Cover - 25
The things that are the most common are often the ones
that people know the least about. This is true of light. Few
boys have even taken the trouble to get the essential facts
about this subject. You can realize how important it is when you
are told that without the sun - our main source of light - there
would be no life at all. There would be no growth of plants,
that's sure. I know as a boy my curiosity was always prompting
me to ask questions. I wanted to know the facts and reasons for
everything. I believe most boys are the same in that respect.
This book has been written so that you can get
information first hand on a mighty interesting subject. It is in
plain language, which you can readily understand, and a study of
it will soon make you familiar with great scientists, who have
made laws of great importance. Their discoveries make possible
the use of a number of instruments that you know so well. The
telescope - the opera glass - the moving-picture
machine - are just a few which can be mentioned, and you know
how necessary they are in the world to-day.
In learning facts about light and some of the inventions
made by great men you will get a knowledge that very few boys
have. You will be able to talk very interestingly about light
and it will be easy for you to explain a great many questions
that come up in connection with it Best of all, you will have a
whole pile of fun and at the same time get a good understanding
of the fundamentals of the science of light.
GILBERT LIGHT EXPERIMENTS
Experiment No. 1.
To obtain more than one million miles of sunshine.
Go outside with your watch in your hand and stand in the
sunlight (Fig, 1) for just six seconds. In that short six
seconds you will have received more than 1,000,000 miles of
sunlight. Light travels at the enormous velocity of 186,000
miles per second, and therefore in six seconds you receive on
your body 186,000 X 6 = 1,116,000 miles of sunlight.
Experiment No. 2.
To receive over two
thousand million million light waves.
Hold your hand in a beam of sunlight (Fig. 2) for six
seconds and then withdraw it.
You will show soon that white light is a mixture of
lights of all colors. Now the red light waves in the sunlight
fall on your hand at the rate of 390 million million per second
and in six seconds 390 X 6 = 2340 million million fell on your
hand. Your hand received many more than this because waves of
all colors fell on it, and of violet light alone it
received twice the above number.
Repeat this experiment with a candle flame, or oil lamp,
or electric light. In each case your hand receives more than two
thousand million million light waves in the six seconds.
6 GILBERT BOY ENGINEERING
TO MAKE YOUR DARK ROOM
Boys, you are going to make many
experiments with a beam of sunlight let into a darkened room, so
prepare for them now thoroughly, as follows:
Select in your home a room with
only one window, facing the south or east or west. Now cover
this window so that no light can enter the room except through a
small slit The best way to do this is to make a solid wooden
shutter, with tar paper on one side, to cover the whole window
and make a slit in it three inches wide and two inches
high. If the window is too large for thin,
make a wooden shutter (Fig. 3) for the
lower part and cover the upper part with black cloth, black
paper, a quilt, a heavy blanket (Fig. 4), or anything that will
shut out light. Make the slit in the wooden part because
you will want to change the
GILBERT LIGHT EXPERIMENTS 7
shape of the slit. To do this
you will make slits of the right kind in black tar paper or
cardboard and then tack these over the slit in the wooden
shutter. Make the shutter so that you can take it down and put
it up quickly, because you will want to experiment many times
and your mother will, probably, not want to leave the window
TO MAKE A DARK BOX
Experiment No. 3
If you cannot make a dark room, you can make a dark box
(Fig. 5), as follows: Take a packing box 1 1/2' X l 1/2'' X
2' or larger, bore a hole 2 1/2 inches in diameter in the center
of one end, cover the open top with a piece of dark light-proof
cloth, 4' X 3', tacked to the ends and one side. Plait this
along the side to leave room for your head and shoulders. Make
the box light-proof, turn it on its side with sunlight entering
the slit, and you are ready to make your experiments.
It improves the box to paint it black on the inside.
When you need a slit, cut it in cardboard and tack the cardboard
over the 2 1/2-inch hole. This hole will just take your large
ring lens holder when you experiment with lenses.
A dark room is more fun than a dark box, and the
directions in this book assume that you have made one. You will
find, however, that you can make most of the dark-room
experiments in the dark box. It is an excellent idea to have
To show that you cannot see a beam of light unless it
falls on some object or directly on your eyes.
