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Teacher Background | STEMscopes
Properties of Visible Light
Teacher Background
Properties of Visible Light
The electromagnetic spectrum consists of the range of all possible wave frequencies of
electromagnetic radiation. Electromagnetic radiation with low frequency and long
wavelengths is found in the microwave, radio, and infrared portions of the electromagnetic
spectrum. High-frequency, short wavelengths are found in the ultraviolet, x-rays, and gamma
rays portion of the electromagnetic spectrum. Visible light occurs in the middle of those two
extremes. Visible light is the small part of the electromagnetic spectrum that we can see.
Colors exist at different wavelengths from lowest energy to highest energy: red, orange,
yellow, green, blue, indigo, and violet.
Light waves can be
re ected, absorbed,
or transmitted
through various
materials. Re ection is the ability of a wave to bounce back but not be absorbed. When a
wave strikes a boundary, it makes an abrupt change of direction, depending upon the
angle the wave strikes; however, the speed of the wave is not changed. One characteristic
of waves is the law of re ection. This law states that the angle at which the wave
approaches a at, re ective surface is equal to the angle at which the wave leaves the
surface. Some surfaces are shiny and re ect a mirror-like image called specular
re ection. When solid surfaces are rough, light can be scattered resulting in diffuse
re ection. Scattering or diffusing occurs when light bounces off an object in a variety of
directions. The amount of scattering that takes place depends on the wavelength of the
light and the size and structure of the object. This phenomenon causes the sky to be blue.
Incoming light rays from the Sun encounter the oxygen and nitrogen molecules in the atmosphere which cause the shortest wavelengths of visible light (blue and
violet) to be scattered in every direction. The longer wavelengths of red and yellow are not affected by these molecules and pass right through.
Re ection also depends on the shape of the surface. If the re ective surface is convex (curves away from the incoming light), the light rays bounce off and spread
out (diverge), like looking at the backside of a spoon. The image in a convex mirror appears to be smaller, upright, and covers a wider eld of view. Security mirrors
on the sides of halls or building walls and side mirrors on trucks and cars take advantage of the wide eld of view created from a convex mirror re ection. If the
re ective surface is concave (curves toward the incoming light), the light rays bounce off and come to a point (converge), like looking at the bowl of a spoon. The
image is reversed but larger. Telescopes, a dentist’s mirror tool, and cosmetic mirrors take advantage of the magni ed image created from a concave mirror
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re ection. When the object is between the center of the curvature and the focal point, the
image is enlarged. The parabolic-shaped concave mirror focuses the parallel lights to a
single point. The behavior of re ected light follows the law of re ection, i.e., the angle of
incidence equals the angle of re ection. This means that the angle of the incoming ray on
shiny surfaces will equal the angle of the outgoing ray.
The speed and
direction of a
wave changes
when a wave
crosses from
one medium to
another. This
change causes
the wave to
bend toward the
denser medium
if it is moving at
a non-perpendicular angle to the boundary. This phenomenon, known as wave
refraction, causes objects in water to appear to be bent when light passes from air
through water. The combination of the refraction of sunlight as it enters a water droplet and then is re ected off the back of the droplet and refracted back out
the droplet causes the sunlight to split into its various wavelengths, which produces a rainbow or spectrum of color. The principle of refraction is used, for
example, in making lenses for cameras, eyeglasses, and refracting telescopes. Different mediums have different refraction indexes. The higher the index number,
the more light is slowed down and therefore bent.
Absorption of light occurs when a ray of light strikes a surface and is not re ected or
refracted. The best example of this is wearing the color black outside. We see the black
color because all the wavelengths of light are absorbed by it, and we feel heat because of
the energy being carried by them. To compare, wearing a white shirt in the Sun is much
cooler because all the wavelengths are re ected, allowing us to see white. Since different
atoms and molecules have different natural frequencies of vibration, they will selectively
absorb different frequencies of visible light. So most objects will selectively absorb light
while also transmitting and/or re ecting some of it. When absorption occurs, heat
energy is generated. For example, a red apple absorbs all frequencies of white light but
re ects shades of red to the observer’s eye.
Visible light rays from light sources such as the Sun or lamps fall onto objects and then
re ect off and enter the eye. The rays are refracted rst by the transparent cornea and
then by the lens which focuses the rays on the retina at the back of the eye. The retina
converts the light to electrical nerve impulses, which are sent to the brain to be interpreted for shape, dimension, color, etc. The eye can only detect wavelengths
of visible light between approximately 750 to 400 nanometers in wavelength in the electromagnetic spectrum, which correspond to the colors and shades of red,
yellow, green, blue, indigo, and violet. The eye cannot see high-frequency wavelengths such as ultraviolet, x-rays, and gamma rays nor low-frequency wavelengths
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such as infrared, radio, and microwave. Because our eyes are able to distinguish between
different wavelengths of light, we perceive color. If the light reaching our eyes contains a
mixture of wavelengths, we interpret it as white light.
