Name_____________________________________ Period_________ Date_________
CONCEPTUAL PHYSICS
Activity
29.12 Reflection and Refraction: Total Internal Reflection
TRAPPING THE LIGHT FANTASTIC
Thanx to
Dean Baird
Purpose
In this activity, you will investigate the behavior of light as it passes from one transparent
material to another.
Required Equipment and Supplies
light beam viewing tank (or equivalent)
opaque white tank insert
stirring rod
access to water
access to scattering agent (Mop-N-Glo™, Pine-Sol™, powdered milk, or equivalent)
laser pointer
LASER LIGHT WARNING: TO AVOID INJURY, DO NOT EXPOSE EYES TO DIRECT
LASER LIGHT. DO NOT AIM THE LASER AT PEOPLE OR ANIMALS.
Discussion
When light passes from one transparent material to another, it undergoes refraction. If the light
crosses the boundary between the materials at an angle, the light changes direction. In this
activity, you will see how the direction changes when light travels from air to water and when
light travels from water to air.
Most optical devices—including glasses, contact lenses, cameras, microscopes, and telescopes—
rely on lenses that refract light in the manner to be explored in Part A of this activity.
The refraction of light as explored in Part B of this activity is employed in fiber optics. Optical
fibers are used in communication and medical technology.
Procedure
PART A: LIGHT BEAM TANK AND REFRACTION BASICS
If you shine a laser beam at a wall on the other side of the room, you will see the dot where the
beam strikes the wall. The light is reflected diffusely and visible from nearly any place in the
room. But the beam from the laser to the wall is not visible. Air is transparent. So the laser beam
passes through the air without being scattered. If smoke or some other tiny particles are added to
the air, the beam becomes visible. The ever-moving particles reflect the light of the beam in all
directions.
Step 1: Fill the tank about half way with water.
Step 2: Shine the laser into water, aim it through the length of the water and record your
observations.
Revised 1/7/08
Step 3: Add a small amount of scattering agent to the water and stir to mix it thoroughly.
Step 4: Shine the laser through the water again and record your observations. What—if anything
is different this time? Describe and explain.
Step 5: Shine the laser beam so that it passes from air to water as shown in figure 1 below.
laser
incident
ray
air
water
Figure 1
The incident ray is labeled in the diagram. Sketch and label the refracted ray (in the water) in
the figure.
PART B: CRITICAL ANGLE AND TOTAL INTERNAL REFLECTION
Step 6: Move the tank a few centimeters over the edge of the table. Shine the laser beam so that
it passes from water to air as shown in figure 2. Sketch and label
· the incident ray (in the water)
· the refracted ray (in the air above the water)
· the reflected ray (in the water)
air
water
table
laser
Figure 2
This arrangement is referred to as “subcritical.”
Trapping the Light Fantastic page 2 of 4
Step 7: Move and rotate the laser to change the angle of the incident ray as shown in figure 3.
air
water
laser
Figure 3
Sketch the result in the figure above and describe it in the space below.
Question: This arrangement is referred to as “supercritical,” The phenomenon it shows is
total internal reflection. Why is this a good name? (Hint: which ray from figure 2 is no
longer represented in figure 3?)
Step 8: Now that you have observed the subcritical and supercritical arrangements, find the
critical arrangement. When the laser beam strikes the water-to-air boundary at the critical angle,
the refracted ray travels along the surface of the water.
Going Further
Step 1: Obtain a clear, 2-liter plastic bottle. Cut off the top of the bottle and put a small, circular
hole in the side of the bottle about five centimeters from the base of the bottle.
Step 2: Fill the container with water so
that a steady stream flows outward from
the hole. Best to aim the stream into a
sink.
Step 3: Shine the laser beam through
the water to the hole as shown in figure
4. What happens to the beam when it
gets to the hole? Add a line to the figure
and describe below.
cut-off 2-L
bottle
laser
stream
Figure 4
Trapping the Light Fantastic page 3 of 4
Summing Up
1. When light passes from air into water, it does not travel continue on its original path. Does it
bend downward to travel deeper into the water or upward to travel at a shallower depth of
water?
2. When light passes from water into air, it does not travel continue on its original path. How
does it bend?
Trapping the Light Fantastic page 4 of 4