Worlds of David Darling > Children's
Encyclopedia of Science > Making Light Work > Chapter 6
MAKING LIGHT WORK:
The Science of Optics
a book in the eXperiment! series by David Darling
6. Over the Rainbow
Our world is filled with color – the green of lush grass, the gray
of a heavy mist, the brilliant golds and yellows of sunset. But what is
color? How does red light, for instance, differ from violet light?
You can think of light as being a kind of wave, similar to waves in the
sea. The distance between one wave crest and the next is known as the WAVELENGTH.
Different colors of light have different wavelengths. The wavelength of
red light is just under one five-thousandth of an inch, while that of violet
light is just over one ten-thousandth of an inch. All the other colors fall
somewhere in between these two extremes.
You will need:
- A large sheet of heavy cardboard
- A bowl of water
- A flat mirror
- A sheet of white card
What to do:
Cut a narrow slit in the sheet of heavy cardboard and place it in
front of a window on a sunny day. The sun must be shining directly
onto the window and you must make sure that light can only get in
through the slit. Put the bowl of water on a table behind the cardboard.
Hold the mirror at an angle against one side and the sheet of white
card above the bowl. Arrange the mirror and card so that a sunbeam
falls onto the white card. What do you see?
The Spectrum of Colors
Sunlight, or the light from an electric light bulb, seems colorless. We
call it "white light." But, in fact, it contains the full range, or SPECTRUM,
of colors from red, through orange, yellow, green, blue, and indigo, to
violet. When these colors pass from air into water or glass, they are slowed
down and refracted by different amounts. Red light is bent the least, violet
light the most.
Normally, when white light passes back into air, all its colors are bent
back by the same amount so that they join together to make white again.
However, if the rays are reflected while still inside the water or glass,
they leave by different paths from those by which they entered. In this
case, the colors are separated and can be seen as a spectrum from red to
The easiest way to produce a spectrum is with a block of glass called a
PRISM. This has three flat faces running in one direction and triangular-shaped
ends. However, if conditions are right, white light can also be split apart
by other arrangements of glass and by water. A rainbow, for example, is
caused by reflections and refractions of sunlight inside raindrops.
What Makes Color?
The colors reflected by an object are those which give it its color. Something
that appears white reflects all colors of light. An object that looks red
reflects only red light and absorbs all the other colors. Something that
appears black absorbs every color and reflects none.
A FILTER is a material, such as colored cellophane or glass, that only lets
light of a particular color pass through. This means that a red object would
appear bright when seen through a red filter, but an object of another color
would seem dark.
Three of the colors – red, green, and blue – are called PRIMARY
COLORS, because they can be combined in different ways to give every other
color. A combination of two primary colors gives a SECONDARY COLOR. Green
and red give yellow, green and blue give cyan (a light blue), and blue and
red give magenta (light purple).
You will need:
- Three flashlights
- Red, green, and blue pieces of cellophane
- Three rubber bands
- A sheet of white card
What to do:
In a well-lit room, hold a piece of red cellophane in fromt of your
eyes. What do you see? What is the normal color of the objects that
seem brightest when viewed theough the cellophane? What is the normal
color of the things that seem darkest?
Make the room as dark as possible and turn on a flashlight. Fix a
blue piece of cellophane to the end of the flashlight with a rubber
band and again look through the red cellophane. What happens? Look
through a piece of blue cellophane, then green. Try different combinations
of cellophane in front of the your eyes and around the flashlight.
Makes notes of your findings and try to explain them.
Fasten a piece of red cellophane in front of one flashlight, a blue
piece in front of another, and a green piece in front of a third.
Darken the room and shine the flashlights onto the card so that their
beams overlap. Try the flashlights in pairs and then all three together.
What do you notice about the areas where the beams overlap?
Taking it further:
Repeat the last part of the experiment involving the three flashlights.
But this time place your finger in various positions in the beams
from the flashlights just before they strike the screen. Look at the
different shadows of your finger that are cast. What colors are they?
How do you explain what you see?
Note: This experiment will work even better if you use three slide
projectors in place of the flashlights, and colored slides instead
Tiny Dot Pictures
Look through a magnifying glass at one of the color photos in a book. You
will see that the picture is made up from a pattern of tiny dots. Each dot
is one of the secondary colors – yellow, cyan, and magenta –
|Close-up of a color photo showing the dots
of which is composed
Also look through a magnifying glass at the image on a color television
screen. Again, the picture is composed of a pattern of tiny dots (or oblongs).
Each of he dots is one of the primary colors – red, green, or blue.