Darken the room on a day when the sun is shining in the
window. Leave the room for five minutes to let the dust settle
and then return. Do you find that you cannot see the beam
8 GILBERT BOY ENGINEERING
the slit and a screen or the wall or floor?
Make a dust near the beam by
shaking your coat or a carpet. Do you now appear to see the beam
(Fig, 6)? You do, because the dust particles reflect light
to your eyes.
On a clear night you cannot see
the beam from the headlight of a locomotive; but when there is
mist, rain, or snow, you appear to see it because particles of
these reflect light to your eyes. In either case you can see any
object the beam falls on because it re-
GILBERT LIGHT EXPERIMENTS 9
fleets light to your eyes. You can also see the light if
it falls directly on your eyes.
Experiment No. 4.
To show that light travels in a straight line.
Experiment No. 5.
Make a dust near the beam. Does the light travel in a
straight line from the slit to the wall or floor or paper
To get a
picture of all out-of-doors.
Punch a nail hole in a piece of black paper or
cardboard, tack the paper or cardboard over the slit in your
darkened room, and hold a sheet of tissue paper about one foot
from the hole. Do you find on the paper a picture (Figs. 7 and
8) of the whole view out-of-doors opposite the hole? Is the
picture inverted and in natural colors? Can you see men, horses,
and automobiles moving?
This is a fascinating experiment, and it shows best when
the sun is shining on the landscape and not on the window.
The picture is inverted because light travels in
straight lines. The sunlight which falls on any part of a cloud,
for example, is reflected in all directions in straight lines,
and a very small part of this light passes through the hole to
the bottom of the tissue paper. Also the sunlight which falls on
any object on the ground is reflected in all directions in
straight lines, and a very small part of this passes through the
hole to the top of the tissue paper, and so on. That is, the
picture is inverted because light travels in a straight line
from each object through the hole to the paper.
The picture is in natural colors because each object
reflects light of its own color and absorbs the remainder. That
is, the blue sky, green grass, and red bricks reflect blue,
green, and red light respectively, and so on.
10 GILBERT BOY ENGINEERING
Move the paper farther from the hole. Is the picture
larger but dimmer? It is larger because the rays from
different parts of the view cross at the hole and diverge
afterward. It is dimmer because only a certain amount of light
passes through the hole, and it covers a larger area the farther
the paper is from the hole. Punch a second nail hole two inches
from the first Do you now get two pictures? Do they blur where
they overlap? Punch many holes. Do you get as many pictures, but
do they blur more and more? Open the slit to its full size. Do
you find that there is no picture at all, but just white light?
There is no picture because light from all parts of the view
falls on all parts of the picture and the combination of all
colors produces white light
Make a hole the size of a lead pencil in a new piece of
black paper and tack the paper over the slit. Do you get a
brighter picture, but is it more indistinct than with the nail
hole? Remove the tissue paper. Is there a picture on the
opposite wall ? This will show only if the sun is shining
brightly on the view, and if your room is completely dark except
for the light which passes through the hole.
Experiment No. 6.
To get a picture of
Allow a beam of sunlight to pass through a nail hole
into your darkened room, catch it on a piece of paper and move
the paper back and forth. Is the image round (Fig. 9) and is it
larger the farther the paper is from the hole?
The image is round because the sun is round. It
increases in size because the light rays from the sun travel in
straight lines and
GILBERT LIGHT EXPERIMENTS 11
cross at the hole. The upper side of the sun sends out
light in all directions in straight lines; a very small part of
this passes through the hole (H, Fig. 11) and makes an image of
itself at the bottom of the picture; similarly light from
the lower side of the sun makes an an image of itself at the top
of the picture, and so on. Reflect the light to the farthest
part of the room by means of a mirror. Is the image larger the
farther it is from the hole? Punch two holes. Do you get two
images (Fig. 10)? Punch many holes. Do you get many images, but
do they overlap?
Open the slit entirely. Do you
get only a bright spot in the shape of the slit? This spot is
made up of many, many round images, and you will notice that the
edges and comers are somewhat blurred and not sharp.
Take a new piece of black paper,
make a triangular hole one-quarter inch on a side, tack it over
the slit and get an image of the sun at one inch from the hole,
then, at greater and greater distances. Is the image at first
triangular and does it become more and more blurred at the sides
and corners until finally it is round? The image is made up of
many small, round images of the sun, and when these are large
12 GILBERT BOY ENGINEERING
compared to the size of the hole they overlap and
produce a round image.
