Light and Matter Teacher Guide

Activity Names

Being Prepared

The activities in this module use a NetLogo model and a PhET simulation. We recommend having a one-to-one ratio of pupil to computer. If this is not possible, try to keep the number of students per computer as low as possible. These activities do not require probes or other equipment beyond computer or tablet.

Prior Knowledge Required

Em-visiblelight

 

Getting Started

Encourage students to pay close attention to the written instructions before interacting with the model or simulation. Clear instructions are provided in the text that precedes the models. Point out the Snapshot tool to the students and give a brief rundown of its features: Annotation, Draw tools, and Saving. Teachers often find it useful to have students share their ITSI Photo Album with other cooperative group members. 

Suggested Timeline:

Allow two class periods for exploration of both activities and follow-up discussion. If you introduce the Further Investigation, allow three class periods.

Thinking About the Discovery Questions

Color Absorption and Reflection: "What creates the color of a material?"  To discuss this question intelligently, students will need to know: 1) The human eye is sensitive to a very narrow band of frequencies that lie between the wavelength range of ~400 to ~700 nanometers within the electromagnetic spectrum (see diagram above). Specific wavelengths within this narrow band are perceived by humans as color, with red occupying the longer-wavelength end and violet the shorter. The retina in a human eye has cones that respond to the specific wavelengths of red, green, and blue (RGB). The rich variety of colors we see result from the light wavelengths emitted or reflected by objects into our eyes. 2) Depending upon the properties of an object, light may be reflected, absorbed, or transmitted from its surface. Some objects reflect the sunlight, but some absorb one or more of the frequencies. Colors perceived by the human eye are affected by reflection and absorption.  

Neon and Fluorescent Lights

Misconceptions

Students in the secondary grades often have difficulty believing that color is not an intrinsic property of an object, but rather is an interpretation of the reflective and absorptive properties of an object's surface and the wavelength of the light it reflects or emits. For example, humans see the wavelength range of 600-700 nm as orange or red; dogs see it as brown or goldish-brown (see chart below). So you and Fido may both be looking at a "red" ball, but it's brown to Fido. So the color isn't innate to the ball; it's the result of wavelength perception in human vs. canine eyeballs. 

Dogcolorperception

Another area of documented difficulty is in understanding that most objects contain atoms that are capable of selectively absorbing or reflecting one or more frequencies of light. When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. The activity "Color Absorption and Reflection" specifically addresses this difficulty. 

Learning Objectives

NGSS

NSES Physical Science - Interactions of Energy and Matter

Each kind of atom or molecule can gain or lose energy only in particular discrete amounts and thus can absorb and emit light only at wavelengths corresponding to these amounts.

Discussion: Setting the Stage

Color Absorption and Reflection Activity:  Ask students to brainstorm this question in groups: "What creates the colors of a material?"  Have them share ideas and journalize them for future reference. Explain that they will exploring a model to help them get more insight into the science of color. If students aren't already familiar with the electromagnetic spectrum, show them a chart. Explain that visible light comprises only a very small portion of the entire spectrum. Within that narrow band of visible light you find different wavelengths that we perceive as color. 

Neon and Fluorescent Lights Activity:  Students consider the question:  "How do neon lights work?"  Most students will have some ideas to contribute, and all ideas should be validated. Teachers can guide the discussion by asking how neon and other gas lights are different from incandescent bulbs (no glowing filament, neon tubes are cool to the touch while incandescent bulbs are hot). Ask students to consider how the energy of a neon light might differ from the energy in an incandescent bulb.

Additional Background:

Human vision is highly complex process that involves both physical and neurological responses. Teachers desiring a more comprehensive grounding in the subject will find it in The Physics Classroom interactive tutorial "Visible Light and the Eye's Response":  www.physicsclassroom.com/class/light/Lesson-2/Visible-Light-and-the-Eye-s-Response

So what's going on at the atomic level as light interacts with matter and is absorbed, reflected, or transmitted? For explicit content support on light absorption and reflection, we again recommend The Physics Classroom's tutorial:  www.physicsclassroom.com/class/light/Lesson-2/Light-Absorption,-Reflection,-and-Transmission

Learning Objectives

NSES Physical Science Interactions of energy and matter

 

Discussion: Setting the Stage

Color Absorption & Reflection Activity:  To prepare students for successfully completing the models, show them a chart of the electromagnetic spectrum that subdivides visible light into the color spectrum (see image above). Ask them if they know the primary colors of light. Be prepared for students to answer "red, yellow, and blue", which are the primary colors of paint pigment. For this activity, they will become familiar with the RGB model of light -- "Red, Green, Blue". In learning groups, ask students to consider why the RGB model is used in this learning activity and give them 10-15 minutes to look it up. They should be able to respond that the human retina has cones that respond to red, green, and blue frequencies. These are known as the primary colors of light -- adding all three together produces white light. Tell the students they will be using models that let them mix colors in a variety of ways to see how absorption of different wavelengths affects the color we perceive. 

