The Science Notebook
Light - Part 2

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 On this page...
LENSES
Changing Image Size By Refraction   
A Practical Example of Magnifying by Refraction - A Magnifying Glass
A Water Drop Magnifier   
Making a Water Drop Microscope
Focusing Sunlight With A Magnifying Glass
Focusing Sunlight With A Concave Mirror

COLOR
 The Prism
Water Glass Prism
Mirror Prism
CD Prism
Using the CD Prism to Study Other Spectra
Garden Hose Rainbow
Mixing Reflecting Colors (Hobby Motor)
Mixing Pigment Colors


LENSES

While mirrors reflect light, lenses refract (bend) light.  A lens may be made of any clear substance that has the ability to refract light - even water.


Changing Image Size By Refraction

   

Materials Needed: Thin piece of clear plastic from the window of a mailing envelope, a "blister pack," or a report cover; page from a magazine or newspaper; eyedropper or soda straw; water.

Procedure: Cut a small square of the plastic and place it directly on top of the printed page.  Using the dropper or straw, place a small drop of water on top of the plastic.  Observe the shape of the drop.

Move the drop over some small print on the page.  What do you see?

What Happened: The water drop beaded up on the plastic creating a curved surface on the top of the drop.  The print under the drop was magnified.

Water will bend or refract light. (See experiments on the first Light page.)  When water takes on a curved shape, as it does here, the water refracts the light just like a lens.  The shape of the water drop is thicker in the middle than along the edges.  The water drop forms a convex lens. 

Going Further: Try experimenting with different size drops.  Do larger or smaller drops magnify better?  If you can get a microscope slide or clear flat plastic from packaging material, place a drop of water on it, and  hold it just above a few grains of salt with the water drop directly over them.


A Practical Example of Magnifying by Refraction - A Magnifying Glass


Materials Needed: A magnifying glass; some small objects to observe.

Procedure: Examine your magnifying glass carefully.  Use it to observe the objects you have chosen.  What happens as you move the magnifying glass farther away from the objects?

What Happened: The common magnifying glass is thicker in the middle than around the edges on both sides.  This type of lens is called a  “double convex lens”.  This shape allows both sides to refract light and magnify. The water drop in the last experiment was convex on one the top only.  The bottom was flat.  

When you brought the lens close to the object, the object appeared to be bigger when viewed through the lens.  As you moved the lens away from the object, it appeared to grow bigger still until the image you saw in the lens became "fuzzy" or out of focus.


A Water Drop Magnifier

   

A water drop is a very crude lens, but it does magnify.  In this experiment, we will improve the water drop lens.

Materials Needed: A short piece of thin wire, such as magnet wire; sharpened pencil; water; magnifying glass; magazine or newspaper page.

Procedure: Twist one end of the wire around the pencil point to make a small loop.  Dip this loop into some water to form a drop of water inside the loop.  Use the magnifying glass to examine the shape of the water drop on the wire.

Look at the print on the page through the water in the loop.  Does it act like a lens?

Shake some of the water off of the loop, and examine it again with the magnifying glass.  What is the shape of the water drop now?  Will it still magnify?

What Happened: The water lens you have just made was an improvement on the water drop.  The top and bottom of the lens were both curved.  How much they were curved depended on how much water was in the drop. If the drop had too much water, it was rounded out, or convex, on both sides, but probably sagged downward due to gravity.  The drop could have been too thick to see anything clearly.  By experimenting, you can get just enough water on the loop to make a fairly good double convex lens.

If you shook enough water off so that only a small film was left, the water lens was thicker on the edges than in the middle.  When a lens is thicker on the sides and thinner in the middle, it is a concave lens.

Lenses may be either convex, concave, double convex, double concave, or convex and concave.  It all depends on how the lens is to be used.

Going Further: Experiment with different size loops and different amounts of water to see which combination makes the best lens.  Keep a record of your tests.


Making a Water Drop Microscope


A water drop may be used to make a very simple, but surprisingly powerful, microscope.

Materials Needed: Small piece of thin flat clear plastic from a "blister pack," report cover (or you can substitute a microscope slide cover slip); clear glass jar and lid, such as a mayonnaise or peanut butter jar; nail; hammer; dropper or straw; water; salt.

Procedure: Wash the jar and lid thoroughly with warm soapy water.  Rinse and let it dry.  

If the lid has a cardboard liner on the inside, carefully remove it.  Then, using the nail and hammer, punch a hole in the center of the jar lid.  Cut a piece of flat plastic about an inch square, and tape it around the edge over the hole in the lid.  Leave the center of the plastic clear.   Place a small drop of water on the plastic directly over the center of the hole.

Turn the jar over.  Sprinkle some salt on the bottom of the jar.  Carefully move the jar lid over the salt crystals.  Place your eye near the water drop and move the jar lid up or down over the salt to bring the crystals into view.  What do you see?