By rapidly changing the brightness of the dots, any colored moving sequence
can be reproduced on the screen.
Eye Sensitivity and Color
You will need:
- A flashlight
- Pieces of cellophane of a variety of colors including red, blue,
green, yeloow, and clear
- A rubber band
- A book with large print
- Several volunteers
What to do:
Are human eyes equally sensitive to all colors of light? For example,
can we see as much by red light as we can by blue light of the same
Fix the clear piece of cellophane to the flashlight with the rubber
band. Set up the flashlight so that it shines onto an opened page
of a book, a fixed distance away. Call in your first subject and ask
him or her to stand behind a line marked on the floor on the opposite
side of the room. The subject should wait about a minute or so that
his or her eyes become adjusted to the light level in the room. Then
ask the subject to begin slowly moving toward the book until he or
she can just read the words. Measure the distance between the book
and the subject at this point.
Replace the clear cellophane with a colored piece. Ask the subject
to return to the starting point, turn to a fresh page, and again allow
a minute to pass. Continue the experiment as before. Repeat it using
all of the other colored filters. Record your measurements for each
filter. Repeat the entire experiment with several other volunteers.
From your results what do you conclude about the sensitivity of the
human eye in different colors?
Taking it further:
Like all the experiments in this book, this one can be developed further
in many ways. For example, you could rearrange it to test how quickly
people's eyes recover when exposed to light of different colors. You
could do this by shining light of various colors into a subject's
eye for a fixed length of time. Then measure how long it is, in each
case, before the person can read a book placed some distance away.
Other questions you might investigate: Does eye sensitivity vary with
age? Does a person's reaction time depend on the brightness or color
of the surrounding light?
The method described for this experiment is intended only as a guide.
A number of improvements could be made. For example, an opticianís
chart showing letters of different standard sizes would probably work
better than a book. Several of these charts (which could be carefully
handmade) would be needed, since each subject is to be tested with
a number of different colored filters.
Another point to consider is whether the different filters let through
the same amount of light. This could be checked using a light meter.
The experiment would also benefit by some means of making sure that
the subject does not lean forward from his or her standing position
to see the chart more clearly. An accurate method of measuring the
distance from the subject's eyes to the chart is important since the
difference in measurements from one filter to the next may not be
This is an experiment that could be improved and developed into a
lengthy project suitable, say, for a science fair.
Special lighting effects play a big part in many of today's live rock concerts.
The most spectacular of these effects use LASER beams.
A laser is a device that produces a special kind of light. Most ordinary
light is a mixture of many different colors. But laser light contains just
a single wavelength. In ordinary light the waves are out of step with one
another and travelling in different directions. But in laser light the waves
are all exactly in step and moving along a single, narrow beam.
Laser light is used, among other tings, in high quality printers (laser
printers), optical fiber telephone links, and precision cutting and drilling
equipment. Lasers have also found their way into hospitals. They make it
possible for surgeons to carry out operations using a needle-thin beam of
light instead of a knife. Heat from the laser beam seals off any small cut
blood vessels quickly. That reduces the amount of blood lost and allows
the operation to be completed in less time.
Compact disk (CD) players use light in the form of tiny laser beam to scan
the mirrorlike surface of a compact disk. To make a compact disk, the sounds
from a studio microphone are first changed into numbers. These numbers are
then represented on the surface of a compact disk by a series of tiny pits
and blanks. In the player, the CD spins around over a laser beam. When the
beam hits a blank or flat part of the disk, it is reflected onto a device
called a photocell, which then gives out a small electrical current. But
when the beam strikes a pit on the disk, it is scattered and no light reaches
the photocell. The resulting electrical signals are amplified, or strengthened,
and then used to produce sounds from the loudspeakers. Since nothing, except
for a beam of light, makes contact with the CD it never wears out and the
resulting sound is free of crackles.
|In a compact disk player, light is reflected from
the flat part of a disk through a photo-detector
Light is a special type of wave called an ELECTROMAGNETIC WAVE. But the
spectrum of light, from red to violet, forms only a tiny part of the complete
electromagnetic spectrum. Other electromagnetic waves include radio waves,
X-rays, ultraviolet rays, and infrared rays. Ultraviolet rays from the sun,
for example, cause sunburn on our skin. We feel the sunís infrared rays,
on the other hand, as heat.
All of these invisible kind so f electromagnetic waves are identical to
ordinary light, except in their wavelength. They each have their own uses
– in science, medicine, and everyday life – and each allows
us to view the universe in a new and different way.