Neon and Fluorescent Lights:  Most high school students will have little prior knowledge of gas discharge lamps or how they work to generate light. They will need a working definition:

Teachers: Other than the definition, let students explore the model to figure out that the electric field accelerates free electrons to an excitation state, causing them to jump to higher energy states. They will see in the model that when the excited atom returns to its previous energy state, it releases energy in the form of a photon. 

Discussion: Formative Questions

Color Absorption and Reflection Activity

Collect Data I:  Were you able to successfully predict what the color would be for the different absorption settings?  Expect students to be surprised by the colors produced in the model. This happens because they may be considering an incorrect model of primary colors. Many of them learned in elementary school that the primary colors are "red, blue, and yellow". While this is true of paint pigment, it is not the correct definition for the primary colors of light. We use the RGB model for light (Red, Green, Blue are the primary colors). 

Red-green

Collect Data II:  If a surface reflects red and green, what color will it appear to be?  Response: The object will appear to be yellow! Expect this to surprise students. When an object reflects red and green, this means it is absorbing blue so your eyes will not receive any light in the blue wavelength. Your retina adds the frequencies of red and green together, and their addition produces the color yellow. The addition of green and blue produces cyan. The addition of blue and red produces magenta. These three colors -- yellow, cyan, and magenta, are called the "primary subtractive colors of light", while red, green, and blue are the "primary additive colors of light".

Collect Data III: If light is sent through a red and green filter, what color will the resulting light be?  Response: Depending on the light intensity setting and the gas number setting, the model will produce a transmitted color in the brown or brownish-tan hue.  (see image below). Students need to play around at least 15 minutes to try out the various settings.  The important takeaway here is for students to gain an awareness that gases can absorb colors and act as a filter. If students are stuck or getting confused: ask them to consider the definition of a light filter below. How would a filter prevent certain frequencies of light from transmission? It absorbs that frequency. For the question above, the sliders need to be set "Absorb Blue-On", "Absorb Green Off", and "Absorb Red Off". 

Filtered-red-green

 

Neon Lights and Other Discharge Lamps

Collect Data I:  Run the model. What do you observe and what don't you understand?  Expect students to have questions about what "ground state" means and to possibly have confusion about how the voltage level is related to the ability of an electron to become excited and emit a photon. 

Collect Data II:  

Question 1: Start with one energy level and "energy at collision" below the energy level. Select continuous energy production. What happens? Why?  Answer: Fired electrons will not be excited, and thus will not jump to a higher energy level. They will not emit photons. If students are stuck: You may need to guide them to see what happens when they change the voltage slider.

Question 2: Now move the "energy at collision" above the energy level. What happens? Why?  ***NOTE: The way to move "energy at collision" level is to change the voltage slider. When "Energy at collision" is above level 2, the electrons will become excited and jump to the next level, emitting a photon. (See image below)

Electronlevel2

Question 3: Add more energy levels. If you have 3 levels (two excited levels plus the ground state), how many frequencies of light can you get? Responses will depend on how high the Energy at Collision levels is set. For the image below, "Sodium" was selected as the gas, voltage was set at 8.25v so Energy At Collision is level 5. In this case, the spectra produced frequencies of ~590-620 nanometers which will show as mostly orange light. Also notice the spectra registers in the infrared and Far IR areas as well. Kids need to play around for 10-15 minutes. Be sure they allow the model to run for at least 60 seconds each time to allow the frequency spectra to show up on the chart.

Collect Data III:  

Question:  What happened when you increased the voltage?  Answer: When voltage is increased to 29 or above, the Energy at Collision allows electrons to jump to levels between 5-6. Below this voltage, you can't see the spectra emitted by an excited hydrogen atom because it falls outside the visible light frequencies. Above 29v, hydrogen electrons reach excitation that jumps them to Energy Level 5 or 6. The resulting light spectra will be visible in the 410-650 nanometer range, allowing you to see violet, blue, cyan, and red shades. 