Electromagnetic
waves do not require a
medium to transfer
energy but they are
affected by the
interface between two
mediums. Light waves
across the
electromagnetic
spectrum behave in
similar ways. As
discussed in previous
sections, when a light
wave encounters an object, it is either transmitted, re ected, absorbed, refracted, polarized, or scattered depending on the composition of the object and the
wavelength of the light. Denser materials generally slow the light more than less dense materials, but the effect also depends on the frequency or wavelength of
the light. If light moves from a less dense medium, like air, into a denser medium, like glass, then the light slows down. The light will bend towards the normal line. If
light moves from a more dense medium to a less dense medium, then the light speeds up and moves away from the normal.
Mediums that are not completely transparent can either absorb light or re ect it. In absorbing
materials, such as dark cloth, the energy of vibrating electrons does not go back to the light.
Instead, the energy goes toward increasing the motion of the atoms, which causes the material to
heat up. The atoms in re ective materials, like metals, reradiate light that cancels out the original
wave. Only the light reradiated back out of the material is observed. All materials exhibit some
degree of absorption, refraction, and re ection of light. The study of the behavior of light in
materials and how to use this behavior to control light is called optics.
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Absorbed
Properties of Visible Light
Picture Vocabulary
To soak up or take something in
Frequency
Light
The number of wave cycles that pass a given
point per unit of time
A form of energy that exhibits wave-like behavior
as it travels through space; part of the
electromagnetic spectrum
Light Path
Reflected
Path followed by a beam of light when passing
through an optical device or a particular part of one
Energy waves bouncing off the surface of an object
(i.e., mirrors or echoes return energy back to the
source)
Transmitted
Transparent
Passed from one place to another
Allowing light to pass through so that objects
behind can be distinctly seen
Properties of Visible LIght
A rainbow is a perfect example of how visible
light is composed of a spectrum of colors. To
see a rainbow, your back must be to the Sun as
you look up at about a 40-degree angle to where
the air has suspended droplets of water. Each
droplet of water acts as a tiny prism that
transmits, refracts, and reflects light to your
eye.
visible light – the range of wavelengths of electromagnetic radiation that our eyes can detect
spectrum – the band of colors
produced when light is separated into its
component wavelengths
transmit – to pass through a medium
refract – to bend or change direction of a wave
reflect – to bounce back
Electromagnetic Waves
Light is made of electromagnetic waves that do not need a medium in which to travel. Actually, light
waves are a combination of electric and magnetic energy that travel as particles called photons.
These photons have different wavelengths along the electromagnetic spectrum. In order of
decreasing wavelengths, they are the following: radio waves (lowest-energy photos), microwaves,
infrared, visible light, ultraviolet, X-rays, and gamma rays (highest-energy photons). Notice that
visible light is only a small portion of the entire electromagnetic spectrum.
In a vacuum, all wavelengths travel at the same speed of 300,000,000 kilometers/second, which is
186,000 miles/second. Frequency refers to the number of waves that pass a given point in a given
period of time (usually 1 second). Electromagnetic waves with longer wavelengths have lower
frequencies; in other words, fewer waves pass a given point each second. Electromagnetic waves
with shorter wavelengths have higher frequencies, which means more waves pass a given point
each second.
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Properties of Visible LIght
Visible Light and Color
Visible light is the only part of the
electromagnetic spectrum that human eyes
can detect. Visible light is made up of
photons of different wavelengths ranging
from about 400 to 700 nanometers (nm). A
nanometer is a billionth of a meter. Each of
these different wavelengths corresponds to
a different color. We see the longest
wavelengths of visible light as red light. We
see the shortest wavelengths as violet light.
White light can be separated into different colors
using a special lens called a prism. As visible light
passes from air into the glass, the change in
medium causes a change in direction and speed.
As a result, violet wavelengths are refracted, or
bent, at a greater angle than red wavelengths.
Refraction causing the separation of white light
into its various color wavelengths is called
dispersion.
Wave Behavior
When electromagnetic waves encounter surfaces of different media, they behave differently in the
form of reflection (bouncing), scattering (incoming photons reflecting in all directions), refraction
(bending), dispersion (when refracting white light can be separated into different wavelengths),
absorption (light energy transferring to another medium), and diffraction (spreading or bending
around the edges of an obstacle). Transmission of waves occurs when light waves pass through a
medium. Sometimes light waves can pass through a medium without being refracted, such as with
clear glass.
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Properties of Visible LIght
Reflection
The most commonly observed wave behavior is
reflection. You observe this phenomenon
whenever you see your image in a mirror.
Reflection of light waves occurs whenever a
light wave strikes a surface and then bounces
back. The angle of incidence equals the angle of
reflection. Scattering is different from reflection
in that scattered light goes back out in all
directions. White surfaces, such as walls, scatter
light in all the colors that strike them.
Absorption
Not all light is reflected or scattered. Black absorbs all
light. Colored surfaces absorb all wavelengths except for
the color reflected. Absorption occurs when all or some of
the light energy from light waves is transferred from one
medium to another. Certain pigments reflect or transmit
the wavelengths they cannot absorb, making them appear
in the corresponding color.
