La enseñanza de las estrategias de aprendizaje

This investigation can be completed in one class period if the equipment is organized ahead of time and if a quick demonstration of the procedure for each station is presented in the previous class. If you use laser light sources for stations 1 and 4, it is vital that you prepare your students and organize the classroom according to the safety recommendations for students that are outlined in the investigation.

In parts of this investigation, you have the option of using a ray box or a small laser pointer to produce rays of light. If you are using a laser pointer, it is important to employ all the recommended safety precautions to avoid having light from this source travel directly to your eyes.

Mandatory Safety Precautions for Working with Laser Light

• Never aim a laser at a person’s eye.

• Avoid having the unprotected eye along or near the beam axis.

– If you are working at a table, this means keeping the laser light parallel to the surface of the table so that your eyes are well above the work surface.

– Anticipate the path the laser light will take and arrange the apparatus so that the beam will not inadvertently be directed near the eyes of other students. One useful strategy is to work around the perimeter of the room, with the laser light directed toward the outside wall. This arrangement also ensures that your eyes are facing away from other groups.

• Keep the room well-lit so that pupils remain small, reducing the “window” available for the entry of laser light. • Avoid having the laser produce light for extended periods of time. Once the apparatus is in place, most measurements

or observations can be made in a matter of seconds, and then the laser can be switched off.

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An efficient way to complete the lab with minimal amounts of equipment is to organize this activity into four stations, one for each of the four properties to be investigated:

• Station 1 – Investigating Refraction: This will take 15 minutes to collect data. The additional time required for the analysis can be completed at the polarization station.

• Station 2 – Investigating Polarization: This will take less than 10 minutes if the apparatus is already set up. The additional time can be used to complete the refraction analysis.

• Station 3 – Investigating Diffraction: This will take less than 10 minutes to collect data if the shield light source and pinhole viewers are already set up. The additional time required for analysis can be completed at the reflection station.

• Station 4 – Investigating Reflection: This will take 10 minutes if the metal is already curved into a parabolic shape. The additional time can be used to complete the diffraction analysis or to start the next station, Investigating Refraction.

If the class is divided into eight groups, then Stations 2, 3, and 4 should be set up with enough equipment to accommodate two groups. Station 1: Investigating Refraction will likely take the most time. This station could be set up to accommodate three groups, since this is the place where bottlenecks are likely to occur. Note that since the activities require slightly different amounts of time, if you rotate the groups the shorter times required at Stations 2 and 4 could be used to complete the analysis portions of Stations 1 and 3.

Investigating Refraction

Although individual student data will vary, the following results are typical.

angle of Incidence 0° 10° 20° 30° 40° 50° 60° 70° 80°

angle of Refraction 0° 7° 15° 21° 29° 36° 36° 41° 49°

OBSERVaTIOnS anD anaLySIS FOR LIgHT PaSSIng FROM aIR InTO WaTER

angle of Incidence 0° 10° 20° 30° 40° 50° 60° 70° 80°

angle of Refraction 0° 6° 13° 20° 24° 32° 36° 39° 43°

OBSERVaTIOnS anD anaLySIS FOR LIgHT PaSSIng FROM aIR InTO gLaSS

angle of Incidence 0° 10° 20° 30° 40° 50° 60° 70° 80°

angle of Refraction 0° 11° 23° 35° 46° 61° 75°

angle of Reflection 70° 80°

OBSERVaTIOnS anD anaLySIS FOR LIgHT PaSSIng FROM gLaSS InTO WaTER

analysis

1. The angle of incidence that produced no bending of light was zero degrees.

2. a. Light bends toward the normal when passing from air into water and from air into glass.

b. Light bends away from the normal when passing from glass into water.

c. The most bending occurred when light passed from air into glass.

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3. a. One trend in the data was that the angles in the water were consistently larger than the angles in the glass. Another trend was that as the angles in the water increased, there was a larger difference between the angles in the water and the corresponding angles in the glass. The result of these two trends is that a point is reached for large angles in the water when there is “no room left” for the corresponding angles in the glass.

b. When the angles of incidence were 70° and 80°, the corresponding light ray reflected from the glass-water boundary back into the glass in each case. Since no light passed into the glass and each ray was totally reflected back into the water, the term total internal reflection describes this phenomenon well.