Repeat with a square hole
one-quarter inch on each side. Are the results similar?
Experiment No. 7.
You have probably noticed that sunlight produces round
images of the sun when it passes through any small opening; for
example, in a shutter or blind, between the leaves of trees, and
so on. The explanation is that given above.
To make a pinhole camera.
Make a nail hole in the middle of the bottom of a
cardboard box, cover the open top with a piece of tissue paper
(Fig. 12), hold the hole toward a brightly lighted landscape,
cover your head and the tissue paper with a black cloth or
blanket to shut out all the light (Fig. 13), and look at the
tissue paper. Do you see a beautiful image of the landscape
inverted and in natural colors?
This is a beautiful experiment and it is explained as
You can actually make
pictures through a pinhole, as follows: Remove the lens
from a camera, cover the opening with heavy tin foil and pierce
the foil with a pin. Now to take the pic-
GILBERT LIGHT EXPERIMENTS 13
ture, cover the pinhole, arrange the plate or film in
position, uncover the pinhole for a short time, cover it, and
develop your negative as usual.
SOMETHING ABOUT LIGHT
Now, boys, before we go any
further let us get some clear ideas about light.
Light is that
which produces on the eyes the sensation of sight.
Medium. A medium
is anything through which light travels; for example, air,
water, glass, and the ether.
ether is supposed to be a very thin and elastic medium which
fills all space, not only the space between the planets,
also the space between the smallest particles (molecules) of
solids, liquids, and gases.
How Light is Produced.
Light is produced by the vibration of very hot particles of
For many reasons, scientists believe that the smallest
particles of all substances are vibrating, that is, moving back
and forth in all directions, all the time, and that the hotter
they are the faster they vibrate. Now in the flame of a candle,
oil lamp, or gas jet there are particles of unburned carbon
which are very hot and are, therefore, vibrating rapidly. These
vibrating particles set the ether in the flame in vibration, and
these vibrations spread out
14 GILBERT BOY ENGINEERING
in all directions in the form of spherical waves in the
ether. These ether waves are light waves or heat waves.
Similarly the light of an incandescent electric arc
light or of the sun is produced by rapidly vibrating hot
particles of matter.
Note. Heat waves
are longer than light waves and do not produce the sensation of
sight, but they are similar to light waves in all other
Waves and Rays.
If the dot in the center of Fig, 14 is a rapidly vibrating
particle, the circles about it will give the position
of its light waves
after equal intervals of time, but the light waves are spherical
instead of circular. The straight arrows drawn from the center
represent light rays.
They give the path along which the light is traveling in all
directions from the center. The light waves are real and produce
the sensation of sight; the rays are not real, they are
imaginary, straight lines which give the direction of the light
and they are always at right angles to the waves.
Waves and Rays. The waves (from the dot are larger the
farther they are from the center, and when they are one hundred
yards or a mile from the center they are very large indeed. If
your eye receives light from any such distant point the small
part of the waves which enter it are nearly parallel straight
GILBERT LIGHT EXPERIMENTS 15
and since the rays are always at right angles to the
waves they are also nearly parallel. This is particularly true
if the distant point is the sun, at a distance of ninety million
miles. Parallel waves and
rays then are those from
a distant source.
Pencil. A beam (Fig. 15) is a group of parallel waves
and rays. A pencil (Fig. 16) is a group of waves and rays
which converge at a point or diverge from it. The eyes
(Fig. 17) are receiving diverging pencils of light from the
candle which is sending out light in all directions.
Non-luminous Bodies. Luminous bodies are those which
give out light, such as the sun, electric light, gas jet, oil
lamp, candle, and match. Non-luminous bodies are those which do
not give out light, and which can be seen only by means of light
from luminous bodies.
Translucent, and Opaque Bodies. Bodies which you can
see through are called transparent;
such as air, water, and glass. Bodies which let light through,
but which you cannot see through, are called translucent; as paper,
ground glass, and cotton cloth. Bodies which do not transmit
light are called opaque;
for example, wood, brick, and metal.
