2003 Virginia SOLs
Motivation for Learning
Students need a basic understanding of the properties of light. Shadows are created because light is reflected or "bounced" off an object. In this way, light acts much like a particle. It is sometimes compared to "little bullets" or "rays" that stream out from a source. Visible light, what our eyes detect, is only a small part of the electromagnetic spectrum. The electromagnetic spectrum ranges from very long waves such as radio waves to very short waves such as gamma rays. Opaque materials absorb or reflect all visible light, and you cannot see objects through them. Transparent materials allow light to pass through them and you can see the object through them. Translucent materials allow some light to pass through.
Reflection occurs with all types of electromagnetic waves, and when light strikes a mirror we are able to see our reflection. Sometimes light waves are bent as they speed up or slow down as their speed changes when they move from one medium to another. Example: Light slows down as it enters water from the air. That's why an object may appear "bent" or "broken" at the water's surface. These are some ways in which light behaves like a wave.
The light that comes from most sources is a wave that vibrates in many directions perpendicular to the direction of propagation. If light is passed through a polarized filter these transverse light waves will only vibrate in one direction. The filter contains molecules that act like parallel slits that allow those light waves vibrating in one direction to pass through. This principle is sometimes compared to venetian blinds where light is blocked from certain angles and allowed to pass through others. If a second filter is placed so that its molecules are at a right angle to the first filter, all the light should be blocked. Light that is reflected off horizontal surfaces (bodies of water, metal surfaces) is partially polarized horizontally. Because of this, polarized sunglasses are made with vertically polarizing filters to block out glare while allowing all of the vertically polarized light to pass through. Photographers also rely on polarized filters for camera lenses to reduce the glare in photographs.
In this activity, students will use polarized filters in different positions to detect the amount of light that passes through them using a light probe with the computer (or a graphing calculator). Teachers need to be sure that all of the other lights in the room are off, if possible. Try to keep the amount of ambient light constant. It is also important that the probe and the light source are in the same position for all of the trials. The teacher could set the components up beforehand if possible. Students also need a basic understanding of the unit being used to measure light intensity. With most sensors it will probably be the lux. The lux (which is Latin for "light") is a unit of illumination equal to the illumination on a surface of one square meter in area on which there is a luminous flux of one lumen uniformly distributed.
Effective demonstrations can be made in front of the class using a transparency projector and large area polarized film. For example, by placing two films with the polarizing axis perpendicular to each on top of the projector glass, one can block out most of the light. Various orientations can be used.Access to a computer or comparable CBL system is needed along with an interface box to connect the computer and probes. Companies such as Vernier sell these items along with selected "start up" kits. Various area polarized film can be purchased from a science supplier at a minimal cost.
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|Light Intensity (lux)|
|1Filter @ 90 degrees|
|2 Filters (2nd @ 90 degrees)|
Students with Special Needs
All students should be able to participate in this activity.
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Have students answer the following on a separate sheet of paper:
Answers to Assessment
Sample Data should look something like this:
|1 Filter @ 90 deg.||
|2 Filters (2nd @ 90 deg)||