Introduction to Light and Color

Introduction to Light
Light, Color, and Their Uses

Light is a form of radiant energy All light can be traced to certain

or energy that travels in waves. Since energy sources, like the Sun, an
Greek times, scientists have debated electric bulb, or a match, but most
the nature of light. Physicists now of what hits the eye is reflected light.
recognize that light sometimes When light strikes some materials,
behaves like waves and, at other times, it is bounced off or reflected. If the
like particles. When moving from place material is not opaque, the light goes
to place, light acts like a system of through it at a slower speed, and it
waves. In empty space, light has a is bent or refracted. Some light is
fixed speed and the wavelength can be absorbed into the material and
measured. In the past 300 years, changed into other forms of energy,
scientists have improved the way they usually heat energy. The light waves
measure the speed of light, and they make the electrons in the materials
have determined that it travels at vibrate and this kinetic energy or
nearly 299,792 kilometers, or 186,281 movement energy makes heat. Friction
miles, per second. of the moving electrons makes heat.

When we talk about light, we usually

mean any radiation that we can see.
These wavelengths range from about
16/1,000,000 of an inch
to 32/1,000,000 of an inch. There are
other kinds of radiation such as
ultraviolet light and infrared light, but
their wavelengths are shorter
or longer than the visible light
wavelengths.

When light hits some form of

matter, it behaves in different ways.
When it strikes an opaque object, it
makes a shadow, but light does bend
around obstacles. The bending of light
around edges or around small slits is
called diffraction and makes patterns
of bands or fringes.

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Introduction to Color

Light, Color, and Their Uses

Color is a part of the electro- Pigment color found in paint, dyes,
magnetic spectrum and has always or ink is formed by pigment molecules
existed, but the first explanation of present in flowers, trees, and animals.
color was provided by Sir Isaac The color is made by absorbing, or
Newton in 1666. subtracting, certain parts of the
spectrum and reflecting or transmitting
Newton passed a narrow beam of the parts that remain. Each pigment
sunlight through a prism located in molecule seems to have its own
a dark room. Of course all the visible distinct characteristic way of reflecting,
spectrum (red, orange, yellow, green, absorbing, or transmitting certain
blue, indigo, and violet) was displayed wavelengths. Natural and manmade
on the white screen. People already colors all follow the same natural laws.
knew that light passed through a
prism would show a rainbow or visible
spectrum, but Newton’s experiments
showed that different colors are bent
through different angles. Newton also
thought all colors can be found in
white light, so he passed the light
through a second prism. All the visible
colors changed back to white light.

Light is the only source of color.

The color of an object is seen because
the object merely reflects, absorbs, and
transmits one or more colors that
make up light. The endless variety of
color is caused by the interrelationship
of three elements: Light, the source of
color; the material and its response to
color; and the eye, the perceiver of color.

Colors made by combining blue,

yellow, and red light are called
additive; and they are formed by
adding varying degrees of intensity
and amounts of these three colors.
These primary colors of light are
called cyan (blue-green), yellow, and
magenta (blue-red).

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 Introduction to Mirrors and Lenses
Light, Color, and Their Uses
Introduction to Mirrors
As we look around the room, we
see most objects by the light that is
diffusely reflected from them.

Diffuse reflection of light takes

place when the surface of the object
is not smooth. The reflected rays from
a diffusely reflecting surface leave the
surface in many different directions.

Light Object
Bulb

When the surface is smooth, such

as the surface of glass or a mirror,
then it can be easily demonstrated
how reflected rays always obey the
law of reflection as illustrated below.

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Law of Reflection

Light, Color, and Their Uses

The angle of incidence is equal to
the angle of reflection.

Smooth
Reflecting
Surface
i

r i = Angle of Incidence (See Glossary,

r = Angle of Reflection page 63.)
r=i

The Image Formed by

Reflection in a Flat Mirror
Every object we see has many rays are reflected by the mirror, the
of light coming from it either by reflected rays appear to come from the
reflection or because it is a light image located behind the mirror at a
source such as a light bulb, the Sun, distance equal to the object's distance
a star, etc. Each point on that object from the mirror. The image is called a
is a source of light rays. In the virtual image since the rays do not
illustration below, the tip of the arrow actually pass through or come from
is used as an example of a point on the the image; they just appear to come
object from which rays of light would from the image as illustrated below.
be coming. As the rays from the object

Mirror
Object Image of Object
(Virtual Image)

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 The Image Formed by a
Concave Mirror
Light, Color, and Their Uses

A concave mirror that is part of a closer to the mirror. This difference in

ball or hollow sphere (that is, it has the axis cross-over points is called
a circular cross section) is a spherical spherical aberration.
mirror. The focal length is approximately
one-half the radius of curvature. A ray If the mirror has a cross section
that is both parallel and very close to that is a parabola instead of a circle,
the optical axis will be reflected by the all of the rays that are parallel to the
mirror so that it will cross the optical optical axis will cross at the same
axis at the “paraxial focal point.” The point. Thus, a paraboloidal mirror does
paraxial focal point is located a not produce spherical aberration. This
distance of one-half the radius of is why the astronomical telescope
curvature from the point on the mirror known as the Newtonian (invented by
where the optical axis intersects the Isaac Newton) uses a paraboloidal
mirror. The word “paraxial” comes from primary mirror.
the Greek “para” or “par” meaning “at
the side of, or beside, and axial.” Thus For demonstration purposes in
paraxial means beside the axis. the classroom, it works out that we can
make the approximation that spherical
Another ray that is parallel to the mirrors behave almost like paraboloidal
optical axis, but not close to the axis, mirrors and determine that the focal
will be reflected by the mirror so that length of a spherical mirror is about
it crosses the optical axis, not at the one-half the radius of curvature of
paraxial focus, but a small distance the mirror.