4. a. The truck turned toward the normal as it passed from the packed gravel into the wet muskeg.

b. When light passed from air into water, the light ray bent toward the normal. This same change of direction was observed when the truck slowed down as it passed from packed gravel into wet muskeg. If the same reasoning is applied to light rays bending toward the normal, then the light rays must also be slowing down. Investigating Polarization

Observations

step 1: Looking through both filters tends to make the light bulb look slightly less bright, due to the absorptive qualities of each filter. If one filter is held in place while the other filter is rotated, the bulb appears progressively darker until the bulb becomes almost completely invisible when the rotated filter is moved through 90°. If the rotation of the one filter continues, the bulb appears to get brighter again, reaching its brightest appearance when the filter has been rotated an additional 90° from the darkest position.

step 2: It is possible to see both the submerged coin and the reflection of the bulb through the polarizing filter, although the reflected light from the bulb makes the coin difficult to see.

step 3: When the polarizing filter is rotated, the reflected light from the bulb can be greatly reduced, making the submerged coin easier to see.

step 4: Looking through a filter at the screen of an LCD monitor or the screen of a graphing calculator tends to make the light from each screen look dimmer. However, when the filter is rotated, the light from the screens gets dimmer until it eventually reaches a point where it disappears. This point occurs after a rotation of 90° from the position of the filter that resulted in the maximum brightness. An additional observation is that the angle of the filter that blocks most of the light from the calculator screen is not the same as the angle that blocks most of the light from the LCD screen. The side of the filter was parallel to the sides of the calculator screen when most of the light was blocked. However, the same filter needed to be held so that its side was at a 45° angle to the sides of the computer screen.

Investigating Diffraction Observations

The following diagram shows the diffraction pattern that was observed when looking at the two sources through the pinhole viewer.

For the viewer with the larger pinhole, the two sources seemed to blend into one at a distance of about 2.5 m. For the viewer with the smaller pinhole, the two sources seemed to blend into one at a distance of about 1.0 m.

At the distance of 1.0 m, the light sources were still distinguishable using the viewer with the larger pinhole. However, when the viewer with the smaller pinhole was used, the two sources seemed to blend into one source at this same distance.

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analysis

If the sources were two distant stars, you would want to use a viewing device that had a large opening to allow as much starlight to enter as possible. This would enable astronomers to better observe the fine details in the starlight, such as the fact that the light is coming from two sources and not one.

Investigating Reflection Observations and analysis

1. The following sample data is typical for this investigation.

angle of Incidence 0° 10° 20° 30° 40° 50° 60° 70° 80°

angle of Reflection 0° 10° 20° 31° 40° 51° 60° 71° 79°

2. The trend suggested by this data is that within experimental error, the angles of reflection are equal to the angles of incidence.

3. The following diagram shows typical student results for this investigation.

1 2 3 4 5 6 6 5 ₄ 3 2 1 incident light rays reflecting surface along this line

4. The reflected paths all come very close to passing through one point. (This point is called the focal point.)

5. One device that utilizes the pattern formed by reflected paths is a satellite dish that is used to receive signals from orbiting satellites. These signals can be decoded to provide a customer with satellite television. In a satellite dish, a sensitive detector is placed at the point where all the reflected waves converge. Another device that utilizes the pattern formed by the reflected paths is the ear of a fox or a cat. In this case, the curved inner surface of the ear reflects the sound waves to the animal’s ear drum.

Practice, page 444

31. a. Kepler designed a refracting telescope. His design uses lenses only.

b. As the light rays pass through the telescope, they switch positions in terms of which one is on the top and which one is on the bottom. This means that if someone was using Kepler’s telescope, the object being observed would appear upside down. Galileo’s telescope did not have this problem.

32. The pupil is the opening that allows light to pass into the eye. Since an eagle’s eyes have such large pupils, the effects of diffraction are reduced, allowing the eagle good resolution for seeing the fine details in distant objects.

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