The Science Notebook
Light - Part 1

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LIGHT
 
LIGHT RAYS
How Light Travels
Direction of Light Rays and a Nearby Light Source
Direction of Light Rays and a Distant Light Source
Camera Obscura
 
REFLECTION & REFRACTION
Reflection of Light
Making a Periscope
Reflection of Light from Other Objects
Refraction of Light - The Bent Pencil
Refraction of Light - The Appearing Coin
Transmission of Light
 
MAGNIFICATION
 Changing Image Size by Reflection
A Practical Example of a Concave Mirror
A Practical Example of a Convex Mirror

LIGHT

A beam of light travels at about 186,000 miles per second.  That is equal to over seven times the distance around the earth in just one second!   But what exactly is light?  We know it when we see it, and we also know that we could not see without it, but that doesn’t tell us what it is.

In these experiments we will learn about the nature of light, and hopefully better understand just exactly what it is.


LIGHT RAYS


How Light Travels


Materials Needed: Three small index cards; modeling clay; flashlight; several books; hole puncher or sharp pencil; plastic coffee stirrer or thin straw.

Procedure: Place the three cards together and punch a hole in the middle of each card at exactly the same spot using the pencil point or the hole punch. Use three small balls of modeling clay to make the three cards stand up as shown.

Turn on the flashlight, and place it in front of one of the cards.  The bulb should be even with the hole in the card.  If it isn’t use one or more books to raise the flashlight.  Once it is in place, you cans secure it with modeling clay if needed. 

Now place the second card about 15 cm (6 in) in front of the first card so that you can see the light through both holes.  Do the same thing with the third card.

When you can see the light through all three holes, run the stirrer or straw through the holes.

What Happened: When the cards were lined up so that you could see the light, the holes were in a straight line, and the stirrer or broom straw fit easily through all three holes. When light moves away from an object, it moves in a straight line unless something acts on it to bend it.

Light rays travel in straight lines, away from the light source.  These next two experiments show how distance from a light source affects the appearance of those rays.


Direction of Light Rays and a Nearby Light Source

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: Comb; candle with safety holder; sheet of cardboard.
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Procedure: Light the candle.  Hold the sheet of cardboard horizontally no closer than 6 inches from the flame.  Hold the teeth of the comb on top of the cardboard as shown.  Notice the shadows cast by the teeth of the comb on the cardboard.

Now move the cardboard and comb a few feet from the flame.  Notice the position of the shadows now.

What Happened: The shadows of the teeth appeared to spread out in a fan shape behind the comb when the comb was near the candle.  This is because light rays move out in all directions from the candle flame.  As these rays come in contact with the teeth of the comb, shadows are cast in the direction of the light rays they block, so the shadows give you a pretty good picture of the direction of the light rays.

When you moved the cardboard and comb away from the flame, you still saw some spreading of the shadows, but it was not as great.  As the rays move farther from the light source, they actually appear to be closer to parallel.  If you could move the comb far enough away, the shadows would appear to be perfectly parallel, although they don’t ever become exactly so.
 
Going Further: Try this experiment using a light bulb instead of the candle. Are the results similar?


Direction of Light Rays and a Distant Light Source

 

Materials Needed: Comb; piece of cardboard; sunny day.

Procedure: Go outside on a sunny day.  Hold the comb so that it faces the sun as shown.  Hold the cardboard at right angles to the comb and observe the shadows cast by the teeth.

What Happened: The shadows of the teeth appeared to be parallel.  The sun is about 93,000,000 million miles away, so you are very far away from the light source.  As a result, the light rays are almost parallel to one another.  In fact, the rays coming from the sun are so close to being parallel, you can assume they are parallel for most purposes.


Camera Obscura


Materials Needed: Small cardboard box, such as a cereal or shoe box; waxed paper or thin tissue paper; tape; pin; aluminum foil; jacket.

Procedure: NOTE: These directions are for a cereal box.  You should adjust for the type of box  you are using.

Cut the top of the box off and discard.  Cover this end of the box with a sheet of thin tissue or waxed paper, and tape it down.  This paper should be kept as flat and smooth as possible.

Cut about a 2 cm (1 in) square piece from the bottom of the box.  Cut a 4 cm (2 in) square piece of foil and tape it over the hole.  With a straight pin, poke a small hole in the center of the foil.

Take the box and a jacket to a window on a bright, sunny day.  Place the jacket over your head.  Point the pinhole to the outside, and place the paper side under the jacket where you can see it.  What do you see?

What Happened: The pinhole allowed only a small amount of light from each point on the object you were viewing to actually reach the tissue paper at the back of the box. This formed an image on the tissue that you could see.

