Unit 11: Light and Color

Unit 11: Light and Color

Unit Menu

Lesson 1 - The Nature of Light

1.1Read 27.6

Quick Lab - Shadows

1.2Read 27.7 and 27.8

CD 27-2

Quick Lab - Polarization

1.3Read 27.1-27.5

Web Walk – Light as a Wave

1.4Knowledge Check – Reflection – Polarization for Car Head Lights

1.5Quiz - The Nature of Light

Lesson 2 – Color

2.1Read Chapter 28 “Color” in Conceptual Physics

Web Walk - Color and Vision

2.2Quick Lab - Mixing Light Colors

2.3Knowledge Check – Conceptual Challenge – Blueprints

2.4Quiz - Color

Lesson 3 - Unit Wrap Up - Light and Color Exam

Learning Goals

By the end of this unit, you will be able to:

* identify components of the electromagnetic spectrum

* describe the dual nature of light

* recognize that light has a finite speed

* describe how the frequency of light affects what happens to it when light is incident upon a material

* explain how light is polarized

* describe how brightness is affected by distance (inverse square law)

* recognize how pigments affect the color of reflected light

* explain/describe what determines whether a material reflects, transmits, or absorbs light of a particular color

* describe/explain color mixing by subtraction and addition

* explain how a spectrum can be used to identify the presence of an element

Check it Out!

Probably the all-time standard physics question is “Why is the sky blue?' Physicists often toss it aside with a few mutterings about scattering. The question deserves better treatment than that and this where you play a role. You have been recommended by your physics teacher to make a presentation to a group of students visiting the Iowa Science Center . As you think about your presentation, consider the following: What part of the sky is the bluest? Why isn't the entire sky a uniform color? Why isn't the sky blue on the nights with a full moon? What is scattering the sunlight to produce the daytime color of the sky?

What is light? Strictly speaking, the only thing we see is light. Can you imagine the world without light? Life just could not exist without light. We would not be able to see, plants would not grow, and our planet would become a frozen, barren wasteland. To understand what light is, we first need to understand how it behaves. The puzzle concerning the nature of light can be expressed very simply without any mathematics. Classical physics, tells us, and experiments confirm, that if you put energy into making waves, more energy goes into making waves with higher frequency than ones with lower frequency. Therefore, the source of all light is accelerating electrons. These electromagnetic waves are not mechanical in nature, like sound waves, but, rather, consist of oscillating electric and magnetic fields. As non-mechanical waves, they do not require a medium and therefore they can travel through empty space. As light and matter interact, it can be reflected, refracted, diffracted, transmitted, absorbed, dispersed, polarized and have interference. Through the course of this unit and the next unit, we will look at each of these interactions.

Why do we see? What is a photon? What is the wave-particle duality of light? What is polarized light? Why does television use only three colors to give full color? Why are sunsets and sunrises red? Why is the sky blue? After completing this unit, you should be better able to answer some of these questions.

Lesson 1: The Nature of Light

Much of our information about the world comes from the light which our eyes and brains process. Light interacts with matter by transmission, absorption, or scattering. Light is a mixture of many different colors, even though it looks white. We know that during the day the primary source of light is the sun. Other common sources of light are flames, hot filaments, and glowing gases. Most everything we see is because of reflected light from these sources. We are able to see an object because it is either illuminated, reflects light, or it is luminous, produces light. This lesson will enable you to describe the dual nature of light, explain the relationship between the visible spectrum and the electromagnetic spectrum, and distinguish between transparent, translucent, and opaque materials.

What is the dual nature of light? In fact, light is both. This "wave-particle duality" is one of the most confusing—and wonderful—principles of physics. How fast is light? Why do you see the shadow of an airplane when it is close to the ground? What is special about polarized sunglasses? Why do wagon wheels in movies act weird? After completing this lesson, you will be able to explain these phenomena!

Light is everywhere in our world. We need it to see: it carries information from the world to our eyes and brains. Seeing colors and shapes is second nature to us, yet light is a perplexing phenomenon when we study it more closely. Here are some things to think about:

Our brains and eyes act together to make extraordinary things happen in perception. Movies are sequences of still pictures. Magazine pictures are arrays of dots. Light acts like particles—little light bullets—that stream from the source. This explains how shadows work. Light also acts like waves—ripples in space—instead of bullets. This explains how rainbows work. Scientists have spent lifetimes developing consistent physical, biological, chemical, and mathematical explanations for these principles. But we can start on the road to deeper understanding without all the equations by acting as scientists do: making observations, performing experiments, and testing our conjectures against what we see.

