- University of Virginia
- Physics Department
Total Internal Reflection
A Physical Science Activity
2003 Virginia SOLs
- refresh their understanding of reflection and refraction;
- learn how total internal reflections occurs;
- observe total internal reflection;
- discuss the important applications of total internal reflection.
Laser light in a Water Tank
- aquarium (fish tank, 5 gallon size is ideal)
- laser pointer (He-Ne, red/orange in color)
- powdered milk (coffee creamer)
- white poster board, at least 12" on a side to stretch across the top of
the fish tank
This is a very nice set up to demonstrate both reflection and refraction.
Look here for
laser pointer safety
- Fill the aquarium (clear glass) almost full of water; leave about 5 cm
- Mix in less than a teaspoon of powdered coffee creamer and stir. You may
have to do this in stages to obtain the optimum amount, but if you put in
too much, you will have to start over.
- Shine the laser pointer from the outside of one end of the aquarium from
near the bottom pointing so the light will come out through the surface of
the water (see photo below). You should be able to see the red path of the
laser as it passes through the water. You will not be able to easily see laser
light pass through clear water, but the powder adds larger masses that will
scatter the laser light so you can see it. You should be able to see the light
reflect off the top water surface. If you shine the laser pointer towards
the side of the aquarium, you may be able to see light reflected off the glass
- You will not be able to see light coming out of the aquarium into air,
because air molecules are too small to effectively scatter visible light.
However, you can place the white poster board over the top and sides of the
aquarium and see the laser light shine on the white board.
the laser light is quickly attenuated inside the aquarium, you have put too
much coffee creamer in the water. Start over. If you can't see the light or
if you can barely see it, then you need to put more creamer in and stir. It
may take some practice to get this just right. Measure what you put in so
you can reproduce the amount later. This photo was produced using a more powerful
laser than a laser pointer, so the light could easily be seen in the 5-gallon
fish tank for the photo. We used less than one teaspoon of coffee creamer.
The laser can be seen in the lower right of the photo (red spot). The path
of the laser light cannot be seen until after it enters the water. The light
moves to the left and is totally internally reflected at the water surface.
Note the straight lines that the laser light makes inside the aquarium. The
fish tank can be seen as outlined by the cloudy area in the photo.
- Shine the laser light so that it reflects off the water surface (from inside
the water) at different angles. Use the poster board to see if light is being
transmitted through the water/air interface (that is, is light being refracted?).
You should easily be able to see the laser light reflect off the glass surface
where it enters the aquarium (use poster board).
- You should be able to find an angle (called the critical angle) where,
as the angle with the water surface gets smaller, all the light will be reflected
from the water surface back into the aquarium and none will be refracted out
into the air. This angle should be about 40 degrees from the water surface
for water and air. Smaller angles should have the light totally reflected
back into the water. You will have to move the poster around on top of the
aquarium to see the red spot, because the direction of the refracted moves
dramatically with entrance angle.
The many website references given in this activity serve as excellent background
There are some fundamental rules to remember:
- Light travels in a straight line.
- Light intensity falls off as the square of the distance from the light
In light reflection, the angle of incidence always equals
the angle of reflection.
- In light refraction, light bends at the interface between two media. If
we measure the angle from the perpendicular to the interface, the angle in
the media with the higher index of refraction is the smallest.
- In an interface between two media, light may be both reflected and refracted.
It is important for students to already understand reflection and refraction
before doing this activity. Even though they should have already studied this
in previous grades, they may have not retained much of their earlier understanding.
If you feel they are unprepared, then it would be best to do a reflection and/or
refraction activity before attempting the present activity. If you think a brief
refresher would be satisfactory, then you can do that before beginning this
activity. You may use your own resources for a refresher, which may consist
of demonstrations or computer simulations, for example. (Remember that a website
in place today may be gone tomorrow!) Some excellent websites have suitable
material such as:
Light travels in a straight line. We can see this by observing shadows from
the sun or from bright lights. We can draw a straight line from the light source
to the edge of the obstruction and note that the edge of the shadow is along
this straight line. We can think of light traveling in rays as if little bullets
were moving along straight lines. This is the origin of the particle theory
of light. We can explain both reflection and refraction using the particle theory
of light. (Do activity with flashlight and holes in cards).
