How is Light Absorbed and Transmitted?

Transcription

1 How is Light Absorbed and Transmitted? Description: Students will examine the absorption and transmission of light by color filters with the help of a light source and a diffraction grating. Student Materials (per group): Light source Set of color filters Lens Diffraction grating/cd/dvd disk White screen/piece of paper Additional Teacher Materials: Other light sources (if available) Background and Misconceptions: White light can be represented by as little as just three colors of an equal proportion the Red, Blue, and Green colors. More often, natural white light contains all the rainbow colors (from blue to red). Many objects appear to have their colors because they absorb some colors, and then reflect or transmit other colors. Color filters are especially made to usually absorb most of the colors and transmit one specific color. A diffraction grating is a special device which can take a white light and separate it into consisting colors, spreading them out into different directions. It also acts as a bad mirror, reflecting some portion of the incident light. Students will be able to examine how color filters affect light coming from a light source and then diffracted by a grating. They will see that the white light from the incandescent lamp (tungsten) consists of a rainbow of colors. The other light sources (such as LEDs) contain only 1-3 relatively narrow colors. First, let the students build a small experimental setup similar to shown on a schematic below. Then allow them experiment with the color filters to see the effect on the spectrum appearing on the screen. The student should fill the table and answer some questions once they are comfortable with their observations. White screen Direction of diffraction Direction of reflection Light source Lens (optional) Diffraction grating (or CD/DVD disk) Teacher Guided Questions to Inquiry: Use these questions to get the students started on their own inquiry!

2 1. If several types of the light source are available (such as tungsten bulb, LED flash light, fluorescent lamp, etc.), ask what students can tell about the nature of the light source and the differences between them by conducting this experiments on each of the light sources. Do all of these light sources have all the colors included in their output? 2. If there are mercury/krypton/argon (calibration) lamps available, ask what is especial in these lamps compared to others. Can students see intense lines in the color spectra of these? Which of the light sources is closer by its nature to our Sun and why? Additional Hints: If the light source is not bright enough, dimming the room light might be necessary for the lab. Lens might be needed to collimate/focus light source on to the grating. Diffraction grating also acts as a regular mirror. Make sure students are looking not at the reflected light but at dispersed colors they should be in a different direction compared to the reflection. How is Light Absorbed and Transmitted? TEACHER ANSWER SHEET Filter Before the Grating After the Grating No Filter Red Blue Green Yellow Blue Green (Cyan) Magenta Orange Before the grating see the white yellowish light coming from the source. On the screen see a rainbow of dispersed colors from blue to red. Before the grating see only the red color red color. Before the grating see only the blue color blue color. Before the grating see only the green color green color. Before the grating see only the yellow color yellow color. Before the grating see only the cyan color cyan color. Before the grating see only the magenta color after the filter. On the screen see only the magenta color. Before the grating see only the orange color after the filter. On the screen see only the orange color. On the screen see a rainbow of dispersed colors from blue to red. except for the red. except for the blue. except for the green. except for the yellow. except for the cyan. except for the magenta.

3 Purple Before the grating see only the purple color purple color. except for the orange. except for the purple. Questions: 1. Based on your observations, what are the main differences between the filter being placed before and after the grating? Answer may vary, but the student should note that if the filter is placed before the grating then only one color goes through it and then grating does almost nothing to it. If the filter is placed after the grating, then if it is inserted in the part of the spectrum different from the color of the filter, this part will be blocked on the screen. The color of the filter is transmitted through it. 2. What do you think color filters do to the light coming from the light source? Answer may vary, but the student should note that the filter blocks (absorbs) all the colors of the light except of the color of the filter, which is transmitted through the filter. This changes white light into the colored light. 3. What do you think color filters do to the color rainbow after the diffraction grating? Answer may vary, but the student should note that all the rainbow colors different from the color of the filter are blocked (absorbed) by the filter, while the color of the filter is transmitted. Depending on where the filter is placed, this can make part of the rainbow or the entire rainbow disappears, leaving only the color of the filter being transmitted. 4. If you combine your ideas from answers (2) and (3), what can you say about the nature of the light from the source? Answer may vary (especially depending on the light source), but the student should note that the color of the light source is a mixture (superposition) of colors seen after the diffraction grating. 5. Based on your observations can you explain why the color filters have their names (red, green, blue, etc.) and why they appear to have color? Answer may vary, but the student should note that the color filters block (absorb) all the colors except of the color corresponding to their name. This color is transmitted through the filter. This makes filters to be colored.

4 How is Light Absorbed and Transmitted? Name Date Description: The light from a bulb, flashlight, Sun, or other non specialized source can be seen as a combination of different colors, called the spectrum. Almost every object absorbs some colors and transmits or reflects other colors. In this lab you are going to observe how this happens, and also understand colors and names of some representative objects set of color filters made to transmit specific colors. Materials: Light source Color filters Lens Diffraction grating White sheet of paper Procedures: 1. Setup the light source to shine into the diffraction grating (orient it initially at almost 0 degrees relative to the angle of light incidence). If the source is not bright, then use a lens to focus more light onto the grating. Using the white screen find the position where the light is dispersed by the grating into a rainbow of colors. Have enough distance between the screen and the grating to clearly see different colors. You might need to adjust position of the screen and the angle of the grating to achieve best results. White screen Direction of diffraction Direction of reflection Light source Lens (optional) Diffraction grating (or CD/DVD disk) 2. Using set of color filters, place each of the filters first in between of the light source and the grating and then in between of the grating and the white screen. Observe colors seen on the white screen and their change compared to when no filter is in place. Record your observations in the table below. Make sure you record colors you see on the screen and the changes.

5 Filter Before the grating After the grating No Filter Red Blue Green Yellow Blue Green (Cyan) Magenta Orange Purple Questions:

6 1. Based on your observations, what are the main differences between the filter being placed before and after the grating? 2. What do you think color filters do to the light coming from the light source? 3. What do you think color filters do to the color rainbow after the diffraction grating? 4. If you combine your ideas from answers (2) and (3), what can you say about the nature of the light from the source? 5. Based on your observations can you explain why the color filters have their names (red, green, blue, etc.) and why they appear to have color?