Science High School Reviewer Physical Science: Light and Its Properties

Properties of light
How does light travel?
Light is a form of energy that travels in waves.
Light waves spread out as they move away from
a source. Light travels in straight lines called rays.
Light waves can travel through empty space, without
needing a solid, liquid, or gas medium. Light travels
through space at the fastest speed matter and energy
can possibly reach: about 300,000 kilometers
(186,000 miles) per second. The speed of light is
represented in scientific formulas by the letter c.

Light has both natural and human-made sources.
Natural sources include the Sun and other stars,
which produce light by continually fusing simpler
elements into more complex ones. Another natural
source is lightning, which is produced by electrical
charges in clouds. Human-made light sources, such
as lamps and candles, rely on chemical reactions or
electricity to produce light. Light rays from any
source always travel in straight lines. However, a
light wave will spread out if it travels past the edge
of a thin object or if it moves through a narrow
opening. Regardless of its source, a ray of light will
not change direction unless it travels through a
different medium or is disturbed in some way.

Matter interacts with light in
various ways. Transparent matter
allows light to pass through with
almost no disturbance. Behind
transparent materials, objects look
clear and crisp, even if the transparent
materials change the color of the light.
A lens, such as the one found in either
side of a pair of eyeglasses, is a piece of
transparent material with at least one
curved surface.
When light rays strike matter that
is translucent (trans•LEW•suhnt), some
light passes through, and some is either
blocked or bent in different directions.
Objects viewed through translucent
materials do not look clear or crisp;
they appear blurred.
Light does not pass through matter
that is opaque (oh•PAYK). Opaque
matter reflects or absorbs all light.

The light that is absorbed is converted
to heat energy. If you tried to look
through an opaque material, you
would not be able to see an object
on the other side. An opaque object
casts a crisp shadow when in front
of a light source. A shadow is a dark
area produced by an opaque object
blocking the passage of light. Since
light always moves in straight lines,
when light is blocked by the surface of
an opaque object, a shadow forms that
is similar in shape to the object that
produces it.
How does light act with mirrors?
Reflection is the bouncing
of waves off a surface. The angle
between an incoming light ray and a
surface is equal to the angle between
the reflected light ray and the same
surface. This relationship is called the
law of reflection.

The law of reflection explains
how mirrors work. A mirror is an
object with a polished surface that
forms reflected images. Light rays that
bounce off a mirror can form an image
of an object. The things you see in a
flat mirror look almost as if they exist
on the other side of a window, with
one important exception. The image
that appears in the mirror is reversed.
For example, if you raise your left hand
in front of a mirror, in your reflection it
appears that your right hand is raised.
When light rays strike a dull or
rough surface, they do not form an
image. The law of reflection still
applies to the light rays, but the
roughness of the surface causes the
rays to reflect in different directions.
The rays still travel in straight lines,
but the lines point in many directions.
The shape of a mirror affects the
appearance of the image it reflects.
A plane mirror has a flat surface.
Plane-mirror images appear as exact
copies, though they are reversed. Most
everyday mirrors are plane mirrors.

A mirror that is concave has a
surface that curves inward. Light
rays are reflected from the surface
of a concave mirror and meet at a
point located in front of the mirror.
The place where the light waves meet
depends on the curve of the mirror. An
object you placed close to a concave
mirror would produce a large image
that was right-side-up. As you moved
the object away, the image would
become blurry and eventually appear
upside down. The image would stay
upside down and become smaller as
you continued to move the object away
from the mirror. Concave mirrors are
used to gather light inside telescopes.
Makeup and shaving mirrors are often
concave mirrors, because they make
the face appear larger and allow people
to see greater detail.
A mirror that is convex has a
surface that curves outward, like the
curve of the outside of a sphere. A
convex mirror produces an image
that is right-side-up and much smaller
than the object. When light rays are
reflected from the surface of a convex
mirror, they spread out, producing
a wide-angle view. This wide-angle
view makes convex mirrors useful
for security in stores and also for
providing a better view for drivers
of vehicles.

How does light act with lenses?

Light waves are refracted
as they pass from the air to the lens.
Refracted light rays stilf travel in
straight lines, but the paths of the lines
change as the light passes into the next
material. You can observe how light
refraction works by placing a pencil in
a clear glass of water. Light waves are
bent as they pass from the air to the
water. As a result, the pencil appears
as though it is broken right at the spot
where it enters the water. Eyeglasses,
telescopes, cameras, and microscopes all
use lenses to produce images.
Convex lenses form images by
refracting light rays together. A convex
lens is thicker toward its middle, and
this gives the lens a shape that bulges
outward. Light rays pass through the
lens and come together at a point on
the other side. The focal point is the
point at which the light rays meet. The
distance between a convex lens and
an object determines the type of image
that forms. If the object is located
between the lens and its focal point,
the image that is formed is right-sideup
and larger than the actual object.
If the object is located beyond the
focal point of the lens, the image that
is formed is upside down and smaller
than the actual object.

How do we correct vision? Contact lenses with concave or convex lenses.
Light Waves and Color
Why Do We See Colors?

