1 class periods of 60 minutes each
Refraction of light is observed in water and through a prism. Polarizing filters amaze students, as they seem to make light appear and disappear. Finally, students use calcite to polarize light and create double images.
Alternative to purchasing polarizing filters: Polarized sunglass lenses are polarizing filters. You can remove the lenses from an old pair of sunglasses and usually see all the same effects.
The sun produces all forms of electromagnetic radiation. Visible light is one of these forms. Visible light can be separated into a spectrum of seven colors which all have different wavelengths. The colors are red, orange, yellow, green, blue, indigo, and violet. Red has the longest wavelength (lowest energy) and violet has the shortest wavelength (highest energy). When all 7 colors are seen together, they form white light.
Why do we see an object as red? When visible light hits a red object, all the colors of the spectrum are absorbed except the wavelength of red. It is reflected back to our eyes and we see the object as red. Objects that look white are reflecting back all 7 colors and objects that look black are absorbing all 7 colors.
When light exists in a single medium it moves in a straight line and at a constant speed. Refraction or bending of light occurs when light moves from one medium to another. For example, when light moves from air to water it slows down because water is denser than air. The same occurs when light travels through a prism. When light slows down it refracts or bends. If you place a pencil in a glass of water you can easily observe the refraction of light. The pencil appears broken at the air/water interface because the light refracts at the interface.
In a prism, the refraction of light is observed because the white light going into the prism splits into its 7 wavelengths of colors when it exits the prism. Shorter wavelength colors, like blue and violet, are bent more than longer wavelengths, like red, so the colors of light always separate into the same pattern.
Light waves emitted from a light source vibrate in all directions. This is referred to as unpolarized light. Normally in unpolarized light, about half the waves are emitted vertically and half horizontally. Light waves in which vibrations occur in a single plane, vertical or horizontal, are referred to as polarized light.
Polarizing filters allow light vibrating in only one plane to pass through the filter. The filter consists of a set of microscopic gates that accept light waves that are either vibrating up and down or side to side. Think of the filter as a picket fence and the only light that will move through the fence must be vibrating in a line that can fit through the spaces of the fence. When light travels through a polarizing filter it has one half the intensity and one half the waves because half of the waves are blocked by the filter. If light passes first through a filter that selects for vertical waves and then through a filter that selects for horizontal waves, no light will be seen coming through the second filter. These filters are referred to as being 'crossed'.
Placing transparent materials, such as plastic or mica, between a set of crossed polarized filters allows the viewer to see certain colors of the spectrum. The first polarizing filter only lets light through that is travelling in a certain direction. The transparent material bends that light, changing the directions of the various light components. As this twisted light travels through the second polarizing filter, only the color that twisted in the same orientation as the second filter will make it through to your eye.
When light passes through calcite, a mineral, the light waves become polarized. One half of the waves become polarized horizontally and one half vertically. This causes a double image to be seen when an image is viewed through a piece of calcite. If a polarizing filter is rotated over the calcite image, the viewer first sees one of the polarized images and then the other polarized image as the filter selects for the horizontal or vertical image.
Polarizing filters are used for many different purposes. Light reflecting off horizontal surfaces like the road, water or snow is horizontally polarized. These horizontal waves cause glare. Polarized sunglasses are oriented vertically and filter out this horizontal glare. Polarizing lenses for cameras fulfill this same purpose. 3D glasses have each lens composed of the different polarizing filters. As each filter allows only that light, which is similarly polarized, and blocks the light polarized in the opposite direction, each eye sees a different image. These two images cause the illusion of 3D.
1a. Observe simple objects, patterns, and events, and report their observations.
1d. Compare things, processes, and events.
1f. Plan and conduct simple experiments.
3a. Know and explain science information specified for the grade level.
4b. Describe or explain observations carefully and report with pictures, sentences, and models.
4c. Use scientific language in oral and written communication.
5a. Cite examples of how science affects life.
Pre-lab discussion: Ask students what types of energy they think we receive from the sun. Explain that it is more than just visible light and heat. Go over the electromagnetic spectrum and other information listed in the background information section of the lab. Demonstrating the filters to the students as you are discussing how filters work keeps them interested in the discussion.
Instructional Procedure: Small groups or set up as stations for rotating groups through the activities
Activity 1: Diffraction glasses and prisms
Activity 2: Polarization
In the other view, the viewer will see through the two filters but only with half the light's original intensity (Fig. 2).
In this case, the filters were aligned and accepted light in the same direction.
Activity 3: Splitting light
Activity 4: Refraction of light in water
Use student pdf to assess student learning
Rio Tinto Hands-on Science Curriculum Team