11.1.1 Quick Lab - Shadows

Problem: Why do shadows have different regions?

Procedure:

1. Arrange a light source so that a solid object casts a shadow on the wall or screen. Sketch the shadow formed, the relative positions of the object and the light source.

2. Move the light source away from the object while keeping the position of the object fixed. Sketch the shadow formed, the relative positions of the object and the light source. What happens to the size of the fuzzy region when the light source is moved farther away?

3. Move the object closer to the light source while keeping the light source and the screen in the same position. Sketch the shadow formed, the relative positions of the object and the light source. What happens to the size of the fuzzy region when the object is moved closer to the light source?

Observations:

Analysis:

1. Which relative positions of the object, screen, and light source result in a sharp distinct shadow with little or no fuzzy region around the edge? What is the name for this dark region?

2. Which relative positions of the object, screen, and light source result in a large fuzzy region around a small or nonexistent central dark region? What is the name for lighter (fuzzy) region?

3. What causes the fuzzy region around the edge of the shadow?

Submit your lab report to your Notebook.

11.1.2 Quick Lab – Polarization

Problem: What are the characteristics of polarized light?

Procedure:

1. Look through two polarized filters (sunglasses) at a light source. Hold one filter, the polarizer, steady while rotating the other filter, the analyzer. Observe and record the results. What is true at the point where the transmitted light is at a minimum?

2. Locate an object with a shiny surface that has a glare. Using one of the polarized sunglasses, look at the object while rotating the sunglasses. Observe and record the results. Look at other reflective objects. What happens to the glare?

3. Take a polarizing filter outside and observe light from many sources such as cars, pavement, clouds, water, etc.. Look at the sky facing north, east, south, and west. Observe and record your observations.

4. Fold a piece of cellophane so that the thickness varies (1 – 6 layers) and with transparent tape, tape it to a microscope slide. Using an overhead projector as the light source, hold the folded cellophane slide between the filters while rotating the analyzer. Record the observations. Try other transparent materials such as plastic tape dispensers, can holders, etc.. Squeeze or press on the transparent while rotating the analyzer. Record the observations.

Observations:

Analysis:

1. How far did you have to rotate the analyzer to go from maximum to minimum transmission?

2. Why are polarizers said to be crossed when the transmitted light is at a minimum?

3. What do your observations from procedure tell you about reflected light?

4. How are polarizing sunglasses different from ordinary sunglasses?

Submit your lab report to your notebook.

11.1.3 Light Waves Web Walk

The Physics Classroom (High School Physics Tutorial) “Light Waves and Color”.

Click on Lesson 1 “How Do We Know Light Behaves as a Wave” and complete Wavelike Behaviors of Light . Note the diagrams, the bold red terms, and the blue links.

Go to

and complete the lesson noting the animation.

Go to and complete Polarization.

Note the diagrams, the bold red terms as well as the blue links, and do the “Check Your Understanding”.

Go to ( Ohio State University – Chemistry Department) “Wave-Particle Duality of Light”. Note the diagrams/illustrations. Does light behave as a wave or as a particle?

Analysis:

Consider the following questions and submit your responses to your notebook.

1. Is light a wave or a stream of particles?
2. What is an electromagnetic wave?
3. What is the speed of an electromagnetic wave?
4. In a medium, what does the speed of an electromagnetic wave depend on?
5. Distinguish between non-polarized and polarized light.

11.1.4 Collaborate Reflection:

Respond to the reflection defending your position. Respond to at least two other people with a thoughtful comment.

What evidence can you cite to support the idea of the dual nature of light?

What evidence can you cite to support the idea that light can travel through a vacuum?

Knowledge Check

As an application of what you have learned in this lesson reflect on this idea and prepare an electronic presentation to communicate your ideas to others at a science seminar. Polarized sheets were first developed to cover car headlights so as to reduce the glare of an approaching car at night. How could this be accomplished, and what would be the best orientation of the polarized sheets? Don't forget that you still want to see the oncoming car, so the light should not be entirely blocked out. Will the tilt of the windshield make a difference?

