The Science Notebook
Gilbert Light Experiments - Part 4

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NOTE:  This book was published in 1920, and while many of the experiments and activities here may be safely done as written, a few of them may not be considered particularly safe today.  If you try anything here, please understand that you do so at your own risk.  See our Terms of Use.  

Pages 76-100

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when these waves enter the air they become more curved (see BC, Fig. 118) because the parts which enter the air first travel faster and get ahead of the parts still in water.



Now your eye estimates the distance of an object partly by the curvature of the waves which enter it from the object. The curvature of the waves which enter your eye from the coin is the same as though the coin were at a point A only three-fourths the depth, and this is the reason the coin appears to be at A.

If you look at the coin in a slanting direction, it appears to be nearer the surface still, because the light is bent more and more the greater the slant of the rays from the coin to the surface.




RELATION BETWEEN ANGLES OF REFRACTION AND INCIDENCE

If the light ray in Fig. 119 is passing from air to water, then the line


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RM is always exactly three-fourths the length of IM no matter how large or small i may be. If the light passes in the opposite direction, the same relation holds until the critical angle is reached. (See page 87 for definition of critical angle.)



If the ray is passing from air to glass, RM will always be two-thirds of IM, and this relation holds if the light passes from glass to air, until the critical angle is reached.

This gives you the relation between the angle of incidence and the angle of refraction in all cases.

FUN BY DAY OR NIGHT

Experiment No. 68.
Magic lead pencil.

Put a pencil in a glass of water in a slanting direction and sight along it (Fig. 120). Does it appear to be bent up? It does, because the light from it is bent as shown in Fig. 121.



Experiment No. 69.
Magic ruler.

Put a ruler vertically in a pitcher of water to a depth of 4


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inches (Fig. 122). Does the part under water appear to be only 3 inches long when viewed vertically? It does, because light travels in water only three-fourths as fast as it does in air.


Does it appear much shallower when viewed at a slant? It does, because light is bent more the greater the angle at which it leaves the water.

Experiment No. 70.
An elastic ruler.

Shove the ruler to the bottom of a pail of water and lift it out Does it appear to stretch?


Experiment No. 71.
Magic glass.

Stand a ruler at one end of the glass prism held on one edge. Fig. 123 (1). Does the bottom appear only two-thirds its real depth when viewed vertically? It does, because light travels only two-thirds as fast in glass as it does in air.

Does it appear even shallower when viewed at a slant? It does, because light is bent more the greater the angle at which it leaves the glass. Repeat this with the prism on end.

Repeat with the glasg plate on its edge. Fig. 123 (2).

Experiment No. 72.
Phantom coin.

Fill a long, deep pan with water. Put a coin on the bottom and view it vertically and then


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at greater and greater slants. Does the coin seem to rise? Why?



Experiment No. 73.
A disappearing coin.

Stand a coin on edge in a tin funnel full of water, ask a friend to stand so that he can just see the top over the edge of the funnel, and then let the water run out. Does he find that he can no longer see the coin from where he stands ? Why ?



Experiment No. 74.
A broken looking-glass.

Play this trick on your family. Take a piece of soap and mark a star with radiating lines near one edge of a looking-glass (Fig. 124). The family will think the glass is broken. A real break shows up because the light is refracted at the break and this gives a fair imitation.

Where is the Fish? The three boys 1, 2, and 3 in Fig. 135 are looking at the same fish and they see it at the three different positions 1, 2, and 3, because the light from the fish is bent more the greater the slant it has when it reaches the water surface. None of them see the fish where it is.


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How Deep is the Water? If you have gone swimming in very clear water you know that it always looks shallower than it is. If the water is at the same depth everywhere it will look to you shallower in the distance, for the reasons given above.

How Tall are You to a Fish? If you are 6 feet tall a fish (Fig. 186) sees you as 8 feet tall, because the curved waves from you are made less curved in water and, therefore, appear to come from a more distant point.

Experiment No. 75.
Breaking a pencil without touching it.

Look at a pencil in a slanting direction through a bottle of water with flat sides or through the edges of the glass plate.

Does it appear to be broken into three parts? Why?


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Experiment No. 76.
Shifting pin.



Put the glass plate flat on a piece of paper on the table. Stick the pins A, B on each side (Fig. 127) and sight from pin A to B through the glass. Does B appear to be shifted? Draw lines around the edges of the plate, aim a ruler at the two pins through the glass, and draw a line along the ruler; then draw a line from B to A and draw a line perpendicular to the edge of the plate at A. This shows that the light which passes from B to A in glass is bent away from the perpendicular when it enters air.