Light traveling in a straight line:
Total internal reflection occurs when light is passing through a medium (call
it 1) like glass or plastic with an index of refraction n1. Outside this material
is a medium like air (call it 2) with an index of refraction n2. For certain
angles of incidence for light striking the interface and going from medium 1
to medium 2, light will not pass through the interface, but will be totally
reflected inside medium 2. This occurs primarily when the light strikes a glancing
blow on the interface. For a thin material of medium 2, like a fiber, these
angles will always be small, and the light will stay inside the fiber. This
process, called total internal reflection, leads to one of the most important
technological applications of physical science, because most of our telecommunications
now passes through fiber optic cables. These fibers can be made very small,
and thousands of fibers can be attached together in a cable.
Total Internal Reflection:
Aswers to Data Sheet:
Answers to Questions (Part I)
- Yes, the holes are lined up in a straight line.
- The light does not pass through when one of the cards is moved out of line.
- Yes, light travels in a straight line.
- You should not see light come all the way through the straw after you bend
Answers to Questions (Part II)
- The path of the light inside the test tube is a straight line.
- Water molecules are too small to effectively scatter light. The powder
provides larger mass that increases the probability of light scattering so
that we can see the path of the light.
- If you do not have too much powder in the milk, you may be able to see
the laser light. Eventually the laser light will be scattered out of the test
tube due to the powder.
- The laser light can travel for many kilometers through clear plastic. We
could not see the path of the light traveling through the plastic, because
it would not be scattered. It would only be imperfections in the material
that would scatter the light.
- This should be able to be done. The laser light will be at an greater angle
than when it was totally reflected.
Answers to Questions (Part III)
- The stream of water has light inside it. The light breaks up with the water
as it hits the water collector.
- Light reflects off the inside of the can and comes out the hole. The light
reflects off the inside of the water stream. The angles are so large that
it is improbable that the light refracts out of the water. This is the effect
of total internal reflection.
- This is what happens with optical fibers where light passes down long thin
To print out the Student Copy only, click
I) Does Light Travel in a Straight Line?
- 4 - 3" x 5" index cards
- means of supporting the index cards
- flexible soda straw
- Punch a small hole in each of the index cards at precisely the same position.
- Stick each card into a white support to hold the card upright, or fold
the card at one end and stand it up leaning on that fold.
- Place the cards about 15 cm apart with holes in a straight line.
- Shine the flashlight so that the light travels through the hole in each
- Move one of the cards a little and observe if the light passes through.
- Now try looking through the flexible straw while it's straight at a light
source at eye level.
- Bend the straw and look at the same source.
- Look down the cards when the light is passing through all the cards. Are
the holes lined up in a straight line?
- What happened when you moved one of the cards a little?
- Can we conclude that light travels in a straight line?
- Did you see light come all the way through the straw after you bent it?
II) Light in a Test Tube
- long test tube (the longer the better)
- laser pointer
- powdered milk or a few drops of liquid milk
- water in container so water can be poured into test tube
- Take a clean test tube and put a small amount of powdered milk in it (only
a pinch). Fill the test tube with water and shake to mix up the powder and
- Make sure the outside of the test tube is clean and dry.
- You will be using the laser pointer to shine into the test tube. MAKE SURE
THAT YOU NEVER SHINE THE LASER INTO YOUR OWN OR ANYONE ELSE'S EYES! Click
here for laser
pointer safety precautions.
- Hold the laser pointer at the bottom of the test tube and shine it up through
the test tube. The powder in the water has large enough particles that the
laser will scatter light into your eyes so that you can see the path of the
laser light through the test tube. If you have time try shining light through
a test tube of clear water. What do you think you will see?