No substance is entirely opaque; for example, even
metals let through some light when they are in very thin sheets.
Lines. Light always travels in straight lines from its
source until it falls on some object or until it
16 GILBERT BOY ENGINEERING
passes into another medium. It then changes direction
but again travels in straight lines from the object or in the
Experiment No. 8.
Carbon particles in a flame.
Experiment No. 9.
Hold a saucer in a candle flame (Fig. 18.) Is
it blackened? The blackening is caused by
the unburned carbon particles.
FUN AT NIGHT
Look along the surface of water in a pan and dip a
pencil in and out. Do you observe circular waves (Fig. 19)?
Throw a stone into a still pond or lake. Do you see circular
waves? These illustrate light waves, but the light waves are
Experiment No. 10.
How you see things.
Boys, when you have attended movie shows where they have
animated cartoons you have perhaps seen a dotted line move from
the eye of the hero (or villain) to the object he is looking at.
You might think from this that you see an object by means of
light which goes from your eye to it. This is not the case,
however, as you will now prove.
Take an unlighted candle into an
absolutely dark room and
GILBERT LIGHT EXPERIMENTS 17
look around. Can you see anything? You cannot because
all the objects in the room are non-luminous, including yourself
and your eyes. This proves that you do not see things by means
of light which goes from your eye to the thing you are looking
at. Now light the candle. The flame is a luminous body and you
see it by means of light which goes from it to your eye.
Can you now see the non-luminous
bodies? You can because light travels from the candle flame to
these objects and from them to your eye (Fig, 20).
You have proved here that you see any object by means of
light which travels from it to your eye and not the reverse.
Experiment No. 11.
To show that light, when not reflected or refracted,
travels in a straight line from the object to your eye.
Note. The thing you are looking at directly or
indirectly is called the object.
Close one eye, look at the flame of a candle, and then move a
book slowly between the flame and your eye (Fig. 21). Do you
find that you cannot see the flame when the book has crossed the
straight line between the flame and your eye? This proves
18 GILBERT BOY ENGINEERING
light from the flame travels in a straight line to your
Close one eye and look at any
part of some other object, then again move the book across the
straight line from the part to your eye. Do you find again that
you cannot again see the part when the book has
crossed the straight line between the part and your eye?
Cut three pieces of cardboard about 5" X 3", punch a
small hole in each at the same height, stand them upright on the
table, place a candle flame in front of an end hole, and look at
the flame through the three holes (Fig. 22). Shift the
cardboards one at a time. Do you find that you can see the flame
only when the holes are in a straight line?
Note. You will
show later that light is bent out of the straight line when
it is reflected from a mirror and when it is refracted
in air to water, air to glass, and so on.
You have shown here that light which is not
reflected or refracted travels in a straight line from
the object to your eye,
Experiment No. 12.
Picture of a candle
Punch a nail hole in card C and arrange as shown in
Fig. 23. Do you sec an inverted
image of the flame and is it larger the farther
GILBERT LIGHT EXPERIMENTS 19
20 GILBERT BOY ENGINEERING
Experiment No. 13.
D is from C? Make the hole larger. Is the image brighter
but more blurred? Can you explain these facts as in Experiments
5 and 6 and as illustrated in Figs. 11 and 23?
You can telegraph to your friends at night by means of
flash-light signals (Fig. 24) and the Morse code. Use a short
flash for a dot and a long flash for a dash. For the Morse code
see page 19.
Experiment No. 14.
Telegraphing with a candle or lamp.
INTENSITY OF LIGHT
Arrange the apparatus as shown in Fig. 25 and have a
friend make a similar arrangement in a window facing you. You
can then telegraph by means of the Morse code. Uncover the light
for a short time to produce a dot and for a longer time for a
If you hold a book 1 foot from a lighted candle, it
receives a certain amount of light; if you hold it 8 feet from
the candle, it receives only one-fourth as much light ; if you
hold it 3 feet from the candle, it receives only one-ninth as
much light, and so on. That is, the intensity of the light on
any object varies inversely as the square of the distance
between the object and the source of light.
In Fig. 26 the light
GILBERT LIGHT EXPERIMENTS 21
which would cover 1 square at 1 foot would cover 4 and 9
equal squares at 2 and 3 feet and would therefore be one-fourth
and one-ninth as intense.