Experiment No. 77.
Shifting line.



Put the plate on a piece of paper (Fig. 128) and draw lines around the edge. Now draw a slanting line AB, sight along a ruler through the glass at this line, and draw the line CD along the ruler. Is the line parallel to AB but shifted? Draw perpendiculars at B and C and draw a line from B to C. The light from A passes into the glass at B and is bent toward
the perpendicular at B ; it passes from glass to air at C and is bent away from the perpendicular at C.

Experiment No. 78.
Things are not where they seem.

Look at a lighted candle through your glass prism (Fig. 129). Does the candle appear to be in a different place?

Does it also appear to be


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beautifully colored? You will experiment with colors soon.



Experiment No. 79.
Bending light around a corner.

Stand the prism on end on paper and draw a triangle around the end (Fig. 130). Now draw a line AB slanting toward one side. Sight along a ruler through the prism at this line and draw a line CD along the ruler. Now remove the prism, draw short perpendiculars at B and C, and join BC.

The light from A enters glass at B and is bent toward the perpendicular; it enters air again at C and is bent away from the second perpendicular. This is why the light is bent around a comer by your prism.



Experiment No. 80.
To see under water from a boat.

You cannot see things under water from air usually, because the light reflected from the surface blinds you to the light coming from beneath the surface. You can easily see through the sur-


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face, however, through a pipe of any kind, as shown in Fig. 131, because the sides of the pipe keep the reflected light
out of your eyes. Try this with a pipe 2 or 3 feet long.


 
Experiment No. 81.
To see the fish you are trying to catch.

If you can fish under a boathouse or under a wharf, you can see the fish deep down under water because the house or wharf prevents surface reflection. You can do this also as follows: Stretch a blanket or tarpaulin between two boats and put your head under it. The surface reflection is removed and you will be able to see to great depths.



Spotting Submarines. Submarines are easily spotted at great depths from a dirigible or airplane at a great height above the surface (Fig. 132) because at these great heights the light re-


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flected vertically upward is not so great as the vertical light received from objects beneath the surface.

Total Reflection. When light passes from water to air in a slanting direction (Fig. 133), part of it is reflected and the part which passes through is bent away from the perpendicular. As the slant becomes greater the bending is greater, and finally the light which passes into air is at right angles to the perpendicular. If the light in water is still more slanting when it reaches the surface, it does not pass into air at all, but is all reflected back into the water. This is called total reflection. The angle at which this takes place in water is any angle greater than 48.5, and in crown glass and hard flint glass, any angle greater than 41 and 37 respectively. These angles are called the critical angles for these substances.

Experiment No. 82.
A phantom pin.

Cut a slice of cork, attach a pin to the under side, float the cork on the surface of water in a full glass, stand the glass on the table, and look at the cork from the level of the table. Can you see a phantom pin above the cork (Fig. 134)? You see it by means of light reflected from the under side of the water surface.

Experiment No. 83.
A broken spoon.

Put a spoon in a glass half filled with water and look at the under side of the water surface through the side of the tumbler. Do you find a brilliant image of the part of the spoon in water? You see this by light reflected at the under surface.



Prism Glass. These prism glasses, Figs. 136 (1) and (2), are used to throw light to the rear of a store, or from the sidewalk into the basement. They are made of glass and have prisms on one side. The light which enters them is totally reflected from the inside surface


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of the prisms and is directed to the back of the building or basement.



Right-angled Prisms. These are made of glass and act as mirrors, in some opera glasses and other optical instruments. Light which enters one right-angled face, AC, Fig. 136, is totally reflected at the slanting face and passes out through the other right-angled face BC.

ATMOSPHERIC REFRACTION

Mirages. A ship at sea sometimes appears upside down (Fig. 137) because the air near the cold water is colder and denser than the air above and the light from the ship is refracted as it passes from each layer of cold air to the warmer layer above and is finally totally reflected. The light which enters the sailor's eye appears to come from the image above.



Mirages on the hot deserts are caused by light from the


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clouds which passes from the upper cold air through warmer and warmer lower layers. It is refracted and finally totally reflected and the clouds look like a lake of water on the ground.

Sunset and Sunrise. You see the sun before it is up and  after it has set because light from it is refracted by successive layers of air which are denser the nearer they are to the earth.



The direct ray SD, Fig. 138, could not be seen at A because the earth is in the way, but the light SB is seen because it is refracted to A.

