Transcript Light and Optics Unit 8: Light and Optics Chapter 21: Light and Color  21.1 Properties of Light  21.2 Vision and Color  21.3 Using Color.

Light and Optics
Unit 8: Light and Optics
Chapter 21: Light and Color
 21.1
Properties of Light
 21.2
Vision and Color
 21.3
Using Color
21.1 Investigation: Properties of Light
Key Question:
What are some useful properties of light?
Objectives:

Discuss properties of light.

Observe interesting examples of reflection and refraction of
light.

Use the speed of light constant to perform simple calculations.
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.
Properties of light
 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 radiates in all directions
 You
can see a bare light
bulb from anywhere in a
room because the bulb
emits light in all
directions.
 Light
emitted from the
Sun or from a light bulb
travels in straight lines
from their surfaces.
Light intensity
 We
use the word intensity
to describe the amount of
light energy per second
falling on a surface.
 Light
intensity is the
amount of energy per
second falling on one unit
of area of a surface.
The inverse square law
 For
a small source of light, the intensity decreases as
the square of the distance from the source increases.
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”.
Speed of light

The speed at which electromagnetic waves travel through
air is about 300 million meters per second.
Speed of light
 The
speed of light is so
fast that when lightning
strikes a few miles away,
we hear the thunder after
we see the lightning.
Light and matter

Light interacts with matter in
four ways. It can:
— pass through almost
unchanged
(transparency).
— pass through but be
scattered (translucency).
— transfer its energy to the
material (absorption).
— bounce off (reflection).
Four ways light is affected by matter
 All
four interactions almost
always happen together.
 Green
colored paper absorbs
some light, reflects some light,
and is partly translucent.
Can you tell which colors
are reflected and which
are absorbed?
Four ways light is affected by matter
 A glass
window is mostly
transparent, but also
absorbs, scatters, and
reflects some light.
 See
if you can identify
where certain colors are
absorbed and reflected
in this image.
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.
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.
Fluorescent light
 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.
CFLs
 Compact
fluorescent lights
are a type of bulb using
fluorescent light.
 A CFL
is actually a thin
fluorescent tube coiled into
the shape of a bulb.
LEDs
 LED
stands for light-emitting
diode.
 A diode
is an electronic
device usually made out of
layers of silicon that allow
electric current to flow in one
direction through a circuit
while blocking current flowing
in the opposite direction.
 Diodes
are found in almost
every electronic device in use
today.
Unit 8: Light and Optics
Chapter 21: Light and Color
 21.1
Properties of Light
 21.2
Vision and Color
 21.3
Using Color
21.2 Investigation: The Additive Color Model
and Vision
Key Question:
How do we see color?
Objectives:

Use flashlights to mix primary colors of light and show that white
light can be made from red, green, and blue light.

Compare sources of light.

Explain how humans see color.
Color and energy
 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
 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).
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.
Photons and light
 Light
energy comes in
tiny bundles called
photons.
 You
can think of a photon
as a wave with a very
short wavelength.
 Each
photon carries the
frequency of the light
corresponding to its
energy.
Energy and light

Like other waves, the frequency of light is proportional to its
energy.

Red light has lower energy than blue light and also has a
lower frequency.
Photoelectric effect

The photoelectric effect is the emission of electrons from a
metal surface when light falls on that surface.

In 1905, Einstein published his explanation of the
photoelectric effect: atoms absorb only one photon at a time.

Einstein’s explanation provided strong evidence for the
quantum theory of light.
This work led to Einstein’s Nobel
Prize for physics in 1921.
Unit 8: Light and Optics
Chapter 21: Light and Color
 21.1
Properties of Light
 21.2
Vision and Color
 21.3
Using Color
21.3 Investigation: The Subtractive Color
Model
Key Question:

How is color created by light?
Objectives:

Mix different pigments and observe how subtractive
color mixing works.

Explain the colors humans see in terms of
subtracting colors from white light.

Distinguish between the additive and subtractive
color processes.
How objects appear as different colors
 Your
eye creates a
sense of color by
responding to red, green,
and blue light.
 You
don’t see objects in
their own light, you see
them in reflected light!
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.
Subtractive color process
 Colored
fabrics and paints
get color from a subtractive
color process.
 Cyan,
magenta, and yellow
are the three subtractive
primary colors.
 By
using different
proportions of the three
pigments, a paint can
appear almost any color.
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.
Ink jet and color printing
 An
ink-jet printer makes tiny
dots of cyan, magenta, yellow,
and black to print a full-color
image.
 Look
at an ink-jet print under a
magnifying glass and the dots
will be visible.
Making an RGB color image
 A television
or computer
monitor 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.
Making an RGB color image
 Like
the rods and cones in your retina, a video
camcorder has tiny light sensors on a small chip
called a CCD.
 There
are three sensors for each pixel of the
recorded image: red, green, and blue.
Color Blindness
 Normal
vision
 Green-weak
 Red-weak
color vision
color vision
Plants use color
 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.
Why most plants are green
 Plants
must reflect some
light to avoid absorbing
too much energy.
 The
leaves of some
plants turn brilliant red or
gold in the fall when the
chlorophyll breaks down
and reveals other
pigments left in the leaf.
Sensing the Quantum World

Most of the digital cameras and
camcorders on the market today use
a charge-coupled device, or CCD.

The CCD was invented in 1969 by
two researchers at Bell Laboratories
in New Jersey.

Willard Boyle and George Smith
were working on creating a new kind
of computer memory when they
realized the technology could be
used to record images.