Chapter 27

Color

1. SELECTIVE REFLECTION

• Most objects "reflect" rather than emit light.

• The spring model of the atom works well in explaining reflection.

• Radiations that match the resonant frequencies of the atoms are absorbed.

• Frequencies of the radiations on either side of the resonant frequencies are “reflected.”

• Objects can only reflect the light that is in

the source illuminating the object.

• Demo – Razorback Football in Cyan Light Demo – Razorback Football in Cyan Light (Next Slide) (Next Slide)

2. SELECTIVE TRANSMISSION

• As light passes through materials some frequencies of light are removed (absorbed) while other frequencies are transmitted.

• The degree of transmission depends on how transparent the material happens to be.

• Color filters are good examples of selective

transmission.

• Demo – Color Filters and White LightDemo – Color Filters and White Light

3. MIXING COLORED LIGHT

• All visible frequencies make up white light.

• Example: The sun emits all frequencies and its

light is white.

• (Actually it is slightly yellowish to us on Earth,

which possibly explains why we are more

sensitive to light in the middle of the spectrum.)

Color Addition

Through color addition you are able to see a wide

range of colors from a color TV or color projector

which actually only emit three different colors.

These colors are red, green, and blue.

They are called the additive primaries.

Your vision system “adds” these together to see

single colors from a single location illuminated by

more than one color.

You even see colors that don’t appear in the

continuous emission spectrum of the sun.

Red, green, and blue are used as the additive

primaries because this set of three will produce the

widest range of colors that you visually

experience.

On the next slide you will see what happens as you add colors to produce other colors.

Note that cyan is the addition of green and blue.

Note that yellow is the addition of red and green.

White

Red

Green

Blue

Yellow

Cyan

Magenta

Colors in White Light

Note that magenta is the addition of red and blue.

You can see that these three add to give white.

To summarize, see color addition circles on next slide.

Color Addition Circles

This is what you would get with three partially overlapping spotlights reflecting off of a white screen.

Complementary Colors

• Any two colors that add to give white are

said to be complementary colors.

• Demo - Complementary ColorsDemo - Complementary Colors

4. MIXING COLORED PIGMENTS

• Subtractive primaries - YELLOW,

CYAN, and MAGENTA

• Example - Mixing paints, zip-lock

sandwich bags, color printing

• Demo - Color SubtractionDemo - Color Subtraction

• Overhead - Foretravel AdvertisementOverhead - Foretravel Advertisement

Color Subtraction

Through color subtraction you are able to see a

variety of colors from printings, paintings, etc.

If you have ever bought printer inks, you will

notice that the ones used to provide a variety of

colors in printing are yellow, cyan, and magenta.

They are called the subtractive primaries.

In subtraction, colors are eliminated by the

absorption of colors that were in the original

illuminating source.

This particular set of three colors, yellow, cyan,

and magenta, will produce the widest range of

colors that you visually experience.

On the next slide you will see what happens as you remove different colors from white light.

White

Yellow

Take away yellow and what is left?

You get blue.

Blue

Colors in White Light

WhiteRed

Take away cyan and what is left?

Cyan

You get red.

Colors in White Light

Magenta

White

Take away magenta and what is left?

You get green.

Green

Colors in White Light

To summarize, see color subtraction circles on next slide.

Color Subtraction Circles

This is what you would get with three partially overlapping transparencies on an overhead projector.

Slide - Raincoat ClosetSlide - Raincoat Closet

• It should be noted from the previous that objects that reflect a particular color are themselves good absorbers of the complimentary color of that particular color.

• For examples:

• A red object is a good absorber of cyan and vice versa.

• A blue object is a good absorber of yellow and vice versa.

• A green object is a good absorber of magenta (blues and reds) and vice versa.

5. WHY THE SKY IS BLUE

• One man’s view.• Just as resonating tuning forks scatter sound, so do

particles in our atmosphere scatter light.

• N2 and O2 scatter high frequencies which are near natural frequencies of N2 and O2.

• (Natural frequencies are in the UV.)• This scattering produces the bluish sky.• The blue end of the spectrum is scattered ten times better

that the red end.

Blue in this direction

Sun

Earth

Top of Atmosphere

6. WHY SUNSETS ARE RED

• If the atmosphere becomes thicker or the paths

of light through the atmosphere become longer,

more of the longer wavelengths of light will be

scattered.

Sunset

• Demo - Blue Sky and Red SunsetDemo - Blue Sky and Red Sunset

• Because of scattering of blue light the sun

appears more yellowish at noon than it really is.

Earth

Sun

Sun

7. WHY CLOUDS ARE WHITE

• Droplet size dictates which colors are

scattered best.

• Low frequencies scatter from larger particles.

• High frequencies scatter from small particles.

• Electrons close to one another in a cluster

vibrate together and in step, which results in a

greater intensity of scattered light than from the

same number of electrons vibrating separately.

• Large drops absorb more and scatter less.

8. WHY WATER IS GREENISH BLUE

• Water quite often looks bluish.

• This is due to reflected “sky light.”

• A white object looks greenish blue when

viewed through deep water.

• Water is a strong absorber in IR and a little in

red.

• Remove some of the red and cyan is left.

• Crabs and other sea creatures appear black in

deep water.

9.9. COLOR VISION ANDCOLOR VISION ANDCOLOR DEFICIENCYCOLOR DEFICIENCY

• Colorblindness (color deficiency) affects

about 10% of population

• Red-green is predominant• Yellow-blue - a few

• Total – some

• Slide – Colorblindness Tests – Slide – Colorblindness Tests – URL

10. AFTER IMAGES

• Slides - After Images - After Images

• After images are due to conal fatigue.

• Cones that have been “firing” for a while will

not “fire” as well as “rested” cones when all

are exposed to white light.

Chapter 27 Review Questions

Most of the light that we see has undergone

(a) selective interference

(b) selective transmission

(c) selective reflection

(d) selective refraction

(c) selective reflection

A mixture of magenta and green lights give white light. These two colors are

(a) additive primaries

(b) secondary colors

(c) complementary colors

(d) fluorescent colors

(e) interference colors

(c) complementary colors

Mixing yellow paint and magenta paint gives what color?

(a) red

(b) green

(c) blue

(d) cyan

(a) red

What color would red cloth appear if it were illuminated by cyan light?

(a) cyan

(b) red

(c) yellow

(d) green

(e) black (e) black

The sky is blue because air molecules in the sky act as tiny

(a) mirrors which reflect only blue light

(b) resonators which scatter blue light

(c) sources of white light

(d) prisms

(e) none of these

(b) resonators which scatter blue light

When you stare at a red object for a long time without moving your head and eyes and then suddenly look away at a white screen, you will see a               image of the object.

(a) red

(b) blue

(c) cyan

(d) green

(e) white

(c) cyan

To Chapter 28