Color and
Vision
The Electromagnetic and Visible Spectra
As discussed in Unit 10 of The Physics Classroom Tutorial, electromagnetic waves are waves
that are capable of traveling through a vacuum. Unlike mechanical waves that
require a medium in order to transport their energy, electromagnetic waves are
capable of transporting energy through the vacuum of outer space.
Electromagnetic waves are produced by a vibrating electric charge and as such,
they consist of both an electric and a magnetic component. The precise nature
of such electromagnetic waves is not discussed in The Physics Classroom
Tutorial. Nonetheless, there are a variety of statements that can be made about
such waves.
Electromagnetic waves exist with an enormous range of
frequencies. This continuous range of frequencies is known as the electromagnetic
spectrum. The entire range of the spectrum is often
broken into specific regions. The subdividing of the entire spectrum into
smaller spectra is done mostly on the basis of how each region of
electromagnetic waves interacts with matter. The diagram below depicts the
electromagnetic spectrum and its various regions. The longer wavelength, lower
frequency regions are located on the far left of the spectrum and the shorter
wavelength, higher frequency regions are on the far right. Two very narrow
regions within the spectrum are the visible light region and the X-ray region.
You are undoubtedly familiar with some of the other regions of the
electromagnetic spectrum.
Visible Light Spectrum
The focus of Lesson 2 will be upon the visible light region -
the very narrow band of wavelengths located to the right of the infrared region
and to the left of the ultraviolet region. Though electromagnetic waves exist
in a vast range of wavelengths, our eyes are sensitive to only a very narrow
band. Since this narrow band of wavelengths is the means by which humans see,
we refer to it as the visible light spectrum. Normally when
we use the term "light," we are referring to a type of
electromagnetic wave that stimulates the retina of our eyes. In this sense, we
are referring to visible light, a small spectrum from the enormous range of
frequencies of electromagnetic radiation. This visible light region consists of
a spectrum of wavelengths that range from approximately 700 nanometers (abbreviated nm) to approximately 400 nm.
Expressed in more familiar units, the range of wavelengths extends from 7 x 10-7 meter to 4 x 10-7 meter. This narrow band of visible
light is affectionately known as ROYGBIV.
Each individual wavelength within the spectrum of visible
light wavelengths is representative of a particular color.
That is, when light of that particular wavelength strikes the retina of our
eye, we perceive
that specific color sensation. Isaac Newton showed that light shining
through a prism will be separated into its different wavelengths and will
thus show the various colors that visible
light is comprised of. The separation of visible light into its different colors is known as dispersion. Each color is
characteristic of a distinct wavelength; and different wavelengths of light
waves will bend varying amounts upon passage through a prism. For these
reasons, visible light is dispersed upon passage through a prism. Dispersion of
visible light produces the colors red (R),
orange (O), yellow (Y), green (G), blue (B), and violet (V). It is because of
this that visible light is sometimes referred to as ROY G. BIV. (Incidentally, the
indigo is not actually observed in the spectrum but is traditionally added to
the list so that there is a vowel in Roy's last name.) The red wavelengths of
light are the longer wavelengths and the violet wavelengths of light are the
shorter wavelengths. Between red and violet, there is a continuous range or
spectrum of wavelengths. The visible light spectrum is shown in the
diagram below.
When all the wavelengths of the visible light
spectrum strike your eye at the same time, white is perceived. The sensation of
white is not the result of a single color of
light. Rather, the sensation of white is the result of a mixture of two or
more colors of light. Thus, visible light -
the mix of ROYGBIV - is sometimes referred to as white light. Technically speaking, white is not a color at
all - at least not in the sense that there is a light wave with a wavelength
that is characteristic of white. Rather, white is the combination of all
the colors of the visible light spectrum.
If all the wavelengths of the visible light spectrum give the appearance of
white, then none of the wavelengths would lead to the appearance of black. Once
more, black is not actually a color. Technically
speaking, black is merely the absence of the wavelengths of the visible light
spectrum. So when you are in a room with no lights and everything around you
appears black, it means that there are no wavelengths of visible light striking
your eye as you sight at the surroundings.
Check Your Understanding
1. A light wave is an electromagnetic wave that has both an
electric and magnetic component associated with it. Electromagnetic waves are
often distinguished from mechanical waves. The distinction is based on the fact
that electromagnetic waves ______.
a. can travel
through materials and mechanical waves cannot
b. come in a range
of frequencies and mechanical waves exist with only certain frequencies
c. can travel
through a region void of matter and mechanical waves cannot
d. electromagnetic
waves cannot transport energy and mechanical waves can transport energy
e. electromagnetic
waves have an infinite speed and mechanical waves have a finite speed
Answer: C
Electromagnetic
waves are able to travel through a vacuum - a region void of matter. Mechanical
waves require a medium in order to propagate from one location to another.
2. Consider the electromagnetic spectrum as you answer these
three questions.
a. Which region of the electromagnetic spectrum has the
highest frequency?
b. Which region of the electromagnetic spectrum has the
longest wavelength?
c. Which region of the electromagnetic spectrum will travel
with the fastest speed?
a. The gamma radiation region have the highest frequency.
b. The radio wave region has the longest wavelength.
c. All regions have the same speed. The speed of a wave is
not dependent upon its frequency and wavelength but rather upon the properties
of the medium through which it travels.
3. Consider the visible light spectrum as you answer these
two questions.
a. Which color of the
visible light spectrum has the greatest frequency?
b. Which color of the
visible light spectrum has the greatest wavelength?
a. Violet waves have the
highest frequencies.
b. Red waves have the
longest wavelengths.