Experiment No. 15.
To prove the law of intensity.
Screen A, Fig.
27, has a hole just 1 inch square and screen B a square 3 inches on each
side, divided into 9 square inches. Place A 1 foot from the
candle and B 2 feet.
Does the light which passes through 1 square inch in A cover 4 square inches on
B? Is it, therefore,
only one-fourth as Intense on B
as it is on A? Place B 3 feet from the candle.
Does the light now cover 9 square inches? Is it, therefore, only
one-ninth as intense? This proves that the intensity of light
varies inversely as the square of the distance.
Experiment No. 16.
A greased spot.
Rub a small piece of butter on the center of a piece of
paper and melt it.
Look at the spot by reflected Iight (Fig. 28). Is the
spot dark? Look at it by transmitted light
22 GILBERT BOY ENGINEERING
(Fig.29). Is it bright? It is darker than the paper in
the first case and brighter in the second, because more light
goes through the greased spot than through the paper.
Experiment No. 17.
Put four candles 2 feet from the greased-spot screen and
one candle 1 foot on the other side (Fig. 30). Trim the wicks
until the flames are of equal size.
Is the greased spot as bright as the paper? It is,
because the light which goes through from one side is exactly
equal to that which goes through from the other, that is, the
one candle throws as much light on the screen as do the four
candles. This is explained by the law of intensity
mentioned above. Each of the four candles is 2 feet from the
screen and therefore throws just one-fourth as much light on the
screen as one candle does at 1 foot.
Experiment No. 18.
Candle power of a lamp.
The candle power of a lamp is the
GILBERT LIGHT EXPERIMENTS 23
number of times greater or less its light is than
that given by a standard candle.
Put the greased spot screen
just 1 foot from a candle (Fig. 31) and move the lamp until the
greased spot is as bright as the paper. Measure the distance
from the lamp to the screen. If this is 2 feet, the lamp
is 4-candle power; if 3 feet, 9-candle power ; 4 feet,
16-candle power; and so on. This follows from the law of
intensity of light.
The instruments used to measure the candle power of
lamps are called photometers, and the one you have here
illustrated is named after its inventor, Bunsen's photometer
(Fig. 32), A is the
greased-spot screen, B
the standard candle, and C
the light tested.
Experiment No. 19.
Four against one.
Place a candle 1 foot from a screen and four equal
candles just 2 feet from the screen, and place an object in
front of the screen as in Fig. 33. Are the shadows of equal
If the candle flames are all of the same size, the
shadows are equally dark because each light illuminates the
shadow produced by the other and because the candle at 1 foot
sends as much light to the screen as the four candles
do at 2 feet from the screen. This is explained by
the law of intensity.
Experiment No. 20.
24 GILBERT BOY ENGINEERING
candle 1 foot from a screen (see Fig. 34, illustrating
shadow photometer), stand a pencil in front of the screen, and
move the lamp back and forth until the two shadows are equally
intense. Measure the distance from the screen to the lamp. If
this is 2, 3, 4, or 4 1/2 feet, the candle power of the lamp is
4, 9, 16, or 20 1/4, and so on.
Each light illuminates the
shadow cast by the other and therefore when the shadows are
equal the lights are equal, and the candle power is calculated
as above from the law of intensity of light. This illustrates
the shadow photometer.
Experiment No. 21.
Move a pencil from the screen toward the candle (Fig.
35). Does the shadow increase in size? It does, because light
goes in all directions in straight lines from the flame, and the
pencil intercepts more light the nearer it is to the flame.
GILBERT LIGHT EXPERIMENTS 25
Boys, yon can have the greatest kind of fun by giving
shadow shows to your friends, and the preparation you need
is very slight. Hang a sheet over a folding doorway as shown in
Now opposite the door put a
strong lamp on a stool, chair, or table, according to the show
(Fig. 37). The audience is in darkness on the other side of the
Show 1. The Dentist.
Dentist seated, bell rings, boy comes in with
bandaged head, dentist seats him and examines tooth, boy howls,
dentist takes very, very large pliers, and pulls out a very
large cardboard tooth (Fig. 38). The tooth, of course, was stuck
in boy's coat collar
"The Science Notebook" Copyright 2008-2018 - Norman