The device you have just made is called a “camera obscura” and it works something like a real camera.  In fact, if you could place a piece of film where the tissue paper is located, you would have a very simple film camera.  A real camera also has a glass or plastic lens instead of a pinhole.  The lens can gather more light and focus it so that it will make a clearer and brighter image than that made by the pinhole.  This is also the same principle behind a digital camera as well, except that the image forms on a light sensitive surface instead of a piece of film.

Going Further: Try changing the size of the pinhole.  What difference does it make?  If you have some black construction paper or spray paint, cover or paint the inside of the box.  Does this improve the image you see?

If you are really ambitious, look up “pinhole camera” in the library or on the Internet.  A pinhole camera works much like a camera obscura, and is fairly simple to make.  It could also be the centerpiece for a great science project on photography.


REFLECTION & REFRACTION

When light rays strike many surfaces, they bounce or ricochet off, much like a billiard ball bounces off the edge of a pool table.  When light rays are bounced off of a surface, they are said to be “reflected”.  These experiments will show how light is reflected off several different surfaces.


Reflection of Light



Materials Needed: Mirror; coffee stirrer or small straw; paper; protractor; ruler; cardboard; modeling clay.

Procedure:  Place a sheet of paper on a tabletop. Position the mirror at the back edge of the paper as shown. The mirror you use should be able to stand up straight.  If it cannot, use a lump of clay to make a support for it.  Using the ruler and the protractor, draw a line on the paper that is perpendicular to the middle of the mirror at the bottom.  Use a small piece of modeling clay, washable marker, or tape to mark the bottom of the mirror where the perpendicular line ends.

Place the stirrer or straw into a small lump of modeling clay so that it will stand up straight, and place it a few centimeters (inches) to the left of the perpendicular line. Next, use the ruler to draw a line from the base of the straw to the perpendicular line.  Position yourself directly behind the straw and this line, so that you see the image of the straw in the mirror directly behind the image of the line you drew from the straw to the mirror.  The line and the image should be perfectly straight.

Move to the other side of the perpendicular line.  Close one eye and look in the mirror from this side.   Move until you see the straw directly over the perpendicular line and the mark on the mirror from this side.  Place the ruler at the edge of the perpendicular line so that it forms a line pointing back to your eye.  Make sure that the image is still over the line as you do.  Now draw a line on the paper along the edge of the ruler. 

Use the protractor to measure the two angles formed by the two lines you drew on each side of the perpendicular line.

What Happened: The first line you drew was the path that a single ray of light from the straw took to a particular spot on the mirror, in this case, the center.  When this line was connected to the perpendicular line, it formed an angle called the “angle of incidence”. The second line that you drew was the path of the light beam from that same spot on the mirror to your eye.  When this line was connected to the perpendicular line, it formed a second angle called the “angle of reflection”.

You were able to see that light ray in the mirror from the opposite side of the perpendicular because the mirror reflected the light ray, or changed its direction.  If you worked carefully, you saw that the angle of incidence was exactly equal to the angle of refraction. If you move the straw, both the angle of incidence and the angle of reflection will change, but the two angles will still be equal to each other.

Many surfaces reflect some or all of the light rays that strike them and change their direction.  However, a plane (or flat) mirror is unique because it reflects all light rays at the same angle, and that angle is just the opposite of the angle at which light strikes the mirror.  As a result, you are able to see an image in the mirror.  Still water will also reflect light in much the same way and will allow you to see a reflection, as will a polished flat metal surface.

If your mirror was thick, you may have noticed that the mark on the mirror was actually a little in front of its image.  If so, did you need to center the straw with the mark, or with the image of the mark to get two equal angles?  Do you know why?

Going Further: Can you design another experiment to measure the angel of incidence and the angle of refraction?


Making a Periscope


A periscope is a device used on submarines that allows the person using it to see above the surface of the water from below.  In this experiment, we will apply what we have learned so far about light to make and use a periscope.

Materials Needed: Two mirrors; cardboard or paper; scissors; tape; modeling clay.

Procedure: Try to find two small mirrors such as makeup or compact mirrors that are exactly the same size.  Make a square cardboard tube about 1 meter (3 ft) long.  It should be as wide as the width of the mirror, but a little shorter than the length of the mirror.  You will eventually tape this tube together, but don’t tape it just yet.

Cut a square hole near the top of one side of the tube about the width of the mirror.  On the opposite side, cut the same size hole near the bottom.  Using tape and modeling clay, secure one of the mirrors at a 45º angle as shown, shiny side down and facing the hole.  Then, mount the bottom mirror, shiny side up and facing the bottom hole.  Tape the tube together.  Use a single piece of tape until you are sure the mirrors are mounted correctly.

Hold the periscope over a fence. You should be able to see over the fence by looking into the bottom mirror.  If necessary, adjust the mirrors for the best result and tape the periscope securely.
  