Concave
Mirror
Radius of
Circle
(1)

(2) (3)
Object c f Optical
Axis

Real
Image

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 In the case where the object is The Image Formed by a
located between the focal point and
the mirror, such that the object Convex Mirror

Light, Color, and Their Uses

distance is less than the focal length
of the mirror, a virtual, upright, and The image formed by a convex
enlarged image is obtained. This is mirror is virtual, upright, and smaller
the case when looking at yourself in than the object. This is illustrated by
a concave “make-up” mirror, which is the ray diagram on the following page.
described below. The diagram depicts the three rays
that are discussed in the following
A ray (1) appearing to come from paragraph.
the focal point strikes the mirror and
is reflected parallel to the optical axis. A ray (1) parallel to the optical axis
A ray (2) parallel to the optical axis is is reflected as if it came from the focal
reflected by the mirror so that it goes point (f). A ray (2) directed toward the
through the focal point. A ray (3) striking focal point is reflected parallel to the
the mirror at the optical axis is reflected optical axis. A ray (3) striking the
so that the angle of reflection is equal mirror at the optical axis is reflected
to the angle of incidence. at an angle equal to the angle of
incidence.
The ray diagram below uses three
reflected rays to illustrate how the
image can appear to be enlarged and
upright. The image formed is a virtual
image.

Concave Mirror

(1)
(2)
Image
(3)
c f Optical
Object Axis

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 Convex Mirror
Light, Color, and Their Uses

Object (1) Image

(2)
Optical i f
c
Axis r
(3)

Introduction to Lenses
A simple lens is a piece of glass or
plastic having two polished surfaces
that each form part of a sphere or ball.
One of the surfaces must be curved;
the other surface may be curved or
flat. An example of a simple lens would A lens thicker in the center than
be obtained if a piece of a glass ball at the edge is called a converging or
were sliced off as shown in the positive lens. A lens thinner at the
following illustration. center than at the edge is called
a diverging or negative lens. In the
illustration shown, lenses 1, 2, and 3
Lens Glass Ball are converging or positive lenses.
Lenses 4 and 5 are diverging or
negative lenses.

(1) (2) (3) (4) (5)

The piece of the ball sliced off

would be a lens with a spherical side
and a flat side. Lenses can be made
in a variety of shapes for various
applications. Some examples of lens
shapes are illustrated here.

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The Image Formed by a
Converging Lens

Light, Color, and Their Uses

Ray #1

f Optical
Axis

Ray #2
Focal
Length
When using a thin lens, that is, the
thickness at the center of the lens is
not too great, a thin lens mathematical
approximation can be used. This
The size and location of an image
approximation assumes the bending of
formed by a lens can be found by using
light occurs in one plane inside the
the information from these two rays
lens.
which is shown in the illustration below.
A ray of light coming from a very
The following illustration depicts
distant object, such that the ray is
two rays, which are defined in the
parallel to the optical axis, will be bent
following text. A ray (1) parallel to
by refraction at the two surfaces of the
the optical axis passes through the
lens and will cross the optical axis at
focal point (f). A ray (2) passing
the focal point (f) of the lens, as seen
through the center of the lens is
in the illustration below. A ray passing
undeviated.
through the center of the lens will
pass through the lens undeviated.
The image is real, smaller than the
object, and upside down. If a piece of
paper is placed at the image location,
a real image can be seen on the paper.
An example of this is taking a picture
with a camera, where the photographic
film is located at the image position.

Object (1)
(2) f
Image

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 A Simple Magnifier
Light, Color, and Their Uses
When the object lies between the After passing through the lens, the
lens and the focal point, a virtual, three rays described above will appear
upright, and enlarged image is obtained, to come from an enlarged and upright
as seen in the illustration below. image. Any other ray leaving the tip of
the object will appear to come from
Three rays are included in the the tip of the image after passing
illustration. Following are descriptions through the lens. The three rays used
of these rays. A ray (1) leaving the in the illustration below were chosen
object parallel to the optical axis will because their paths are always known.
bend at the lens and go through the Two rays are actually enough to locate
focal point (f). A ray (2) leaving the the image, while the third ray is used
object going through the center of for an additional check of the location
the lens will be undeviated. A ray of the image.
(3) leaving the object as if it came
from the front focal point of the lens
will bend at the lens and travel in a
line parallel to the optical axis.

(3)

(1)
Optical f f
Axis
Object
Virtual (2)
Image

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