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Warm Up. Where does all light come from? Do different colors of light have different intensities? If so, what color has the greatest intensity? When two of the same colors interfere each other, what color is produced?. What are the three primary colors of light?

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warm up
Warm Up
  • Where does all light come from?
  • Do different colors of light have different intensities? If so, what color has the greatest intensity?
  • When two of the same colors interfere each other, what color is produced?

What are the three primary colors of light?

What color do we get when we mix those three colors?

What colors is white light composed of?


The different colors (frequencies) of light travel at different speeds through glass. Red travels through faster so it is ‘bent’ less than the other colors. Violet travels through the slowest so it is ‘bent’ the most. The other colors are in between as shown.


If white light is passed through one prism, it is separated into all the colors of visible light. If the rainbow of colors is then passed through another prism, it goes back to white light.


Mixing Colors of Light: Color by Addition

The three primary colors of light arered, greenandblue. When mixed in equal intensities they produce:

red + green + blue = white

red + green = yellow

red + blue = magenta

green + blue = cyan


Mixing Colors of Light for Color TV

A beam inside the TV scans very rapidly across the back of the TV screen and lights up a tiny pixel of a particular color (red, green or blue). When the colors are seen together in an area they appear as all one color.


Each beam lights up a tiny pixel of a particular color.

We see the tiny pixels all together as one color.


Different types of light bulbs create light with slightly different frequency ranges. Incandescent light bulbs are richer in the lower frequencies of light so objects under incandescent light will look slightly _____________.

Fluorescent light is richer in the higher frequencies, so objects under fluorescent light will look slightly _______________


Sapphire in incandescent light.

Sapphire in fluorescent light.


But look what happens when we shine different colors of light on the flowers. Can you tell what color the flowers are now? Can you see the real color of the leaves?

Why not?


What color are the roses down in the right hand corner?


What color could the flowers that look green be?



What color could the flowers that look red be?


What color do the leaves appear to be?



Were you right about the colors you thought the flowers were? What color of light is shining on the flowers now?


The three secondary colors of light are yellow, magenta and cyan.

Which color of flowers show up best in the yellow light?… in the magenta light?… in the cyan light? Why?


Complementary colors are…

yellow + blue = white

green + magenta = white

red + cyan = white

Why does it work out

this way?


Remember that mixing colors of light is called color by addition. To create a new color, another color of light is added. An example is adding equal intensities of blue light to green light to produce cyan.

But how are colors other than the three primary and three secondary colors produced?

By changing the intensities of each of the colors, still more colors can be produced.



When light hits an object, what three things can happen to that light? Does just one of these happen on a particular object, or does a combination occur?


Now let’s look at color by subtraction. Color by reflection is a type of color by subtraction.

An object appears a particular color because of the colors it reflects.


Why does a blue object look blue? What colors are being absorbed and what colors are being reflected?


How can objects appear yellow, orange, magenta or some other color that is not a primary color of light?

More than one color is reflected.

Why do some objects appear black?


If almost all the color is absorbed then an object appears black.

What would happen if absolutely all the color was absorbed by an object?


Which square reflects the most colors? What are the colors reflected?

Which square absorbs the most colors. What are the colors absorbed?

Which square would become warmer if left out in the sun? Why?


Normally daylight shines on a red object, it reflects the red light so it appears red. But what would happen if another color of light was shining on the object?

If green light was shining on a red object what color would it appear to be? Why?


The object would appear black because a red object absorbs green light and there is no red light there to be reflected.


Color by Transmission; Another Type of Color by Subtraction

An object appears a particular color because of the colors of light that come through the object. This is why glass can be different colors.


Remember that sunlight contains all the colors (frequencies) of visible light. However, the intensity (brightness) of each frequency is not the same.


Which colors are most intense in sunlight?

What color are humans most sensitive to?


You should have noticed that the lower frequencies (which are…?) and higher frequencies (which are…?) are not as bright as the yellow-green frequencies.

Humans are most sensitive to yellow-green. Why would some fire trucks now be yellow-green?


Mixing Colors of Pigments: More Color by Subtraction

When colors of inks, paints or dyes are mixed, only the colors reflected are seen. Cyan, magenta and yellow are often called the subtractive primary colors.

cyan + magenta + yellow = black

cyan + yellow = green

cyan + magenta = blue

magenta + yellow = red

The colors that are absorbed are subtracted from the light shining on a object, so are not seen.


White light is shining on each of these pieces of paper. Each paper has been printed with one or more colors of ink.

Look at each one and decide which colors of light are absorbed (subtracted) and which are reflected. What color will each paper appear? Why?

There is a monster that only eats yellow M&Ms…and you, what color would the monster “absorb”? “Reflect”?
  • What if you were out of yellow M&Ms, and the monster were hungry. What could you do with light to “make” yellow M&Ms?

So how are these colors of pigments, the subtractive primaries, used to print a book with color photographs?

By overlapping layers of yellow, magenta, cyan and black ink a color photo can be printed.


The layers of ink are added to create the color image. Check your printer at home.

It will have yellow, magenta

cyan & black.

This can be demonstrated using each color of transparency on our overhead projector.


Pictures in a magazine or newspaper use dots of cyan, magenta, yellow and black to make pictures of all colors. A computer printer does the same thing.


Compare color by addition and color by subtraction.

This is color by ___________. All the colors combine to create ___________. Colors of _______ are combining.

This is color by ___________. All the colors combine to create _________. Colors of _____ are combining.


The human eye contains three different types of color receptor cells called cones. Each cone cell is sensitive to a different color, either red, green or blue.


If the cones don’t function properly, then a person might be color blind. A color blind person won’t be able to see the number in this circle.


Why is the sky blue?

Molecules & small particles in the earth’s atmosphere scatter the shorter wavelengths of light the most.


Why are clouds white?

Water droplets are of all sizes so all wavelengths of light are reflected.


Late or early in the day, the sunlight must go through more atmosphere to reach us so more short wavelengths are scattered leaving mostly reds.


The electrons within an atom can be made to emit light but it will be for a specific color for each element in its pure gaseous state. For example, neon gas emits a brilliant red light when energized with electricity. Mercury vapor emits bluish violet light and helium gas glows pink.

These colors can be analyzed to see just what frequencies of light each element is emitting.


An instrument called a spectroscope is used to split the light into its different frequencies.

Each element has its own unique set of frequencies which can be used to identify the element.

Here we see; the sun, hydrogen, helium, mercury & uranium spectra (top to bottom).