A First Course in Physics · Ch. XXI, §§ 565–574 · Chapter 21: Color Phenomena
66. Color and wave length; mixing colors
Goal: Your student can explain that each color is a wave length, that white light is a mixture, why objects have color, the difference between mixing colored lights and mixing paints, and why soap films are colored.
You'll need
- The book, open to these pages
- A glass prism, or a glass of water on a sunny windowsill, or an old CD
- Red, green and blue cellophane or plastic and a flashlight
- Crayons or paints (yellow and blue)
- A cardboard disk, markers and a pencil (a Newton color wheel)
- Soap-bubble solution
- A notebook and pencil
What's in the book
Soap-film fringes in red and green light show red has the longer wave; a table of wave lengths of the colors; a prism spreads white light into a spectrum; the color of a body is the light it does not absorb; colors under colored light; compound colors and Newton's color disk; complementary colors; retinal fatigue (afterimages); mixing pigments subtracts colors; the colors of thin films. Seven questions follow.
The lesson, step by step
Hook question
SayA red apple under a pure blue light looks almost black. Where did its red go?
Read together
Read §§ 565–574. The rule for colored objects:
the color which a body has in ordinary daylight is determined by the wave lengths which the body has not the power of absorbing.
Try it: make a spectrum
Set a glass prism (or a glass full of water) in a sunny window and find the band of colors on the floor or wall: red, orange, yellow, green, blue, violet (§ 567). A CD tilted in sunlight also spreads the colors. Never look at the Sun itself.
Try it: colored light
Cover a flashlight with red cellophane and shine it on a red, a blue and a white object in a dark room: red looks bright, white looks red, blue looks almost black (§ 569). Now stare for 30 seconds at a bright red square and then at white paper: a greenish-blue afterimage appears (retinal fatigue, § 572).
Try it: lights versus paints
Color a cardboard disk half yellow and half blue, push a pencil through the center and spin it like a top: the colors blend toward gray or white, because spinning adds the lights (§ 570–571). Now mix yellow and blue paint: green, because each paint subtracts colors (§ 573).
Explain the idea
Red waves are longest (the book's table: red about .000068 cm), violet shortest (about .000042 cm). Soap films and oil slicks are colored because each wave length cancels at a different film thickness (§ 574). Blow a bubble and watch its colors swirl.
What we know now: wave lengths of light are usually given today in nanometers: the book's .000068 cm is 680 nm. Screens make every color by adding red, green and blue light; printers subtract with cyan, magenta and yellow ink.
Say it back
Your student explains why yellow and blue lights can make white but yellow and blue paints make green.
Questions
Work questions 1–7 on book page 450, using the table on page 444.
Afterwards
Look closely at a phone or TV screen with a magnifying glass: you will see the tiny red, green and blue dots that make every color.
Tip
Colored cellophane from gift wrap or candy wrappers works well; stack two layers if the color is pale.
Answers to the book's problems
1. The yellow fringes (wave length .000058 cm) are closer together than the red (.000068) and farther apart than the green (.000052). 2. Nearly black. 3. The blue glass lets only blue light through. 4. Nearly black, since blue glass absorbs a pure yellow. 5. The blue and violet. 6. Purple (the red, blue and violet left over). 7. Thinner (where the film is thin the short blue waves are destroyed and red is left).
Worked out for this site from the scan. If a number in your copy differs, trust the book.