What Happened: When the mirrors are at a 45º angle to the tube, light coming into the top of the periscope is reflected 90º.  This causes it to go down to the bottom mirror, which is also mounted at a 45º angle, but opposite of the first.  This reflects the light another 90º into your eyes, and you see the image reflected from the top mirror.

This works the same way as a periscope on a submarine, except that submarine periscopes use prisms rather than mirrors to, and most also use lenses to magnify and sharpen the image.  (More about prisms a little later!) 


Reflection of Light from Other Objects

Most materials reflect light to some extent, but some materials reflect light better than others.

Materials Needed: Flashlight; dark room with a white or light colored wall; mirror; aluminum foil; tape.

Procedure: Stand in the dark room with a flashlight.  Stand to one side of the mirror and shine the light on the mirror.  Observe the reflection of the light.

Tape a piece of aluminum foil on a convenient spot with the shiny side up.  Stand to one side of the foil and shine the light on the foil as you did with the mirror.  Again, observe the reflection.

Remove the foil, crumple it up and smooth it out again.  Put it back up in the same spot and repeat the last step.  What does the reflection look like now?

Finally, shine the light on the wall at an angle.  Do you see any reflection of this light.  (You may have to look carefully.)

What Happened: The mirror reflected the light almost perfectly.  The smooth foil did not reflect the light quite as well.  The reason is that the foil is smooth and polished, but not quite as smooth and flat as the mirror.  Thus, the light rays were reflected, but they were not all reflected in exactly the same direction.  Then, when you crumpled the foil, you made the foil much less smooth, and the light rays were reflected in many different directions.  You may have been able to see a “fuzzy spot” of light, but it was not nearly as clear as the mirror or the smooth foil.  Finally, the white wall reflected light, but it scattered the light rays in many different directions.  If you looked carefully, you probably could see a fuzzy concentration of light, but it was even fuzzier than the crumpled foil.  The more light rays are scattered, the less well the surface will reflect light.

We have seen that rays of light may be reflected or bounced off of a surface. These next experiments show us how light rays may be bent.  The process of bending light rays is called “refraction”.


Refraction of Light - The Bent Pencil


Materials Needed: Clear drinking glass; pencil; water.

Procedure:   Place the pencil in the glass and allow the top portion of the pencil to rest on the edge of the glass.  Fill the glass about 3/4 full of water. Look at the pencil through the side of the glass. What do you see?  Move the pencil upright.  Now what do you see?

What Happened:  The pencil appears to be sharply bent at the surface of the water.  As light rays pass from the air to the water, they are bent or refracted.  Since the image you see comes from light rays reflected off the pencil from above and below the surface, and since the water bends the rays below the surface, the pencil appears to be bent.  Only when the pencil was stood straight up did you see little or no bending.

Going Further: Look at any distant object through an empty glass.  It will almost certainly look distorted.  Glass will also bend or refract light.  How much the light is refracted depends on the thickness of the glass, how smooth the surface is, and the shape of the glass.  The image may look distorted because the rays of light are not all bent in the same direction.  In smooth flat glass, such as a window pane, there is very little distortion, but even here, if you look carefully, you may see some distortion.


Refraction of Light - The Appearing Coin

Materials Needed: Coffee cup with straight sides; coin; water.

Procedure:   Place the coin in the bottom of the cup on the side opposite from you.  Look into the cup from above.  Move your head back until the coin just disappears.  Now, without moving your head, slowly fill the cup with water (or have someone fill it for you).  What do you see?

What Happened: The coin again came into view as you filled cup.   As light rays passed from the air to the water, they were bent or refracted.  This allowed you to see the coin from an angle that you could not see it when there was no water to bend the light.  In the same way, when you walk along the edge of a pond, the refraction of light by water prevents you from seeing objects where they really are.


Transmission of Light

Not all objects allow light to pass through them.  We can group objects into three different categories by how light passes through them.

Materials Needed: Piece of clear glass or plastic from a picture frame (or clear plastic packing material); piece of white paper; piece of cardboard; flashlight.

Procedure: Place the flashlight behind the glass or plastic and shine it through the plastic.  Do the same thing with the paper and the cardboard.  Notice what you see in each case.
  
What Happened: When you shined the flashlight through the clear glass or plastic, you were able to see not only the light, but also the flashlight, and anything on the other side. In other words, you could see through the glass or plastic.  This is because the glass or plastic allowed all, or nearly all, of the light to pass through without being refracted.  When an object allows light to pass through without interference, it is said to be “transparent."

