Light and Color Unit Overview

The Light and Color Unit is intended to engage students in myriad experiences with hands-on and computer-based materials that will help them modify their existing ideas and construct new ideas about light and color. The focus on the unit is on geometrical optics. Physical optics is not covered in this unit. (A separate Waves and Sound unit will focus on wave motion. Light waves could be incorporated as application or extended activities in that Unit.) Extensive use is made of both hands-on equipment and the various Light and Color Simulators throughout the entire unit.

The Unit consists of four cycles:

Cycle I: Illumination, shadows and pinholes

Cycle II: Reflection and image formation with mirrors

Cycle III: Refraction and image formation with lenses

Cycle IV: Color

Target Ideas for Unit

The Light and Color Unit was designed to provide the opportunities for students to construct ideas which are closely aligned with the ones listed below. We have developed Idea Journals for each cycle, with each idea prompted by a set of questions. The target ideas listed below correspond to the individual "pockets" in the Idea Journals where students write their own ideas, and provide supporting evidence. At the end of each activity in the Development Phase, students are asked to add or modify an idea in their Idea Journal, based on evidenced gathered within that activity. We have found this semi-structured approach for development of a common set of ideas to work well. Naturally, as part of their consensus discussion for each cycle, the students will probably develop these ideas in their own words. However, the conceptual content of their own ideas should be similar to these. The Teacher Guide for each cycle provides examples of the kinds of statements students actually develop in the class. After the class agrees on a set of ideas the teacher should introduce appropriate terminology and conventions so that the students' are more closely aligned with the corresponding ideas they would find in textbooks or when they talk with other students.

Target Ideas for Cycle I

1. Light travels idea: Light travels in straight lines. Scientists represent this idea diagrammatically by drawing straight lines with arrows pointing in the direction that light travels. These diagrammatic representations are called light rays. A diagram showing how light behaves in a particular situation in terms of light rays is called a light ray diagram. Light ray diagrams show representations of the set-up and how light is behaving. There are almost always drawn from either a SIDE VIEW or TOP VIEW perspective. Sometimes they also include a drawing of the phenomenon that is observed (for example, a front view of the screen).

2. Point source idea: A point source sends out light in all directions.

For simplicity, in representing a point source in light ray diagrams, scientists usually just draw a tiny circle and ignore the remaining structure of the source. This is done whether you are using a Side view or Top view representation. Scientists also just draw a few light rays, carefully chosen to emphasize critical features of the explanation. For example:

3. Screen illumination idea: The brightness of a region on the screen (including a partial shadow region) depends on the intensity and number of light sources sending light to that region, and on the distance between each light source and the screen. The screen brightness increases as the number and/or intensity of light sources increase, and the screen brightness decreases as the distance between the light source(s) and the screen increases.

4. Extended source idea: An extended source consists of a continuous sequence of point sources. (Each point on an extended source sends out light in all directions.)

5. Intensity-distance idea: The intensity of light decreases with distance from a source. (This idea is suggested in the way we represent light traveling outward in all directions from a source. Adjacent light rays get further part the greater the distance from the source. )

6. Reproduction of a Source idea: To produce a pattern of illumination on a screen that looks like a reproduction of the source (although perhaps inverted), light reaching each point on the screen must have come from only one point on the source.

Target Ideas for Cycle II

1. Reflection from a mirrored surface idea: Light reflects off a mirrored surface at the same angle that it hits the surface. This is also called regular (or specular) reflection. If the surface is curved, the light reflects off each point as if there was a flat mirror at that point tangent to the curve of the mirror.

2. Reflection from a non-mirrored surface idea: Light reflects off a non-mirrored surface in all directions. This is also called diffuse reflection.

3. Vision idea: To see a point, light spreading out from that point must enter your eye. The point can be its own light source, a non-light source (in which case light from a source must diffusely reflect off it), or an image.

4. Concave mirror image idea: Light spreads out from each point source in front of the concave mirror, reflects off all parts of the mirror, and comes together at another point in front of the mirror called the image point. The location of this image point depends both on the distance between the source and the mirror, and also on the curviness of the mirror. An entire image, made up of many points, will be upside-down and left-to-right reversed.

5. Plane mirror image idea: Light spreads out from each point in front of a plane mirror, reflects off all parts of the mirror, and spreads out so as to appear to come from another point behind the mirror called the image point. A plane mirror can form an image that appears to be an equal distance behind the mirror as the object is in front. This image cannot be formed on a screen.

Target Ideas for Cycle III

1. Refraction of light idea: (a) When traveling from air into a solid, the light bends (refracts) partly towards the perpendicular line. As the incoming angle increases, the bending angle decreases. (b) When traveling from a solid or liquid into air, the light bends (refracts) partly away from the perpendicular line. For incoming angles greater than about 45 degrees, as the incoming angle increases, the bending angle decreases. For incoming angles less than about 45 degrees, all of the light will be reflected back into the solid or liquid, and none will emerge into the air. (This phenomenon is called total internal reflection.)

2. Distortion of objects idea: Objects immersed in a transparent solid or liquid material will appear distorted or displaced because of refraction. Light leaving the object will change direction in going from the material into air, and hence enter your eye as if originating from a different point.

3. Lens image idea: Light spreads out from each point in front of a lens, refracts at both surfaces of the lens, and comes together at another point on the other side of the lens called the image point. (In drawing ray diagrams, for simplicity, we usually only show light bending at the mid-plane of the lens.) The location of the image point depends both on the distance between the source and lens and the bulginess of the lens. The closer the source is to the lens, the further away the image is formed (and conversely). The bulgier the lens the closer the image is formed (and conversely). An entire image, made up of many points, will be upside-down and left-to-right reversed.

Target Ideas for Cycle IV

1. Colored components of white light idea: White light consists of a mixture of all the colors of the spectrum. As a simple approximation it can be considered to consist of three main components: red, green and blue.

2. Color filter idea: A colored filter produces colored light by removing other colored components from the light entering it. This process is called color subtraction. Colored plastic, paints, inks, dyes and crayons behave like colored filters. For ideal filters:

• a red filter subtracts out the green and blue components (also known as a -G-B filter)

• a green filter subtracts out the red and blue components (also known as a -R-B filter)

• a blue filter subtracts out the red and green components (also known as a -R-G filter)

• a cyan filter subtracts out the red component (also known as a -R filter)

• a magenta filter subtracts out the green component (also known as a -G filter)

• a yellow filter subtracts out the blue component (also known as a -B filter)

3. Color addition idea: Overlapping different brightness of red, green and blue lights can produce any colored light, a process called color addition. The simple rules of color addition (for equal brightness of colored lights) are:

• red + green = yellow

• red + blue = magenta

• green + blue = cyan

• red + green + blue = white

4. Overlapping filters idea: When colored filters are overlapped, the resulting color perceived is that left over after each filter subtracts out its colored components. Overlapping different amounts of yellow, cyan and magenta filters can produce any other color.