11.1.5 Lesson Wrap Up – Quiz - The Nature of Light

Lesson 2: Color

The eye is sensitive to only a tiny portion, the visible spectrum (less than 0.1%), of the electromagnetic waves that radiate from objects. Scientific instruments have extended our view of the electromagnetic spectrum. The infrared detecting devices as well as the microwave and radio receivers allow us to explore the lower frequency end of the electromagnetic spectrum. The ultraviolet, X-ray, and gamma ray detectors let us “see” the higher frequency end. What we see without unaided eyes is a tiny part of what's out there in the world around us. The visible portion can be divided into thirds – a low frequency section that averages to red, a medium frequency section that averages to green, and a high frequency section that averages to blue. These three sections correspond to the three regions of color to which the retina of our eye is sensitive. Hence we see that the primary colors of white light are red, green, and blue.

Did you ever look at a beautiful painting or witness a gorgeous sunset and wonder, `How is it that I am able to see that?' What enables us to see the light and experience such wonderful shades of color during the course of our everyday lives? Some may take seeing for granted, but if the process is looked at closely, you can see what a wonder it really is.

11.2.1 Web Walk Color and Vision

Go to

Click on Lesson 2 “Color and Vision” and complete. The Electromagnetic and Visible Spectra Note the diagrams and the bold red terms.

Click NEXT to go to Light Absorption, Reflection, and Transmission and complete the lesson. Note the bold red terms, the blue links, the diagrams, check out the examples, and complete the “Check Your Understanding”.

Click NEXT to go to Color Addition and complete the lesson. Note the bold red terms, the blue links, the diagrams and complete the “Check Your Understanding”.

Click NEXT to go to Color Subtraction and complete the lesson. Note the bold red terms, the blue links, the diagrams, the examples, and complete the “Check Your Understanding”.

Click NEXT to go to Blue Skies and Red Sunsets and complete the lesson. Note the bold red terms, the blue links, and the diagrams.

Analysis:

Consider the following questions and submit your responses to your notebook.

1. What is the common color of tennis balls and why?
2. Compare and contrast color mixing by addition and by subtraction.
3. Why is the sky blue and sunsets red?
4. If you shine a red light on a red rose, why does the temperature of the leaves increase more than the temperature of the petals?
5. Television screens have red, blue, and green spots that are illuminated with different intensities of light to produce a full spectrum of colors. Which spots are activated to produce yellow? Magenta? White?

11.2.2 Quick Lab - Mixing Light Colors

Problem: Does the color we see when mixing light depend on the brightness of the primary colors?

Procedure:

. Go to and read about additve colors.

Go to

(Exploratorium Online Exhibits) Mix and Match. Follow the “To Do and Notice” directions. Play around with this activity and see how many colors you can create. Read “What's Going On?” after you have tried to match the color of the circle to the background a few times.

10. Go to read about subtractive colors

Go to:

(R. Truscio) Mixing Colors: Mix It!. Mix the buckets of paint to see what colors are formed when they are mixed.

Analysis:

Consider the following questions and submit your responses to your notebook.

1. What is a complimentary color?

2. Two primary colors will produce what color?

3. Why are the mixing of colors of light called additive and the colors of mixing pigments called subtractive?

11.2.3 Knowledge Check

Conceptual Challenge 1: You have probably noticed that the color of an object can appear different under different lighting conditions. Now is the time to reflect on the concept of seeing color. Blueprints are blue drawings on a white background. If a blueprint is viewed under blue light, will you be able to perceive the drawing? What will the blue print look like under yellow light? Submit your reflection in your notebook.

Conceptual Challenge 2: You probably have noticed that the medical staff in an operating room wear blue or green scrub suits rather than white. Why? Submit your reflection in your notebook.

11.2.4 Lesson Wrap Up – Quiz – Color

End Notes

Nicely done! You have just experienced another opportunity of “Physics at Work!”. Besides being pretty to look at, colors and the light they come from really do have the power to impact people in many ways. Along with the aesthetics of light and color, there is real science behind each and every sight we see. Each flash or ray of light, each shade of color that light makes visible, and each time our eyes receive the messages to see them, we are reminded of a special relationship—one that is often overlooked because we simply take seeing for granted. We miraculously experience a bright, vivid world because of the workings of our eyes, the wonders of light, and the brilliance of color . During the next unit we will continue to look at how light behaves.