COLOR



Spectrum. When a beam of sunlight passes through a glass prism as shown in Fig. 139, it is spread out into a colored band called the spectrum. This spectrum contains all the primary colors, of which those most easily rec-
ognized are in order: red, orange, yellow, green, blue, indigo, and violet. 


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White Light made up of all Colors. The experiment above shows that white light is made up of lights of all primary colors. This is proved again by passing the spectrum through a prism turned in the opposite direction (Fig. 140); the colors are recombined to produce white light



It can be proved also by turning the prism back and forth quickly (Fig. 141). The colors overlap at the center and produce white light.

Dispersion. You know that light is refracted or bent when


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it passes from air to water or glass or the reverse, because it travels more slowly in water and glass than it does in air. Now the waves of red light are longer than those of orange, the waves of orange are longer than those of yellow, and so on, the waves of each light beginning at the red end of the spectrum are longer than those next to it until we get to the very shortest, namely, the waves of violet light. It has been found by experiment that the shorter the waves, the more slowly they travel in water or glass and, therefore, the more they are refracted or bent when they pass from air to water or glass, or the reverse. When white light passes through a prism then, the shorter waves are bent


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or refracted more than the longer waves and as a result the white light is spread out into the spectrum. This spreading of light is called dispersion.


Spectrum by Reflection. Another beautiful method of producing a spectrum is illustrated in Fig. 142. A mirror is placed in a slanting position under water in a pan and a beam of sunlight is allowed to fall on the mirror. The sunlight, in going through the water to the mirror and back, really passes through a prism of water and it is spread out or dispersed into a beautiful spectrum.

If, after the spectrum is formed, the surface of the water is stirred, the colors of the spectrum are mixed and the reflected beam is white. This proves again that white light is made up of all the colors of the spectrum.



Interference. In a water wave the particles of water simply move up and down, but the wave moves forward. A wave length is a hill and a hollow. If, now, two waves of exactly the same length come together in such a way that one is one-half wave behind the other (Fig. 143), the hill of one coincides with the hollow of the other, the particles of water do not move at all, and one wave destroys the other. This is called interference.

The same thing occurs in light waves; two streams of waves may come together and destroy each other, that is, produce darkness.


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Colors by Interference. If a beam of sunlight is allowed to fall on a soap film held in a vertical position on the end of a lamp chimney (Fig. 144), it is found that the soap film when viewed by reflected light is crossed by horizontal colored bands. These colors are formed by interference as follows: The soap film has two surfaces with water between, and when it stands on edge the water runs toward the bottom and the film becomes a narrow prism. Now the light is reflected partly from the front film and partly from the back film, and where the films are 1-4, 3-4, 5-4 waves of red light apart, the red waves from the rear are 1-2, 1 1-3, 3 1-3 waves behind the red waves from the front when they enter your eye. These two sets of waves, then, interfere and destroy each other, and all that your eye sees is blue. Similarly a little above and below these points the blue waves destroy each other, and you see red light

FUN WITH SUNLIGHT



Experiment No.84.
The prism spectrum.

Allow a beam of sunlight to pass through


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the slit in your darkened room and fall on the prism supported between blocks as shown in Fig. 145. Cut a piece of cardboard of the exact size of one face of the prism and put it on the upper face. Do you find a beautiful spectrum on the wall or ceiling.  Do you find that the violet end is nearest the base of the prism and the red end nearest the angle, that is, is the violet end the most bent? Turn the prism over. Do you get a spectrum on the floor?



Get the spectrum on the wall or ceiling again and rock the prism quickly. Is the center of the spectrum white? This proves that white light is made up of all spectrum colors because they mix at the center.

Experiment No. 85.
Spectrum by reflection.

Place a mirror in a slanting position under water and arrange it so that the beam of sunlight falls on the mirror (Fig. 146). Do you find a beautiful spectrum on the wall above the slit? Stir the water. Do the colors mix and produce white light?

Experiment No. 86.
Colors by interference.

Make soap suds as yon would for blowing soap bubbles. Put the suds in a saucer. Dip the end of a lamp chimney in the suds and support the chimney on its side in sunlight (Fig. 144). Look at the film by reflected light. Do you find that the film at the top is crossed by beautiful horizontal colored bands? These colors are produced by interference. The colors in a soap bubble and in a fihn of oil on water are produced by interference.


GILBERT LIGHT EXPERIMENTS  95

WHY OBJECTS ARE COLORED

An object has a certain color because something in the object absorbs all other colors. For example, a blue dress is blue because the dye in the dress absorbs all the other colors of the spectrum. Also a red dress is red because the dye absorbs the colors in the blue end of the spectrum, and so on.

