Motivation for Learning
The Disappearing Penny
Explain that in the following experiment students will be trying to measure how much light bends when it enters one substance from another.
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.
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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?
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.
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.
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