all four
events:
The amount of light reflected at the surface either by scattering or ____________________
reflection depends on the type of material, the smoothness of the material, and the angle
the light strikes the surface. The ratio of light reflected from a surface to the light
striking the surface is called __________________, and it ranges from less than 5% for very
black objects to >99% for high quality mirrors. There are three types of mirrors: flat or
__________; and two curved ones, ____________ and _____________. They are designed to
reflect as much light as possible so they must be very smooth. ____________ are formed
when reflected light rays either _____________ in front of the mirror or ____________ ____
____________ behind the mirror. If the image is formed by converging light rays it forms in
front of the mirror and are called ________ images. ________ images are always
______________ (upside-down), can be _______________ on a screen, and can be of any size
compared to the original object. _____________ (not real, phony, fake) images are formed
when light rays seem to converge behind the mirror. These images are always
_____________, can be any size but cannot be projected on a screen.
(1) __________ mirrors
always produce virtual images which are the same
size as the original object.
(2) ___________ mirrors
always produce virtual images which are smaller
than the original object.
(3) ____________ mirrors
can produce both real or virtual images of any size
depending on how far the object is from the mirror.
Color
There are two reasons that we can see an object’s color: luminous objects emit a
particular set of photons with wavelengths our eyes can detect or illuminated objects
reflect wavelengths our eyes detect and absorb other colors. Our eyes have rods and
cones which detect these photons. While more sensitive, rods do not detect color. The
cones detect three colors better than all others and these colors are called the primary
colors of light: ________, __________, and ____________ (or _______). When all three of these
are equally present our eyes detect this as ___________. When none of these are present
we see ___________. Combinations of the primary colors gives us all of the rest of the
millions of colors we can see. Equal amounts of any two primary colors produces the
three complementary colors:
Red + Green =______________ Red + Blue = ______________ Green + Blue =______________
Televisions and computer monitors are often called RGB devices. Why?
So, if equal amounts of the following are present what would you get?
Blue + Yellow = ___________ Green + Magenta= ____________ Red + Cyan=_____________
A primary color plus its complementary color always produce ___________.
Pigments
Pigments (paints or inks) subtract color because they absorb all colors except the colors
they reflect. A red sign appears red because it absorbs all colors but _______. White
objects reflect all colors so we see the combination of R+G+B as white. Black objects
absorb all colors so no light reaches our eyes from that object and it appears black. The
primary pigments are the complementary colors: ________, ______________ & ____________.
The same rules apply to pigments as colors:
cyan + magenta = ____________ cyan + yellow = _____________ magenta + yellow = _______
Printers are also called cmyk devices? Why?
1) what color would a green object appear under (a) blue, (b) yellow; (c) cyan; (d) red
light?
(a)
(b)
(c)
(d)
2) what color would a magenta object appear under (a) blue, (b) yellow; (c) cyan; (d) red
light?
(a)
(b)
(c)
(d)
R
G
B
C
M
Y