What Happened: This is a practical use for the water drop lens. It will magnify small objects many times.  However, you may have to experiment with the size of the hole, and of the water drop, to get the best possible image. 
 
Going Further: Try to improve your microscope by wrapping the sides of the jar with dark construction paper and putting a small light source inside the jar. You can use a small flashlight, or you can make your own light source using a miniature white Christmas tree light.  (See the Magnetism and Electricity pages)


Focusing Sunlight With A Magnifying Glass


One property of a lens is its ability to focus or concentrate light energy.

CAUTION!  This experiment poses a small fire hazard.  Think safety, and do this with adult supervision only! 

Materials Needed: Magnifying glass; paper; a sunny day; pail of water.

Procedure: Find a sunny spot on a driveway or other area that is free from leaves, dry grass, or anything else that can burn.  Keep the pail of water nearby.  Hold the flat part of the lens toward the sun, and hold the paper behind the lens as shown.  You should see a bright spot on the paper.  Move the lens back and forth until the spot is as small as possible.  At this point, you may be able to see that the bright spot is actually an image of the sun.

Hold the paper in place until it just begins to smoke.  What happens?  Dip the paper into the pail to put out any fire.

What Happened: All of the energy from the sun striking the entire area of the lens was refracted by the lens and focused into a small spot.  The distance from the lens to the paper where the spot is focused is called the “focal length.” 

Because all of the energy going into the lens was concentrated into such a small spot, there was enough energy present to raise the temperature of the paper so that it was hot enough to burn. 


Focusing Sunlight With A Concave Mirror


A concave mirror can also focus or concentrate light energy.  In this experiment, we will make a crude concave mirror from an old umbrella and aluminum foil and use it to focus sunlight.

CAUTION!  This experiment poses a small fire and burn hazard.  Think safety, and do this with adult supervision only! 

Materials Needed: Old umbrella; aluminum foil; a sunny day.

Procedure:   Open the umbrella.  If possible, have an adult to remove the handle of the umbrella.  However, if you can’t safely remove the handle, just leave it.

Line the inside of the umbrella with aluminum foil.  Use the shiny side and keep the foil as smooth as possible.

Look inside the umbrella.  If you have worked carefully, you can probably see a very crude reflection.  You have just made a crude concave mirror.

Take the umbrella outside and point the shiny inside toward the sun.  Do not look directly inside the umbrella.  Hold a piece of paper over the umbrella and see if you can find a bright spot.  Move the paper back and forth over the center of the umbrella until you find the brightest spot.  Hold the paper there and see if the paper will begin to smoke.  (If you feel your hand starting to get warm, move it away!)

If the paper is not hot enough to smoke, place your hand over this spot.  How hot is it?

What Happened: There was a point above the umbrella where the energy was most concentrated.  How hot this point got depends on the shape of the umbrella and just how smooth the foil was.  Some of these umbrella mirrors can get very hot!

As light fell on the surface of the foil, it was reflected.  Because the foil surface was curved, the light rays were all reflected at a slightly different angle.  In a well constructed concave mirror, all of the light will be reflected to a single point, the focal point.  Since all of the energy reflected by the mirror is focused in this one spot, it can become very hot.  In this crude congave mirror, the light was focused to a relatively small area, but because the surface was not smooth and the reflector was not perfect, the light was not focused to a single point.

A concave reflector is sometimes called a “parabolic reflector” because if its curved shape.  It reflects energy from the entire surface and focuses it on a small spot.  Satellite dish antennas are parabolic antennas that focus radio waves just as parabolic mirror focuses light rays.  They concentrate the small amount of radio energy striking the whole dish into one spot to get the strongest possible signal.

Going Further: If you have a good reflector, you may want to use a candy or oven thermometer to see how hot it is at the focal point.  Can you design a device to hold the thermometer safely in place?


COLOR

When we see colors we are seeing one or more colors of light.  A beam of white light is actually made up of different colors of light. We can see these colors by using the properties of mirrors, glass, water, or other substances that can reflect or refract light. The most common device used to do this is the “prism.”   A prism is a triangular shaped piece of plastic or glass that will refract a beam of light which passes through it.  Your school science lab probably has at least one that you can borrow.  Also, many museum and school stores sell very inexpensive ones. 


The Prism


Materials Needed: A prism; a window through which the sun is shining; a sunny day; white paper; a friend.  

Procedure: Locate a window where the sun is shining.  If there are curtains or blinds on the window, close them so as to get as small a sunbeam as is possible.  Also, you should close as many other blinds or curtains on other windows as possible, in order to darken the room.  A dark room is not essential, but the darker the room, except for the single sunbeam, the better you will be able to see the results.
 
