- University of Virginia
- Physics Department
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Refraction
A Physical Science Activity
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- 2003 Virginia SOLs
Objectives
Students will
- observe that light travels in a straight line;
- observe the bending of light as it enters a new material;
- measuring the angle of refraction of light between air and water;
- use skills such as: observing, inferring, measuring, and isolating variables.
Motivation for
Learning
Discrepant Events
- The Bent Rod
- Materials
- Wood dowel or Metal rod
- Fish tank (nearly filled with water)
Procedure
- Place the wood dowel or metal rod in a fish tank that is
nearly filled with water.
- Let the dowel rest such that most of it is inside the water. The portion
of the dowel that is in the water will appear enlarged and distorted due to
the refraction of the light in the water. Explain to the students that the
light reaching their eyes from under the water is bent relative to the light
reaching their eyes from outside the water.
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The Disappearing
Penny
- Materials
Procedure
- Place an empty cup on the table and drop a penny in it. Ask a
student to come up and look down into the cup so that he can see
the penny.
- Then have him move back away from the cup slowly until the
penny just disappears from his sight.
- Slowly pour water into the cup and the penny will come back
into sight.
- Explain that this is due to the refraction of light.
Explain that in the following experiment students will be trying to measure
how much light bends when it enters one substance from another.
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Background
Information
The fact that light bends when it moves from one material to
another is a strong indication of its wave properties. Light will
travel in a straight line as long as it is moving through one medium.
However, when it is leaving one material and entering another at an
angle, it will bend. If a ray of light travels from a less dense
medium to a more dense medium (traveling from air to water), it will
bend toward the normal (perpendicular line to the boundary of the two
materials). If it is leaving the denser material into the less dense
material, it will bend the other way, away from the normal.
Bending occurs because the speed of the light ray or wave is
retarded in the new material. A portion of the wave enters the new
material before the rest of the wave does and is slowed down, while
the remaining portion of the wave that has not entered the new
material is still traveling at the original speed. Part of the wave
will be traveling at the original speed and the other part of the
wave at a slower speed. This causes the wave to bend as it enters the
new material.
Student
Activity
To print out the Student Copy only,
click
here.
Materials
- Transparent, rectangular container at least 4 cm deep, or
rectangular glass plate
- Cardboard at least 10 cm longer and wider that the plastic container
- 4 straight pins
- Ruler
- Protractor
Procedure
- Place water in a transparent rectangular container to a depth
of 3 cm.
- Place the container on the cardboard and mark the outline of
the container on the cardboard.
- Place a pin in the cardboard behind the container. The pin
should be as close to the container as possible.
- Place a second pin 4 to 5 cm behind the first. The two pins
should form a straight line which should form an angle of less
than 45° with the side of the container.
- Bend down in front of the container, so that you are looking
at the two pins through the water in the container. Place a third
pin in front of the container close to it. This pin should appear
to line up with the two pins behind the container.
- Place a fourth pin 4 to 5 cm in front of the third pin so that
it lines up with the other pins. Readjust the positions of any of
the pins so they all appear to be in a line when you look through
the water in the container.
- Carefully remove the container of water, leaving the pins in
their position.
- Use a ruler to draw a line connecting the positions of the two
pins that were behind the container. Repeat this procedure for the
two pins that were in front of the container.
- Draw a third line inside the area were the water container was
located connecting the other two lines. This line represents the
path of the light ray through the water.
- With a protractor, measure the angle between the two sides of
the container and the lines connecting the two pins that were
behind the container.
- Repeat step 11 for the line connecting the two pins in front
of the container.
Data Sheet
To print out the Data Sheet only,
click here.
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Angle Measured
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Measurement of Angle
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Angle between pin line and the front side of the
container
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Angle between pin line and the back side of the
container
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Angle between the line inside the tray
connecting the two pin lines and the side of the
container
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Questions:
- 1. Did the light bend toward or away from the side of the container
as it entered the water?
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2. Did the light bend toward or away
from the side of the container as it left the water?
3. Were the angles measured in steps 10 and 11 equal?
4. How did the direction of the light ray entering the
water compare to the direction of light leaving the water?
5. A light ray enters a piece of glass from air at an
angle of 35 degrees. Use the diagram below to answer the
questions.
a. At what angle did the light ray leave the glass?
b. Draw the line inside the glass representing the path
of light inside the glass.
c. Are the angles between the light rays and the surface
of the glass smaller inside or outside the glass?
d. When the angles between the light rays and the surface
of the glass become larger, does that represent light
speeding up or slowing down?
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Answers to Data Sheet
Answers to Questions and Assessments:
1. After the light entered the container it bent away from the
container's surface.
2. As the light left the container, the ray bent toward the surface of the
container making the angle outside smaller than the angle inside.
3. The angles measured should be equal.
4. The directions should be the same.
5. a. 35 degrees, c. The angles are smaller outside. d. Larger angles represent
the slowing down of the ray.
Extensions
Repeat this investigation using a glass triangle instead of a rectangular container
of water or glass plate. Measure the angles of the incident rays and the refracted
angles as you did in this experiment. How do these results compare to this experiment?
How are the angles of incidence and refraction different? Form a hypothesis
to explain the differences.
Students with Special Needs
Some students may have difficulty manipulating small objects (like the pins).
They may work with partners or in small groups.
Click here for further
information on laboratories with students with special needs.
Assessment