Light and Optics
Part One: Color and Energy
Light = straight path
An atom:
emits light when an electron moves from
a high to a low energy level.
absorbs energy as its electrons
move from a low to a high energy
level.
Learning Goals
 Describe the properties of light.
 Explain the relationship between
energy and the colors of light.
 Describe waves included in the
electromagnetic spectrum in terms of
energy, frequency, and wavelength.
LIGHT
 The fluorescent bulb uses high-voltage
electricity to energize atoms of gas. These
atoms give off UV which is absorbed by
atoms in the white coating inside the bulb
which re-emits white light we see..not heat
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
 The light from a gas flame is blue (high
energy) and the light from a match is redorange (low energy).
The speed of light
 The speed at which light travels through air is
about 300 million meters per second. 3x108 m/s
 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.
 The speed of light is so fast
that when lightning strikes a
few miles away, we hear the
thunder after we see the
lightning. (5 sec. per mi.)
Wavelength and
Frequency of Light
 wavelength is so small,
scientists measure it in
nanometers.
 One nanometer (nm) is one
billionth of a meter
(0.000000001 m).
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.
The electromagnetic spectrum
 Light, like sound and heat, is a form of
energy.
 The visible light we see is part of the
electromagnetic spectrum.
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.
The entire range of electromagnetic waves
Light and Optics
Part Two: Color and Vision
Learning Goals
 Explain how humans see.
 Demonstrate knowledge of the
additive and subtractive color
processes.
 Apply knowledge of the behavior of
light to explain why plants have
certain colors.
The human eye
 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
 Rods respond to the
intensity of light…“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.
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.
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.
How objects appear to be 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.
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.
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.
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!
Light and Optics
Part Three: Optics and Reflection
Learning Goals
 Explain how basic optical devices
function.
 Compare and contrast the
interactions of light and matter.
 Distinguish between concave and
convex lenses.
 State the law of reflection.
Optics is the study of life
 Optics is the study of
how light behaves.
 As light moves through
a material such as air,
the light normally
travels in straight lines.
 A light ray is an
imaginary line that
represents a thin beam
of light.
Bending light rays
 Light does not always
go straight from an
object to your eyes.
 The curved surface of
a magnifying glass
bends light rays so
they appear to come
from a much larger
thumb.
Basic optical devices
Three useful optical devices are:
1. lenses
2. mirrors
3. prisms
Basic optical devices
 A magnifying glass is
a converging lens
(convex lens) that can
be used in survival
situations to make a
hot spot.
 Mirrors can attract the
attention of rescue
teams from great
distances.
Optical devices
 A diverging lens (or concave lens) bends
light so it spreads light apart instead of
coming together.
 An object viewed through a diverging
lens appears smaller than without the
lens.
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?
Reflection
 Reflection occurs when light bounces off a
surface and when light bends while crossing
through materials.
Reflection
 There are two types of reflection; but not all
reflections form images.
 Rays light that strikes a shiny surface (like a
mirror) create single reflected rays.
 This type of reflection is called specular
reflection.
Reflection
 A surface that is dull or uneven creates
diffuse reflection.
 When you look at a diffuse reflecting surface
you see the surface itself.
Ray diagrams
 A ray diagram is an
accurately drawn
sketch showing how
light rays interact with
mirrors, lenses, and
other optical devices.
Light and Optics
Part Four: Refraction
Learning Goals
 Use the index of refraction to
determine how much light rays
bend.
 Describe total internal reflection
and it’s applications.
 Explain the role of refraction in
prism and rainbows.
Refraction
 Materials with a higher index of refraction bend
light by a large angle.
 The index of refraction for air is about 1.00.
 Water has an index of refraction of 1.33.
Angle of refraction
 The angle of refraction is the angle
between the refracted ray and the
normal line.
Refraction
 Vegetable oil and glass
have almost the same
index of refraction.
 If you put a glass rod into
a glass cup containing
vegetable oil, the rod
disappears because light
is NOT refracted!
Total internal reflection
 As the angle of incidence increases, there is
a point at which the light will not enter the air
but reflect back into the water!
 This effect is called total internal reflection.
AIR
WATER
Fiber optics
 If glass rods are made
very thin, they are
flexible, but still trap light
by total internal
reflection.