When you shined the light through the paper, you could see the light from the other side, but you could not see through the paper.  That is because some of the light was allowed to pass through the paper, but the light that was able to pass through was refracted, or bent, in many different directions.  The more the light is scattered, the less clear the image is.  An object that allows light to pass through it, but refracts it in many directions so that you cannot see through it, is said to be “translucent.”   Many glass shower doors, the glass surrounding most light bulbs, and “frosted” glass are examples of translucent objects.  Some light can pass through, but it is scattered, and you can’t see through these objects.

Finally, light could not pass through the cardboard at all.  An object that will not allow any light to pass through is said to be “opaque”.

Going Further: Try to identify at least three objects around your home that are transparent, translucent, and opaque.


MAGNIFICATION

So far, you have seen that light rays may be reflected or refracted.  You have already seen at least one useful object that takes advantage of reflection, the mirror.  We can also use refraction and reflection to make images larger or smaller.  This has many practical uses.  These next few experiments will demonstrate how reflection and refraction change an image size.


Changing Image Size by Reflection (Spoon)

Materials Needed: A shiny metal spoon; straight pin.

Procedure:   Hold the spoon a couple of feet away from you.  Look at your reflection on the outside of the spoon.  Turn the spoon over and look at your reflection on the inside of the spoon.  Is there any difference?

Hold the point of the straight pin inside the spoon.  What does the reflection look like?

What Happened: The bowl of the spoon forms a mirror on both the inside and the outside. The outside of the spoon bowl bulges outward in the middle.  It forms a convex mirror.  The image you saw of yourself was very small.  The convex mirror will reduce the size of an image.

The inside of the spoon bowl is curved inward and forms a concave mirror.  You could see yourself in this spoon, but your image was upside down or inverted.  The image was also reduced in size.  A concave mirror will invert distant images and will make them appear much smaller.

However, when you placed the straight pin inside the bowl of the spoon, its image was actually magnified.   A concave mirror may be used to magnify nearby objects.


A Practical Example of a Concave Mirror (Makeup Mirror)


Materials Needed: A makeup or shaving mirror that magnifies; a flat mirror.

Procedure: You will need to find a makeup or shaving mirror that magnifies the image of your face when you look into it.  Many makeup mirrors magnify on one side and have a plane (flat) mirror on the other, so if you can find a friend who has one, ask that person to let you examine it.

Feel the surface of the magnifying mirror.  Can you feel a curve?  Look at the mirror from the side.  Can you see that it curves inward slightly in the middle?

Compare the size of the image of your face when you look in this mirror to the size of your image in the plane mirror.

Hold the mirror up so that you can look at the image of a distant object behind you in this mirror.  What do you see.

What Happened: When you examined the surface of the magnifying mirror, you should have been able to observe a very slight inward curve toward the middle.  Since a magnifying mirror is curved inward, it is a concave mirror, just like the inside of the spoon. However, because it is only slightly curved, it will magnify clearly to a greater distance than the inside of the spoon.

The image of a distant object appear upside down in this mirror, as in the concave side of the spoon, or, if the curve is very slight, it may just appear fuzzy. Which does your mirror do?

Going Further: Look into the mirror while slowly backing away.  Can you find the point at which your image becomes inverted (turned upside down)?


A Practical Example of a Convex Mirror (Passenger Side Car Mirror)


Materials Needed: A car with right and left rear view mirrors; a friend.

Procedure: Examine both side mirrors carefully.  Does either mirror have a curved surface.  If so, how does the mirror curve?  Do you see any writing on either mirror? If so, what does it say?

Sit in the driver’s seat and adjust the mirrors so that you can see out of both.  Have your friend to stand far enough back so that you can see him or her out of both mirrors at the same time. If the person can’t move that far back, have them stay the same distance back, but to move sideways so they can be seen, first in the driver’s side mirror, and then in the passenger side mirror. Do you see any difference in the two images?

What Happened: Unless the car you are looking at is a very old car, the mirror on the driver’s side will be flat, and the mirror on the passenger side will be curved outward slightly.  This is a convex mirror.  It will usually have the words,”Objects In This Mirror Are Closer Than They Appear”, or something similar printed on it.

Your friend appeared smaller in the concave mirror on the passenger side than the flat mirror on the driver’s side.  The purpose of this mirror is to reduce the size of the image so that the driver can actually have a wider field of view on the passenger side.  This allows the driver to see more of what is behind the car than could be seen in a plane (flat) mirror.  There is a “blind spot” where you cannot see beside the car through the mirror on that side, but the convex mirror allows you to see more than a plane mirror would, and so makes the “blind spot” smaller.  However, a car coming from behind will have a smaller image in this mirror, and so will appear to be farther away than it actually is.  This could be dangerous, so car manufacturers print the warning on this mirror.  By the way, we call a flat mirror a “plane mirror” because a flat surface is sometimes called a “plane” surface.  


There are more light experiments on the Light - Part 2 page, or you can find links to lots more stuff on the Experiments page.


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