Hold one side of the prism in the sunlight and look around the room.  You should see a light spot somewhere in the room, and you should also see a rainbow pattern in another part of the room.  You may have to look carefully for the pattern, but it will be there.  If you don’t see it at first, move the prism around a bit.  If you have sunlight, you will be able to find the pattern.

Once you have found the rainbow pattern, have your friend to place the white paper in it’s path.  Move the prism and the paper so as to make the brightest and widest pattern possible.  Study the colors that you see.  How many can you identify?

What Happened: The glass or plastic in the prism refracts the light going into it until it reaches the edge of another side.  The other side then reflects some or all of the light out through the third side.  (See diagram) Different colors of light are refracted at different angles, so the white light separate into the colors of the rainbow pattern.  This rainbow pattern is called the “visible spectrum.”

If you look carefully, you can see many different shades of color, but scientists often name seven colors in the spectrum.  You should be able to see them in the following order: red, orange, yellow, green, blue, indigo (a bluish purple) and violet (purple).  Sometimes, you will not be able to see each of these colors clearly, depending on a number of things.  The brightness of the light and the quality of the prism are just two.  Also, the colors will gradually change from one to the other.  There are not sharp bands of color.

However, these colors will always be seen in this same order, and you can remember the order just by remembering the name “Roy G. Biv”.  Each letter in the name is the first letter of the color of the spectrum in its proper order!

Going Further: Try using the prism to view other types of light such as from a flashlight, an electric light bulb, and a fluorescent light.  Are you able to see a spectrum?  If so, how many different colors do you see?


Water Glass Prism


Materials Needed: A clear smooth water glass or jar; water; a sunny window; white paper.

Procedure: This is not always the easiest experiment to do, and you may not initially see a spectrum when you do it. The best spectrum will be produced by a glass container that has smooth sides and a flat bottom with little or no curve from the side to the bottom, such as a plain drinking glass, but if you don't have one, try what you have.  You should see a spectrum with most any clear glass container.

Fill the container with water about halfway to the top and set it on the edge of a window sill with about a third of the bottom hanging over the edge of the sill,  Place it so that the sun is shining on it.  When the water is no longer moving, look on the floor below and behind the container.  You should see a circle of light on the floor and on the edge of that light, a spectrum.  This spectrum may be made clearer by placing a white sheet of paper on the floor.  What you will see and where you see it will depend on a number of things including how much water is in the container, how smooth the glass is, and the angle of the sun when it hits the glass.  Depending on these factors, the spectrum from the water glass may not be as bright or colorful as the spectrum from the prism, but you should be able to see that it is there.  If you don't see a spectrum at first, try changing some of the things shown in "Going Further" below.  However, be sure to vary only one thing at a time, so you'll know what you changed to make it work.  This will work.

What Happened: The light was refracted as it passed through the rounded surface of the glass and water.  This separated the sun light into it’s different colors, just as the prism did.

Going Further: Try different sizes and shapes of clear glass containers.  Also try viewing the spectrum with the sunbeam striking the glass at different angles, and with different depths of water.


Mirror Prism


Materials Needed: A small mirror; a small bowl; modeling clay; a sunny window.

Procedure: Use a small ball of modeling clay to make support the mirror in the bowl at about a 45 º angle as shown.  Fill the bowl with water and place it in the windowsill with the mirror facing the sunlight.  Look on the ceiling for the visible spectrum.  If you don’t see it right away, adjust  the angle of the mirror until you get a good spectrum.

What Happened: The water and the mirror created a triangular shaped surface that refracted and then reflected the light, much like the prism.  Again, the spectrum may not be all that colorful, but it should be visible.


CD Prism


Materials Needed: An old compact disc; a sunny location.

Procedure: Hold the shiny side of the CD so that it faces the sun.  Move it to one side, and you should immediately see one or more sets of visible spectra (plural of spectrum).

What Happened: There are many thousands of tracks in a CD that are used to store information.  Each of these individual tracks acts like a miniature prism.  When combined together they produce a very brilliant and colorful spectrum.


Using the CD Prism to Study Other Spectra

It is very difficult to see the spectra of light other than sunlight just using a prism.  You need a dark room and a way to focus the light into the prism.  However, the CD gives a much brighter spectrum, and so can be used to study these spectra.

CAUTION!  Always be careful to follow all safety precautions when using fire, and use with adult supervision only!  Keep your candle in an aluminum pie pan, and keep the flame at least three feet away from anything that can burn, unless otherwise instructed. 

Materials Needed: An old compact disc; flashlight; candle with safety holder; dark room.

Procedure:    In a dark room, shine the flashlight at an angle on the CD.  You should see its spectrum very clearly in the CD.  Also look for the reflection of the flashlight on the wall. Carefully move the CD until you see the spectrum of the flashlight on the wall.  How does it compare to the spectrum of sunlight?