- Now rotate the laser so its light reflects off the side of the test tube.
What do you see? Have one of your group members put a finger near the test
tube where the light hits the test tube. Do you see laser light on the finger?
If you do, try to shine the laser at a smaller glancing angle onto the side
of the test tube. Eventually all (or almost all) the light will be reflected.
THE INTENSITY OF LASER POINTERS IS SMALL ENOUGH THAT IT WILL DO NO DAMAGE
ON YOUR SKIN, BUT DO NOT SHINE THEM IN ANYONE'S EYES.
- Try to shine the light on the sides so that it reflects several times before
coming out of the top of the test tube. You have demonstrated total internal
- If the light disappears inside the test tube, you have most likely added
too much powdered milk. Pour this out and start over. If you don't see the
laser light inside the test tube, you need to add a little more powdered milk.
- Can you conclude that light travels in a straight line inside the test
- What is the effect of the powdered milk? Why is it needed?
- What do you think would happen if the test tube were ten times longer?
Would you still be able to see the zigzag path of the light? Why or why not?
- Why would a thin, clear plastic fiber be better than the test tube with
water and powdered milk? Would we be able to see the path of the laser light
in the plastic?
- Were you able to shine the laser inside the test tube at an angle such that
a lot of the light came outside the test tube? If so, draw a diagram showing
the angle of the laser light path through the test tube.
III) Water Stream of Light
- soda can
- pan to collect water
- can opener
- Use a can opener to take off the lid of the soda can that has the pull-tab.
Clean out the inside of the can.
- Use the nail to make a hole in the side of the soda can near the bottom
of the can. It is important that this hole have very smoot edges so that the
stream of water is not broken up.
- Put a piece of tape over the nail hole. Fill the can almost full of water.
- Use the nail to make another hole in the can toward the top as an air hole
to allow the water to flow more freely.
- Place a flashlight on top of the open can with the light pointing towards
the bottom. It might be helpful to tape the flashlight in place using black
electrical tape or duct tape.
- Place the can over a dish or bucket to catch the water.
- Turn out the room lights and turn on the flashlight
- Hold the can above the collector and take off the tape letting the water
pour out slowly into the dish. If you have difficulty seeing the stream of
light lower the can so the stream is short and gradually lengthen it. Or put
your hand in the path of the water and move it along the stream to watch the
light spot on your hand.
- Observe carefully the water coming out of the soda can.
1. What do you observe happens in the stream of water?
2. Why does the light appear in the water stream?
3. Can you think of any application of this technique?
To print out the Data Sheet of questions only, click
A very nice and useful extension to do is to allow students to handle and use
fiber optics. You can obtain a sampler kit of fiber optics from Edmund Industrial
Optics (Optical Grade Fiber Sampler, Catalog #K53-883 (year 2000) for $31; 101
East Gloucester Pike, Barrington, NJ 08007-1380, 800-363-1992). The kit contains
a range of sizes and lengths of fibers that have outer covering (called jacketed)
and nonjacketed. The students can shine flashlights into one end and see light
out the other end even when the fibers are turned at angles. They can place
them on top of overhead projectors and see light shining through. They can also
shine a laser pointer into the end, and it should come out the other end. They
must be careful when using laser pointers to not let either the direct light
from the laser pointer or the light coming out of the fiber optic to shine in
anyone's eyes. It is fun to place various color filters over the flashlight
and see that the colors will pass through the optical fibers. Color plastic
can be obtained from art supply stores.
Students with Special Needs
Some students may need help in handling the fish tank, putting water and creamer
in, and in handling the laser pointer. Safety is very important in using the
laser pointer. Teachers may want to do the laser pointer activity as a demonstration.
Click here for further
information on laboratories with students with special needs.
The students should each write down what they observe in these activities on
a Data Sheet and discuss their answers to the questions. It is okay for the
students to discuss the questions and answers, but the students should write
out the answers in their own words.