Visible light from the Sun comes to Earth as white
light, traveling through space in the form of waves.
Visible light contains a mixture of wavelengths that
the human eye can detect. When these wavelengths
are separated, we see them as different colors. This
happens when light waves are refracted as sunlight
passes through raindrops. Different wavelengths are
refracted in different amounts. Long, red wavelengths
are bent the least, and short, violet wavelengths are
bent the most. Recombining all the wavelengths of
visible light produces white light.
A prism, a triangular piece of glass or plastic,
bends light. This refraction separates visible light
into the red, orange, yellow, green, blue, and violet
wavelengths that make up white light. Another way
to bend light waves is to use a diffraction grating. A
diffraction grating is usually made of glass, plastic,
or metal, and it contains many thin, parallel slits.
The compact disc
acts like a diffraction
grating, separating
white light into a
spectrum of colors.
Light rays passing through these slits
interfere with each other, separating
the white light into colors. Diffraction
gratings, like prisms, enable scientists
to study properties of light.
In the late 1600s, Sir Isaac Newton
observed that sunlight passing
through a prism emerged as bands of
different colors. Newton hypothesized
that sunlight was naturally made of
different colors of light. He called
these colors a spectrum, Latin for
“appearance” or “apparition.” We now
know that each wavelength is refracted
at a different angle and that this is what
produces the different bands of color.
Sunlight striking an object may
be reflected, refracted, or absorbed.
The light that is reflected determines
the color of an object. For example,
when sunlight strikes a leaf, many
wavelengths are absorbed and used in
photosynthesis. Green light is reflected,
so the leaf appears green. An object
that reflects all visible light appears
white. An object that absorbs all visible
light appears black.

How many kinds of light are there?
Are there waves other than visible
light within sunlight? In 1800, a
scientist named William Herschel
answered this question with an
experiment.

Today, we know that energy from
the Sun travels in many types of waves.
Diagram of the electromagnetic spectrum

Visible light makes up only a
small portion of these waves. The
electromagnetic spectrum contains the
full range of wavelengths. The spectrum
is arranged from long waves, with the
lowest amount of energy, to short waves,
with the highest amount of energy. It
consists of radio waves, microwaves,
infrared waves, visible light, ultraviolet
rays, X rays, and gamma rays.
Radio waves have the longest
wavelengths and include transmissions
of AM radio, shortwave radio,
television, and FM radio. In the next
part of the spectrum are microwaves.

Microwaves are used in radar and
satellite systems as well as ovens that
cook food quickly. Infrared waves, next
in the spectrum, are typically felt as
heat. Infrared waves are given off by
the Sun and other sources of heat, such
as electric-stove burners and active
volcanoes. The waves that Herschel
discovered were infrared waves.
Near the middle of the spectrum are
the wavelengths of visible light. We see
these wavelengths as colors that range
from red to violet.
After visible light in the spectrum
are ultraviolet rays. Ultraviolet, or UV,
rays carry more energy than visiblelight
waves do. Overexposure to
ultraviolet rays and other high-energy
waves can damage people’s skin and
eyes. The ozone layer in Earth’s upper
atmosphere provides some protection
against these electromagnetic waves.

Microwaves are used in radar and
satellite systems as well as ovens that
cook food quickly. Infrared waves, next
in the spectrum, are typically felt as
heat. Infrared waves are given off by
the Sun and other sources of heat, such
as electric-stove burners and active
volcanoes. The waves that Herschel
discovered were infrared waves.
Near the middle of the spectrum are
the wavelengths of visible light. We see
these wavelengths as colors that range
from red to violet.
After visible light in the spectrum
are ultraviolet rays. Ultraviolet, or UV,
rays carry more energy than visiblelight
waves do. Overexposure to
ultraviolet rays and other high-energy
waves can damage people’s skin and
eyes. The ozone layer in Earth’s upper
atmosphere provides some protection
against these electromagnetic waves.

After ultraviolet rays in the spectrum
are X rays and gamma rays. X rays
can pass through many substances,
including soft human tissue. Because of
this property, X rays are used to make
images of hard parts of the body, such
as teeth and bones. X rays are also used
in airports to screen luggage and other
cargo. Gamma rays have very short
wavelengths and have so much energy
that they can even pass through some
metals and concrete. Gamma rays have
many applications in science.

How do colors mix?
Different color models are used to
understand the relationships between
colors. Each color model is named
after its primary colors. Primary colors
are not produced through the mixing
process. Secondary colors are produced
by blending primary colors.
The traditional color model is the
RYB (red, yellow, blue) color model.
While it is useful in art, this model
does not include all colors, such as
some shades of green, cyan (SIGH•an),
and magenta. The RYB model is still
referred to in art classes, but scientists
now use more accurate color models.
In the RGB (red, green, blue) color
model, primary colors of light combine
and produce almost all colors. The RGB
color model is an example of additive
color mixing. In this color model, the
three primary colors can combine,
reflect all colors, and produce white.

How do colors mix?
Different color models are used to
understand the relationships between
colors. Each color model is named
after its primary colors. Primary colors
are not produced through the mixing
process. Secondary colors are produced
by blending primary colors.
The traditional color model is the
RYB (red, yellow, blue) color model.
While it is useful in art, this model
does not include all colors, such as
some shades of green, cyan (SIGH•an),
and magenta. The RYB model is still
referred to in art classes, but scientists
now use more accurate color models.
In the RGB (red, green, blue) color
model, primary colors of light combine
and produce almost all colors. The RGB
color model is an example of additive
color mixing. In this color model, the
three primary colors can combine,
reflect all colors, and produce white.