Repeat this with the lighted candle.  Be very careful when working with the candle!

What Happened: You should have seen the same colors as in the spectrum for sunlight, and in the same order.  (They always are.)  However, you probably also noticed that the reds and oranges were more visible than the blue and green end of the spectrum, particularly with the candle.  This is because the light given off by the flashlight and the candle has more color from the red end than from the violet end.

Going Further: Try this with other light sources.  If possible, enclose the light source inside of a box with a hole in it that lets out only a small portion of the light.


Garden Hose Rainbow

When you see a rainbow in the sky, it has the same colors as the visible spectrum.  In this experiment, you will make an artificial rainbow.

Materials Needed: Garden hose with sprayer nozzle; a sunny day.

Procedure: Set the sprayer to spray as fine a mist as possible.  With your back to the sun, spray the water in front of you.  You should see a rainbow in the mist.

What Happened: You have already seen how a container of water can act as a prism.  Each drop of water in a rainstorm, or from the garden hose is in the shape of a ball or sphere.  Each sphere of water acts as a prism to refract light.  When you see a rainbow, you are actually seeing the spectra produced by many thousands of raindrop prisms. 

You may also recall that the glass of water produced a prism, but that the spectrum was in the shape of a semicircle.  The rainbow has a semicircular shape because the raindrops that produce it are round as well. 


 Mixing Reflecting Colors (Hobby Motor)


We see objects because light is reflected from those objects to our eyes.  The color of an object depends on the color or colors of light it reflects.  An apple appears to be red because when light strikes it, the red portion of the light is reflected.  The other colors are absorbed as heat energy.  This experiment will show that the different colors of light combine to produce white light.

Materials Needed: Small hobby motor with battery and battery holder; modeling clay; new pencil eraser; pliers; straight pin; strong glue; compass; white cardboard; markers or crayons.

Procedure: Using your compass, draw a 7 cm (3 in) diameter circle on the cardboard and cut it out.  Use your protractor to divide the circle into 45 segments of 8º each.  Color the segments as follows:

Color
******
red
orange
yellow
green
blue
purple
# of Segments to Color
***************************
5
4
7
7
7
15

ALTERNATE METHOD

Use your protractor to divide the circle into six segments as follows, and color the segment as indicated.

Color
******
red
orange
yellow
green
blue
purple
Degrees in Segment
************************
40
32
56
56
56
120

Use the pliers to pull a new eraser from a pencil.  Using the straight pin, punch a hole from the top all the way through the center of the eraser. Try to make this hole as straight as possible.  Glue the bottom end of the eraser to the back center of the cardboard circle and let it dry thoroughly.


Prepare a hobby motor and battery holder.  Secure the bottom end of this motor to a small board with a lump of modeling clay to hold it in place.  Push the end of the eraser over the motor shaft using the hole you made earlier.  It should fit tightly!  Check to make sure it is secure.

Hook up the battery to start the motor and observe the colors on the circle.

What Happened: The colors on the disc blended to white, or nearly white.  As the wheel turned rapidly, the light from all of the colors were reflected back to your eyes and appeared to blend together.  Since the reflected colors are all part of white light, it should not be surprising that the disc should appear white.
 
Going Further: Once you have made your own wheels, you can try a couple wheels that have already been done for you by clicking on the links below.

Wheel # 1       Wheel # 2

How do these compare to the one you did?

Also, try combinations of colors an see what they combine to produce.  For example, make a disc that is half blue and half yellow.  What color does the spinning disc produce?


Mixing Pigment Colors

Materials Needed: Crayons or markers; paper.

Procedure: There are three primary colors that may be used to produce many others when their pigments are mixed together.  Pigments are the materials used to color inks, dyes or paints.  The primary colors that you will be using are red, blue and yellow.

On a piece of paper, combine two of the primary colors by making a mark with one of them and marking over that mark with the other.  Notice what color is produced.  Fill in your results on the chart below. 


RED BLUE YELLOW
RED X

BLUE
X
YELLOW

X

What Happened:
You were able to produce other colors by mixing two primary ones. You have probably done this  in an art class or when coloring Easter eggs with food coloring.  The precise shade of color is determined by how much of each pigment is used. In any case, the color you see is still determined by the color of light that is reflected to your eyes.

Going Further: Try mixing all three primary colors.  What do you get? 

Also, if you have some food coloring, mix a drop or two of each in a small amount of water and see how many different colors you can produce.  Most boxes of food coloring have a mixing chart that will assist you in dong this.

In case you missed it, there are more light experiments on the Light - Part 1 page.  If you've already been there, check out our other experiments on the Experiments page.

"The Science Notebook"  Copyright 2008-2017 - Norman Young