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Properties of Visible LIght
Law of Reflection
The reflection of waves from a boundary is similar to
the way a billiard ball strikes and bounces away from
a wall. If a ball strikes a wall head-on (meaning it is
traveling perpendicular to the wall), the ball will
bounce back in exactly the same direction from which
it traveled. However, if a ball strikes a wall at an angle
to the perpendicular (called the angle of incidence), it
will bounce away from the wall at the same angle to
the perpendicular (the angle of reflection). Waves
behave in the same way, as shown below. In fact, this
is called the law of reflection. While a wave’s direction
changes during reflection, its speed does not.
The reflection of hot-air balloons is
seen on the surface of a still lake.
Some of the light that has been
reflected from the hot-air balloon
travels through the air to the lake.
When this light strikes this boundary,
some of it reflects back into the air.
This causes an image of the hot-air
balloon to appear on the
lake’s surface.
A wave (red arrow) strikes a barrier (blue line) at an angle θi to the perpendicular line. This is the
angle of incidence. The wave bounces away at an angle θr. This is the angle of reflection. The
law of reflection states that the angle of incidence is equal to the angle of reflection.
Not all materials transmit light clearly. For example, transparent objects allow all light to pass.
What do you think are good uses of transparent materials? Translucent objects transmit only
some light, resulting in objects on the opposite side appearing fuzzy, such as some office
windows or shower doors. Opaque surfaces do not allow light to pass through them. Light striking
an opaque surface is either absorbed by the surface or reflected from it.
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Properties of Visible LIght
Refraction
A wave can also be transmitted through a boundary, meaning that it passes through the boundary
from one medium to another. In fact, at most boundaries, part of the wave is reflected and part of it
is transmitted. Most waves move at different speeds through different media. For example, light
moves quickly through air but more slowly through water. Waves change speeds as they transmit
from one medium to another. This change in speed usually causes a wave to refract, or bend, as it
passes through the boundary.
Refraction can be thought of as the black dots in this
diagram, which represents a group of students marching
across a room. The students represent a light wave. The
left side of the room represents air (a relatively fast
medium), and the right side represents water (a relatively
slow medium). When the students pass from left to right,
one student crosses the boundary first. This student
begins moving slowly, but the rest of the group still travels
quickly, causing the students to change direction, or bend,
as they move into the new medium. This is known as
refraction.
Refraction causes light from the Sun to spread out into
different colors as it passes from air into new media,
such as water or glass. For example, the different
wavelengths of sunlight all refract at different angles
when they pass through a raindrop. This causes
sunlight to spread out into a rainbow as it refracts
through raindrops after a storm.
Refraction through concave or convex lenses
allows correction for poor vision or
magnification of objects. Refraction through
water bends the image. Refracting telescopes
use lenses to gather light from distant
sources, such as planets, stars, or galaxies,
and focus that light through an eyepiece.
.
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Properties of Visible LIght
Diffraction
Have you ever watched water waves pass through a
narrow slit in a barrier? When the waves pass through,
they spread out radially. This is known as diffraction. For
example, when a light bulb is turned on in a dark room,
light waves spread out radially from the light bulb.
However, once the waves have traveled a certain distance,
they can be thought of as plane waves that move in
Plane waves (left) spread out radially
parallel sheets, as shown in the image on the right. When a
(right) when they pass through
plane wave passes through a narrow slit or a barrier, the
a slit in a barrier.
wave will act as though it is originating from a point source
again. Thus, the wave spreads out, or diffracts.
This can also happen when a wave approaches a solid barrier. In these cases, when the wave
passes along any edge of the barrier, it will spread out radially around the barrier. In this way,
waves will appear to bend around barriers. This is the reason you can hear sounds that are emitted
from behind a wall or large building.
Everyday Life: Examples of Diffraction
You can see an example of diffraction when light passes through
tiny grooves on the surface of a CD and the light is separated
into a rainbow of colors. This effect is also seen when light is
diffracted off the surface of oil. Diffraction is also used in
producing special holographic images.
Special photographic techniques
can diffract light and record it in
3-D. Diffraction images can be
used as holograms as a security
measure on identification cards or
credit cards. These images can
only be read by an optical scanner.
Supermarket checkout scanners
use holographic optical elements
(HOEs) that can read a universal
product code (UPC) from any
angle.
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Properties of Visible LIght
In this exercise, you will show what you know about
reflection and refraction. In the diagram on the right, the
double lines represent a pane of glass and the single line
represents a mirror, both shown edgewise. The arrow is a
ray of light.
Draw the path of the light ray until it reaches point X.
Show how the change in the speed of the light ray affects
its direction as it passes from one medium to another.
Remember, light travels more slowly in glass than it does
in air. Hint: The light will bend both when it enters and
when it exits the glass!
Explain how a rainbow is produced.
Why do you see something as white, black, or another specific color?
What are electromagnetic waves, and how can they be seen?
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