SOL 5.3 Light Notes
April, 2013
Light is form of energy and does NOT need matter in which to travel.
We cannot see without light. In order to see, there must be a source of light that
gives off light (sun), or an object that reflects light (moon) from another source.
To produce light, heat is necessary. Nuclear reactions (sun), chemical reactions
(burners), and electricity (bulbs) all are involved in making light.
Light has properties of both a wave and a particle. A recent theory identifies light
as a small particle, called a photon. A photon moves in a straight line. In both the
wave and photon descriptions, light is energy. Because light has both electric and
magnetic fields, it is referred to as electromagnetic radiation. Light waves move as
transverse waves and travel through a vacuum at a speed of approximately 186,000
miles per second. Compared to sound, light travels extremely fast. It takes light
from the sun less than 8 ½ minutes to travel 93 million miles (150 kilometers) to
reach Earth.
Unlike sound, light travels in straight paths called rays, and do not need a medium
(matter) through which to move. A ray is the straight line that represents the path
of light. A beam is a group of parallel rays. This is evident when objects cast
shadows when light reflects off objects. Light rays can also bounce or reflect off
objects. Light rays that bounce off polished, shiny surfaces can reflect a picture
of the light source called an image. When we look into a flat mirror we see our
reflection, or our image.
Light waves are characterized by their wavelengths and the frequency of their
wavelengths. Like the sound wave, the size of a wave is measured as its wavelength,
which is the distance between the crests of the wavelength. Also, the crest of the
light wave is the top of the wave; the trough is the bottom of the light wave.
Frequency is the number of waves passing a given point every second; the greater
the frequency, the greater the amount of energy. Light waves are waves of energy.
The amount of energy in a light wave is related to its frequency; high frequency
light has high energy; low frequency light has low energy. The more wavelengths in
a light wave in a given time, the higher the energy level. (Gamma rays have the
most energy and radio waves have the least energy).
The entire range of electromagnetic radiation (light) is called the electromagnetic
spectrum:
The only difference between the different types of electromagnetic radiation
(light) is the amount of enery. Only sunlight consists of the entire electromagnetic
spectrum.
We see visible light as the colors of the rainbow. White light is a combination of all
colors. When light passes through a prism, a band of rainbow colors are revealed.
This band of colors is called a spectrum. The colors of the visible spectrum are
red, orange, yellow, green, blue, indigo, and violent (ROYGBIV). Red has the
longest wavelength and violet has the shortest. Red is refracted the least and
violet is refracted the most. Primary colors are red, green, and blue light. These
colors, when mixed in equal amounts, make white light. A rainbow is formed when
water drops, acting as both prisms and mirrors, bend rays of sunlight at different
angles, causing the colors to spread out and reflect off the back of the drops into
our eyes. When lights strikes an object, pigments (colored substances) in the
object reflect some colors but absorb others. Magenta, cyan, and yellow are called
primary pigments.
Black and white are not spectral colors. Black is when a material absorbs all the
visible light and no light is reflected back. Black is a total absence of reflected
light. White is a reflection of all visible light together.
Concave mirrors curve in on the shiny side, while convex mirrors curve out on the
shiny side. Images formed by a concave mirror depend on how far the object is
from the mirror. Objects very close to the mirror produce enlarged, right-side up
images. Objects a little bit farther back produce enlarged, upside-down images.
Images formed by a convex mirror always form reduced upright images that are
much smaller than the object. Convex mirrors are used as side rearview mirrors on
vehicles because they give a wide-angle view. Keep in mind, though, the objects in a
convex mirror seem to be farther away than they really are. Convex mirrors are
also used in stores as security mirrors to give a wide view of what’s going in the
store.
Light travels in straight paths until it hits an object, where is bounces off
(reflects) is bent (refracted), passes through an object (is transmitted), or is
absorbed as heat. Light is blocked by some objects and is able to pass through
others. Opaque materials completely block light from passing through. Examples
of opaque materials are a book, stuffed animal, wood, and metal. Transparent
materials allow light to pass through with almost no disturbance. Objects can be
seen clearly through them. Examples are clear glass, clear plastic, and windows.
Translucent materials allow only part of the light to pass through, while also
bouncing it in many new directions, giving only a blurry view. Examples are shower
doors, tinted glass, kaleidoscopes, and frosted glass and windows. Polarization is a
way of controlling the brightness and glare of light. A pair of sunglasses is a good
example of polarization. The lenses are transparent, but contain light-absorbing
chemicals that shield our eyes.
The bending of light rays as they pass from one substance into another is called
refraction. When light enters a new material at a 90˚ angle, the rays slow down at
the same time and the light continues in the same direction, however, when light
enters material at a different angle, some rays slow down before others and the
light changes directions.
Lenses are pieces of transparent materials with curved surfaces that use the
refraction of light to make images. Convex lenses curve outward while concave
lenses curve inward. Convex lenses form images by refracting light rays together;
concave lenses form images by refracting light rays apart. Concave lenses are used
to correct nearsightedness; convex lenses are used to correct farsightedness.
How the Eye Works: Light from an object reaches the eye and is refracted by
the cornea. The refracted light then enters the eye through the pupil and travels
to the lens. The lens of the eye bends the light even more, so that it forms an
image on the retina. These images are upside-down. The optic nerve sends a
message to the brain, which then turns the image right-side up. If we didn’t have a
brain, we would see the world upside-down.
Electromagnetism refers to forces that come from electricity and magnetism.
Light is electromagnetic energy. All the wavelengths of light that we see and
cannot see are called the electromagnetic spectrum. Lasers are devices that
produce thin streams of light.
Contribution to Light
Sir Isaac Newton – Discovered the colors of the spectrum.
Alessandro Volta – Produced the first steady electric current.
Thomas Edison – Invented the first popular light bulb in the United States.
Lewis Howard Latimer – Invented an improved bulb two years after Edison. He
later worked with Edison.
Irving Langmuir – Invented a way of improving the light bulb to make it brighter by
filling it with a special gas.
James Clerk Maxwell – Discovered that light is electromagnetic energy.