An object is white when all of the colors of the spectrum are partly absorbed and all are reflected.

An object is black when all the colors are completely absorbed and none reflected.

Experiment No. 87.
Changing colors.

Darken your room and allow sunlight to enter through a slit smaller than your colored-glass plates. Hold the red glass over the slit and hold colored objects in the. red light. Are red objects red, but all other colored objects dark or black? They are dark or black because the dye in them absorbs the red light. Repeat with the blue glass. Are the results similar?

Note. The blue glass lets through a little red, yellow, and green, as you will now show.

Experiment No. 88.
Changed spectrum.

Get the spectrum with the prism and then put the red glass against the prism. Does the red glass absorb all colors except red? Repeat with the blue glass. Does it absorb nearly all colors except blue, but does it let through a small amount of the other colors?

Experiment No. 89.
Changing colors in spectrum.

Get the spectrum with the prism and hold colored objects in the different colors. Do they change colors according to the part of the spectrum they are in? They are black in the part of the spectrum which they absorb completely.

FUN BY DAY OR NIGHT

Experiment No. 90.
A colored strip.

Cut a strip of white paper about 1-16 inch wide and 2 inches long and pin it to a


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black object. Put it in a good light and look at it through the prism (Fig. 147). Do you find a spectrum instead of the white paper?



Do you find also that the spectrum is reversed, that is, that the red is nearest the base of the prism and the violet nearest the angle? This is so because your eye sees an object in the direction the light enters it from the object The red is least bent but appears to be most bent, and the violet the reverse.

Experiment No. 91.
Combining spectra.



Cut a strip of white paper 1 inch wide and 2 inches long and look at it through the prism (Fig. 148). Do the edges appear colored, but is the center white? The center is white because the spectra formed by the edges overlap at the center and this combination of all the colors of the spectrum produces white light.

Experiment No. 92.
Colored candle flame.

Look at the flame of a candle through a prism. Is it beautifully colored, but does the center tend to be white and are the colors reversed as above? 


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COMPLEMENTARY COLORS

Complementary colors are those which, when combined, produce white light. If any colors are taken out of the spectrum, the remaining colors are complementary to those taken out, because together they produce white light

MIXING PAINTS

A paint which absorbs the colors in the blue end of the spectrum is red in color and a paint which absorbs the colors in the red end of the spectrum is blue in color. If, now, these paints are mixed, they do not produce white paint but black paint because together they absorb all the colors.

FUN WITH SUNLIGHT

Experiment No. 93.
Colored glasses.

Stand the red and blue glasses side by side on a piece of white paper in sunlight. The red absorbs the blue end of the spectrum and lets through red light The blue absorbs the red end of the spectrum and lets through blue light. Now place one behind the other. Do they absorb all the light and is the shadow black?

THE RAINBOW



The rainbow (Fig. 149) is formed by the internal reflection and dispersion of sunlight by falling drops of water. You see it when the sun is behind you and not over 48 above the horizon. The first or primary rainbow is formed by two refrac-


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tions at A and C and one internal reflection at B (2); it is violet below and red above and the angle at which the light enters your eye is about 4 to the direction of the sunlight. The secondary rainbow is formed by two refractions A and D and two internal reflections B and C (3); it is red below and violet above and the angle of the light is about 52.



Experiment No. 94.
An artificial rainbow.



Place a glass full of water (Fig. 160) on a table in sunlight and projecting beyond the edge. Do you get two or more beautiful rainbows on the floor? Stand the glass on a mirror. Do you get two beautiful rainbows on the ceiling? These bows, however, are not reversed. This


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experiment will show best in your darkened room.

FUN AT NIGHT


Experiment No.95.
A changing devil.

Cut a little devil out of cardboard and arrange as shown in Fig. 151. Hold the red glass in front of the candle at the right. Is the devil at the right red and is the devil at the left very dim but of the complementary color, green? Use blue glass. Is one devil blue and the other very dim but of the complementary color, orange?



Experiment No. 96.
A tricolored star.

Fold a piece of cardboard. Cut a four-pointed star in one half. Fold the points back, make a tracing on the other half, and cut out a star very carefully with the points exactly between those of the first. Arrange as shown in Fig. 158 and hold the red glass in front of one candle. Do you get an eight-pointed star with the points alternately red and green and with a white or pink eight-pointed star inside? Repeat with the blue glass. Are the star points blue and orange?


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Experiment No. 97.
A ghost party.

Mix a half teaspoonful of salt in three or four teaspoonfuls of alcohol in a saucer, stand the saucer on a cup on the table (to prevent burning the table),


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