 Fiber optics are thin glass
fibers that use total
internal reflection to carry
light, even around bends
and corners.
Refraction and colors of light
 A glass prism splits
white light into its
spectrum of colors
because each color is
bent slightly differently.
 The order of colors in
the visible light
spectrum is red, orange,
yellow, green, blue,
violet (or ROY-G-BV).
Dispersion
 The “rainbow” you see when
light passes through a prism
and a real rainbow in the sky
are examples of dispersion.
 Rainbows in the sky occur
when white light from the sun
passes through water droplets
in the atmosphere.
 Like a prism, each drop splits
white light into the spectrum of
colors.
Light and Optics
Part Five: Mirrors, Lenses and Images
Learning Goals
 Distinguish between how we “see”
objects and images.
 Explain the difference between how an
image forms in a mirror and from a
lens.
 Find the focal point of a lens.
 Measure the focal length of a lens.
Mirrors, Lenses and Images
 Objects are real physical things that
give off or reflect light rays.
 Images are “pictures” of objects that
are formed in space where light rays
meet.
Images
 Images are created
by collecting many
rays from each point
on an object and
bringing them back
together again in a
single point.
Cameras and images
 A camera works by collecting the rays from an
object so they form an image on the film.
 Many rays can be focused to a single point by
a camera lens, forming the image of that part of
the railing.
 A camera captures some but not all of the rays.
Images in mirrors
 The arrow on the
graph paper is an
object because it is a
physical source of
(reflected) light.
 The image of the
arrow appears in the
mirror.
Virtual and real images
 The image in a mirror
is called a virtual image
because the light rays
do not actually come
together to form the
image.
 Real images, such as
those from cameras,
form where light rays
meet.
Lenses
 An ordinary lens is a
polished, transparent
disc, usually made of
glass.
 The shape of a
converging lens is
described as being
“convex” because the
surfaces curve
outward.
Lenses
 The distance from the center of the lens to
the focal point is the focal length.
 Light can go through a lens in either
direction so there are always two focal
points, one on either side of the lens.
Lenses
 For a converging lens, the first surface (air to
glass) bends light rays toward the normal.
 At the second surface (glass to air), the rays
bend away from the normal line.
Drawing ray diagrams
 Step 1: Draw a light ray passing through
the center of the lens.
 Step 2: Draw a light ray that starts parallel
to the axis and bends at the lens to pass
through the far focal point.
 Step 3: Draw a light ray passing through
the near focal point.
Real images
 A converging lens can form a real
image.
 The place where the light comes back
together again is called the focus.
Real images
 The ray diagram shows
how the real image is
formed.
 To make an image, a
lens collects rays from
every point on an
object.
 Rays from each point
on the object are
brought back together
again to make each
point of the image.
Real images and ray diagrams
 To make an image, a
lens collects rays from
every point on an
object.
 Rays from each point
on the object are
brought back together
again to make each
point of the image.
Magnification and telescopes
 Images may be smaller
than life size, or equal to or
larger than life size.
 The magnification of an
image is the ratio of the
size of the image divided
by the size of the object.
Telescopes and images
 To get higher magnification, microscopes and
telescopes use more than one lens.
 A refracting telescope has two converging
lenses with different focal lengths.
Microscopes
 A compound microscope
uses two converging
lenses.
 The lens closest to the
object has a very short
focal length and makes a
real, larger, inverted
image of the object inside
the microscope.
Images and converging lenses
 A converging lens
becomes a
magnifying glass
when an object is
located inside the
lens’s focal
length.
Images and diverging lenses
 A diverging lens
always has the
same ray diagram,
which shows a
smaller image.
 It doesn’t matter
where the object is,
the image will
always be smaller.
Image Summary
Optical systems
 Optical systems are built from lenses,
mirrors, and prisms.
 Optical systems do two things:
 collect light rays
 change the light rays to form an image, or
process light in other ways.
Simple optical system
 A simple optical
system can be made
with a pinhole in a box.
 The image inside the
box forms because
light rays that reach a
point on the box
surface are restricted
by the pinhole.
Lenses in optical systems
 The larger the lens,
the brighter the
image.
 This is because a
larger lens collects
more light rays.