Chapter Twenty-Five: Light
• 25.1 Properties of Light
• 25.2 Color and Vision
• 25.3 Optics
25.1 Properties of light
• Light travels fast over long
distances and carries
energy and information.
• Light travels in straight
lines, but can be bent by
lenses or reflected by
mirrors heat and warmth.
• Light has color and can be
bright or dim.
The electromagnetic spectrum
• Light, like sound and heat, is a form of energy.
• The visible light we see is part of the
electromagnetic spectrum.
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The Electromagnetic Spectrum
• Visible light is a very small part of the
electromagnetic spectrum.
4
Where does sound fit into the
electromagnetic spectrum?
?
5
Properties of light
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• You see book pages because light in the
room reflects from the page to your eyes.
• Your eyes and brain use the information
carried by the light to make a mental
picture.
Light is produced by atoms
• Most light is produced by atoms.
• When you put some energy into the atom,
it excites the atom’s electrons.
• Light is produced when the electron
releases this energy.
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Incandescent light
• Making light with heat is
called incandescence.
• Atoms in the filament
convert electrical energy
to heat and then to light.
• Incandescent bulbs are
inefficient, but their waste
heat can be useful.
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Fluorescent light
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• To make light, fluorescent
bulbs use high-voltage
electricity to energize
atoms of gas in the bulb.
• These atoms release the
electrical energy directly
as light (not heat), in a
process called
fluorescence.
Color and energy
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• When all the colors of the rainbow are combined,
we see light without any color.
• We call the combination of all colors white light.
Color and energy
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• Compare the hot, blue flame from a gas stove to
the orange flame of a match.
• The light from a gas flame is blue (high energy)
and the light from a match is red-orange (low
energy).
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25.1 The speed of light
• The speed at which light travels through air is
about 300 million meters per second.
• The speed of light is so important in physics
that it is given its own symbol, a lower case “c”.
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25.1 Speed of light
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• The speed at which electromagnetic waves
travel through air is about 300 million meters
per second.
• The speed of light is so fast that when lightning
strikes a few miles away, we hear the thunder
after we see the lightning.
Wavelength & Frequency of Light
• Because the wavelength of
light is so small, scientists
measure it in nanometers.
• One nanometer (nm) is
one billionth of a meter
(0.000000001 m).
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Wavelength and Frequency of Light
• One THz is one trillion Hz.
• 1,000,000,000,000 Hz
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What kind of wave is light?
• A sound wave is a oscillation of air.
• A water wave is an oscillation of the surface of
water.
• An oscillation of electricity or magnetism
creates electromagnetic waves.
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Electromagnetic waves
• If you could shake the
magnet up and down 450
trillion times per second,
you would make waves of
red light with a frequency
of about 450 THz.
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Electromagnetic spectrum
• The entire range of electromagnetic waves,
including all possible frequencies, is called the
electromagnetic spectrum.
• This spectrum includes visible light and
invisible waves:
– radio wave
– microwaves
– infrared light
– ultraviolet light
– X-rays
– gamma rays
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Electromagnetic spectrum
End Section 25.1
Begin Section 25.2
The human eye
• The eye is the sensory
organ used for vision.
• The retina contains lightsensitive cells called
photoreceptors.
• Photoreceptors convert
light into nerve impulses
that travel through the
optic nerve to the visual
cortex of the brain.
Photoreceptors
• The human eye has two
types of photoreceptors:
cones and rods.
• Cones respond to color and
rods respond to the intensity
of light.
• Rod cells “see” black, white,
and shades of gray.
How we see color
• Our eyes work according
to an additive color
process
– 3 photoreceptors (red,
green, and blue) in the eye
operate together so that
we see millions of different
colors.
Cone Cells
• There are three types of cone cells.
• One type responds best to low-energy
(red) light.
• Another type responds best to mediumenergy (green) light.
• The third type responds best to higherenergy (blue) light.
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Cones
• If the brain gets a signal • If there is a strong red
from only a green cone, signal and a weak green
signal, we see orange.
we see green.
•
All of the light is still there. We just see orange.
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The Additive Primary Colors
• They're called primary because any color can
be made from a suitable combination of red,
green, and blue.
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An object appears the color it reflects!
White object
Black object
Red object
Green object
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Reflection
• An object is the color of the light reflected.
• Red reflects red light.
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Reflection
• Blue reflects blue light.
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Reflection
• Blue light on a red object.
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White light
• Illuminated by white light (a mixture of all
colors) all parts of the girl's clothing show their
colors because each reflects its own part of the
spectrum.
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Red Light
• Illuminated only by red light, the girl's red top
shows up red because it reflects red light. Other
clothing absorbs red light, so reflects nothing
(and therefore looks black)
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Complementary Colors
•
•
•
•
Red + Green = Yellow
Red + Blue = Magenta
Green + blue = Cyan
(Red + Green) + Blue = White
Additive Color Mixing
(mixing of light)
• Colors which combine to produce white are
said to be complementary.
• Therefore, yellow and blue are
complementary colors.
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Wavelength & Frequency of Light
25.2 Making an RGB color image
• A television makes
different colors by lighting
red, green, and blue pixels
in different proportions.
• Color images in TVs and
computers are based on
the RGB color model.
Red
Green
Blue
White
• TV’s
• Computer screen
• Spot lights
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25.2 Subtractive color process
• A blue shirt looks blue
because it reflects blue
light into your eyes.
• Chemicals known as
pigments in the dyes and
paints absorb some
colors and reflect other
colors.
25.2 The CMYK color process
• The subtractive color
process is often
called CMYK for the
four pigments it
uses.
• CMYK stands for
cyan, magenta,
yellow, and black.
25.2 The CMYK color process
The CMYK color process
• The full color image is
a combination of all
four images!
Mixing Colored Pigments
1. Magenta:
reflects red and
blue; absorbs
green.
2. Yellow:
reflects red and
green; absorbs
blue.
3. Cyan:
reflects green
and blue;
absorbs red.
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Mixing Colored Pigments
4. If magenta, 5. Addition of
yellow, cyan are black.
combined, the
picture looks
like this.
6. Combined
finished results.
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25.2 Why plants are green
• Plants absorb energy
from light and convert
it to chemical energy in
process called
photosynthesis.
• Chlorophyll is the main pigment of plants absorbs
red and blue light and reflects green light.
25.2 Why plants are green
• Plants must reflect some
light to avoid absorbing
too much energy.
• A plant will die if placed
under only green light!
Dustyn
McKnight