Lesson 2

Description: In this lesson, students will explore the properties of both visible and IR light, as well as how we perceive light and color. This is intended to be the second lesson in the series of four.

Topics: Electromagnetic Spectrum, Physics, Biology

Learning Goals

• Understand the properties of the Electromagnetic spectrum
• Experience the ways our eyes perceive light and color
• Explore mixing of light vs mixing of pigments

Supplies

Advanced Prep

Create Boxes with Red Filters (1 per student group)

• Cut hole in side of box­ approximately 3” by 3” hole
• Cover hole completely with red colored film
• If time allows, have students help with the construction of these boxes

Introduction

5 minutes

Begin class by going over classroom expectations, and reminding students of the information learned in Lesson 1: [xxx].

In our last lesson, we learned about the wetlands loss occurring in the gulf and the importance of protecting these lands. Today, we’re going to study light, and how we see light, so that we can learn more about how to observe and protect our environment around us.

Electromagnetic Spectrum

10 minutes

What is light? Light is electromagnetic radiation that is visible to the human eye. You may have heard of the term “visible light”­ does that mean there is also “invisible light”? What other types of light are there? Electromagnetic radiation that is outside the visible range has a wavelength that is too long or too short for the human eye to detect. Humans can only see a small portion of the electromagnetic spectrum!

Electromagnetic radiation is classified by its wavelength. The human eye can typically see wavelengths between approximately 400­700 nanometers, but this range is just a small fraction of the entire spectrum. Below 400 nanometers is ultra­violet, or UV, light, X rays, and gamma rays. Above 700 nanometers is Infrared, or IR, microwave, radio waves, and long radio waves. Today we will be taking a closer look at visible light, as well as IR light that is very near the visible light spectrum, also known as near­infrared.

Visible light is made up of all the colors of the rainbow­ red, orange, yellow, green, blue, and purple. When all these colors combine, we see “white light”.

Colored Boxes

20 minutes

For this activity, arrange students in small groups of 2­4 students.

Materials

• Boxes with red filter
• Small colored objects
• Flashlight
• Filtered light worksheet

When we look at a object, why does it look a certain color? When light hits an object, the object will either absorb or re­emit that light. The color of an object is just a combination of all the wavelengths of light that were re­emitted by the object. A red object appears red because it is absorbing the orange, yellow, green, blue, and purple wavelengths, and is re­emitting the red wavelengths.

Why do white objects look white? What about black objects? White objects are reflecting all wavelengths of visible light. Black objects are absorbing all wavelengths of visible light. _ Pass out the filtered light worksheet to each group and have them draw what happens to the visible light wavelengths in Section A. After each group has had a chance to complete Section A, review with the class to check for understanding._

Pass out a box with red filter and a flashlight to each group.

In front of you, you each have a box with a red filter on the side. What do you think happens to light as it enters this filter? Only the red wavelengths of light are able to pass through this filter.

In a moment, I will be passing out a few colored objects, including a red, black, and white object. On your worksheet in Section B, make a prediction about what color each of these objects will appear when viewed through the red filter. Once you receive your objects, place them in the box and use the flashlight to shine through the filter and view your objects one at a time.

Pass out small colored objects to each group. After students have a chance to test out each of their objects, review as a class and discuss why each object appeared as it did.

Mixing Light

20 minutes

Materials

• Flashlights
• Squares of colored film (blue, red, and green)
• White paper
• Worksheet

You likely learned about mixing colors when you were in elementary school. However, most of our experience with mixing colors usually comes from mixing pigments. This is very different from the results of mixing light. For example, when you mix all colors of the rainbow with paint, you get a lovely shade of brownish black. However, when you mix all colors of the rainbow in light, you get white light.

We are going to experiment with mixing light using the colored filters and flashlights in front of you.

Pass out the materials listed above to each group. First, have students use the worksheet to predict what color will form when they mix each of the colors of light, and what color will form when mixing all three colors. Then, instruct students to place a colored film on each flashlight and shine it onto the white piece of paper. Have them begin by mixing two colors at a time, then all three colors at once. Students should color in the light color wheel on their worksheet with the results. Discuss their predictions and the results of their experiment.

Students should observe that mixing red and green light results in yellow light, mixing red and blue results in magenta, and mixing green and blue results in cyan. Mixing all three results in white light.

These three colors of light are known as the primary color because when they combine they can create all other colors of light.

Biology of Our Eyes

We now know how visible light can be reflected or absorbed and combined to form different colors. But how do our eyes detect those colors?

Our eyes have millions of light­receptor cells of two types: rods and cones. Each of these types have a very different purpose. Cones can detect color, while rods cannot. Rods are extremely sensitive and can detect light at very low levels. If you’ve looked around a dark room, chances are there was still enough light for you to make out different objects, but not enough light to tell what color the objects were. This is because at low levels of light, only your rods are active.

Cones need a much brighter amount of light to activate. There are three types of cone cells­ those that respond to long wavelengths, medium wavelengths, and short wavelengths. These allow us to see red, green, and blue light. We know from our light mixing experiment that these three colors of light can be mixed in different amounts to produce all other colors of light.

Optical Illusions

20 minutes

Materials

• Optical Illusion Prints
• False Color Photographs
• White Paper
• Markers

Pass out optical illusion prints to students. Ask them to stare at the middle of the print for one full minute, then look at their blank white paper. Students will observe an after­image on the white paper that has different colors from the print.

What did you observe? Why do you think you see the same image when you look away? What you observed when you looked at the white paper is called an afterimage. You may have seen this effect before if you’ve experienced seeing spots of lights in your vision after looking at a camera flash. This happens because the rods and cones in your eyes lose sensitivity if they are over stimulated.

Were the colors the same in the afterimage as they were in the original image? The colors were not the same. Your task is to figure out how the colors in the afterimage are related to the colors in the original image, and why they appear different. You can use the markers and white paper to test various colors and make your own optical illusions.

After allowing time for the students to experiment, discuss their results. They should discover the following connections:

Encourage students to refer to the color wheel of light they created in the previous activity. They will notice a pattern in the colors of the afterimage: The afterimage is of a color across the color wheel from the original color. For example, if the original image is green, the afterimage will be magenta. This is because the green photoreceptors are fatigued, making the signal from the red and blue photoreceptors stronger by comparison. Sensing red and blue light together forms a magenta image. Similarly, an original yellow image is made from both red and green light. As the cones that sense red and green are fatigued, the blue photoreceptors are stronger by comparison and create a blue afterimage.

If time allows, let the students draw their own optical illusions keeping in mind what colors the afterimage will appear. Have them trade pictures with their neighbors and view their optical illusion.

Wrap Up

Let students guide the discussion about the day’s lesson and present their hypotheses before discussing explanations.

What is the electromagnetic spectrum? How much of the electromagnetic spectrum can humans see? How do we see light? What surprised you about today’s lesson? What would you like to learn more about from today’s lesson?

Resources

Public Lab Website Wikipedia entry on the electromagnetic spectrum Wikipedia entry on additive color Wikipedia entry on afterimages

Vocabulary