Sodiumgasdischarge

Analysis:

Color Absorption and Reflection Activity

1. If a surface absorbs red and green, what color will it appear to be?  Answer: blue. If the surface is absorbing all frequencies of red and green, this means it is reflecting only light in the blue wavelength. Blue is the color that will be perceived by the human eye.

2. If light from a red filter is combined with light from a green filter, what will the resulting color be?  Answer: The result will be some shade of brown or tan, depending on the light intensity. (See image directly above with accompanying explanation.)

3. What colors do you think are used in the chlorophyll reaction, if leaves filled with chlorophyll appear to be green?  Responses may vary because leaf colors can be variant shades of green. Acceptable responses should say that green is reflected by the plant, so the colors absorbed into the leaf cells must be red and blue. 

4. What's the difference between a surface that absorbs red and a filter that transmits red?  This may be difficult for students. This model depicts filtering by a gas in Data III. In photography, filters are transparent objects that allow electrons to pass through to neighboring atoms and be re-emitted on the opposite side of the object. Color filters are typically made of transparent pieces of dyed glass, plastic, or gelatin that have been treated to selectively transmit desired wavelengths and restrict others. This is completely different from the interaction that happens when light hits an opaque object. When opaque objects interact with light, they absorb some wavelengths, but reflect other wavelengths. This is a far different phenomenon from transmission of light. The filter that transmits red will produce a red hue on photographic paper because it is filtering OUT frequencies of green and blue. An object that absorbs red and reflects blue & green equally will appear to be the color cyan. 

5. Why is the sky blue?  Think about the gas absorption model. Blue light is scattered in all directions by tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time. This phenomenon of selective scattering of light in the atmosphere is known as "Rayleigh scattering".

6. Sometimes pigments (paint colors) are called subtractive, and light from filters are called additive. Why? Use the two diagrams to explain this.  Responses will vary. Pure paint pigments absorb only a single frequency (color) of light. The resulting color we perceive will "subtract" out this frequency. Using multiple filters, on the other hand, will add frequencies of transmitted light. To take a deeper dive, see The Physics Classroom interactive tutorial on Color Subtraction:  http://www.physicsclassroom.com/class/light/Lesson-2/Color-Subtraction

Analysis: Neon and Fluorescent Lights

1. Using this model, can you explain why sodium discharge lamps appear orange-yellow?  Answer: Sodium discharge lamps produce light in the frequency of ~590-620 nanometers which will show as mostly orange light. Also notice the spectra registers in the infrared and Far IR areas as well, which will not be visible to the human eye. ***NOTE: The voltage will need to be about 8.25 or greater to allow sodium gas electrons to jump to energy levels of 5 or 6, which will produce the colors we can see. See image above for "Collect Data III". 

2. Why are these lamps more efficient than incandescent lights?  Answer: Discharge lamps are more energy efficient because they are cooler than incandescent lights. For example, fluorescent lights don't require continual heating of a filament to operate, as do incandescent bulbs. Discharge lamps require a higher operating voltage to ionize the gas, but are not heating a wire to produce light. In incandescence, a large amount of the energy is dissipated as heat to the surrounding environment. 

3. So-called "neon lights" for advertising actually use other gases besides neon. Explain how a mixture of gases could be used to get a variety of colors. This question can elicit a number of correct responses. Primarily, students should recognize that different gases produce specific frequencies on the light spectra, so mixing the gases could be a useful way to produce a greater variety of colors.

Further Investigation:

Color Absorption and Reflection Activity

What system do you think is better -- the HSB (Hue-Saturation-Brightness) or RGB system (Red, Blue, Green)?  There will be no definitive correct response. Beginning students may feel that the HSB system conforms more to their intuitive sense of color perception, and may prefer it. On the other hand, the RGB system has crucial applications for color television, computer monitors, and mobile device displays.  

Neon Lights and Other Discharge Lamps

Task:  Choose multiple atoms, then run the model with different gases and voltages. Note the number of visible spectral lines and their color. Record your observations.  Acceptable responses: Students should notice that the model provides a chart showing the number of electrons at different energy levels. This is most apparent when the voltage is cranked up. Ask them if they can figure out patterns for each gas: at what levels are the greatest number of atoms excited?  See image below:

Mercurydischarge