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What gives color to objects? How do light waves interact with materials to produce red and green or white

and black? In this lesson, learn how white light is composed of all colors and how absorption and

reflection influence our color perception.

Introduction To Color

What makes a plant green? What gives a male cardinal his bright red feathers? Why is an orange, well,orange? All of the colors in the visible light spectrum are characterized by different frequencies. Green-

colored light lives between 540 and 610 Terahertz. For red, the frequencies are a bit lower. For violet,

they are higher. But what determines the color of an object? How do the frequencies get divided out

between red, green, and violet objects? To answer these questions, we first need to look into the

mysteries of white light.

White Light And The Color Spectrum

If you pass a beam of white light through a prism, you will see the light split into the colors of the

rainbow. There's red, orange, yellow, green, blue, and violet all spread out. You can also see a rainbow

when sunlight passes through a crystal decoration or even the peephole in your front door. Sunlight and

other white lights are actually made of many different frequencies. The sun emits infrared and ultraviolet

waves in addition to the full spectrum of visible light. We don't see the infrared and UV waves. But we

do see the full range of visible color, all bundled together in the form of white light. White lightis the

combination of many different frequencies of visible light from all parts of the visible spectrum. The only

way we can see these frequencies as separate colors is to separate them into a rainbow.

So if sunlight is white light, and

sunlight reflects off of objects

like plants and birds and

oranges, then why don't we see

those objects as white? It turns

out that the different color

frequencies are absorbed and

reflected differently in every

different object. Let's learn more

about selective absorption and

how it relates to color.

Selective Absorption And Reflection

Let's say you've got a nice ripe orange in your hand. You walk outside, and the sunlight beams down onthe orange from above. The sunlight is white light, so it contains all the different frequencies in the

visible light spectrum. Many different waves of light beam down on the orange, each wave a different

frequency. When a light wave strikes any object, it can do one of three things: it can be transmitted, it

can be absorbed, or it can be reflected. We know that none of the light waves are transmitted through

the orange. If they were, then the orange would appear transparent. Since we can't see through

oranges, that means they are opaque, and that means all the light must be reflected or absorbed.

Sunlight emits the full spectrum of visible light in the form of white light.

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Absorption of a light wave occurs through resonance. When the frequency of a light wave matches the

resonant frequency of an object, then the object vibrates at that frequency. The energy of the light wave

stays in that object as thermal or vibrational energy. In other words, you never see that light wave again!

Let's take this one little light wave here, which happens to be the frequency of the color violet. When this

violet light wave strikes the orange, there are some molecules in the orange that resonate at that

frequency. The violet light wave is absorbed by those molecules, so we never see the violet reflecting

off the orange's surface. What about green? When a green light wave strikes the orange, it also

resonates with some of the molecules in the skin. It gets absorbed, and so we don't see green, either. In

fact, most of the frequencies inside the white sunbeam are absorbed by the orange. The only frequency

that ISN'T absorbed is that of the color orange!

So what is different about the orange

light? Instead of being absorbed into the

orange like the other colors, orange light

is reflected by the skin of the fruit. In

reality there are a range of frequencies for

the color orange. Some waves are more

reddish-orange and some are more

yellowish. But all those frequencies are

reflected together, giving the orange the

appearance of being orange. The

appearance of color is due to the

selective absorption of light waves. Orange wasn't the only color to be beamed down on the fruit. But it

WAS the only color to be reflected. Selective absorptiondescribes the tendency of an object to absorb

some frequencies of light more than others. An object that appears a certain color reflects the light

frequency that corresponds to that color, and it absorbs all the other frequencies in the visible light

spectrum. An orange absorbs all of the frequencies but orange. A banana absorbs all of the frequencies

except for yellow.

Pigments In Living Things

Selective absorption occurs in just about everything that has color. It happens because of the specific

properties of the molecules that make up objects. The molecules in red paint are slightly different from

the molecules in blue paint. The molecules in red feathers are different from the molecules in blue

feathers. In fact, most living things use special chemicals called pigmentsto make their bodies appear

a certain color. A pigment is a chemical that alters the color of a light wave by selectively absorbing one

or more light frequencies. The feathers of a male cardinal are made red by special organic pigments

called carotenoids. Birds get carotenoids from the plant materials they eat. They can make red, yellow,

or orange colors in their feathers this way. Another pigment they use is melanin, the same pigment that

colors human skin.Melaninis used to make the blacks, browns, and tans in the feathers of birds and

the hair of mammals. Structures that are totally black absorb all of the frequencies of light. In the case of

white structures, there ARE no pigments. So white feathers and white hair appear that way because

ALL frequencies of white light are reflected from their surfaces.

Colored light waves are either reflected or absorbed by an object.

http://education-portal.com/academy/lesson/what-is-melanin-definition-production-function.htmlhttp://education-portal.com/academy/lesson/what-is-melanin-definition-production-function.htmlhttp://education-portal.com/academy/lesson/what-is-melanin-definition-production-function.htmlhttp://education-portal.com/academy/lesson/what-is-melanin-definition-production-function.html

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One very important biological pigment is the plant pigment chlorophyll.Chlorophyllis what gives plants

their green color. More importantly, it's what allows them to absorb the sun's energy and make food

through the process of photosynthesis. Chlorophyll is found mostly in the leaves and stems of plants. It

absorbs a huge amount of energy from white sunlight. It absorbs the waves in the red frequencies, and

the blue, and the violet, orange, and yellow frequencies. But it doesn't do a great job of absorbing light

in the green frequencies. So the green frequencies are reflected instead! And that's why plants look

green.

The common theme

here is that color is

not something that

exists in objects. We

perceive things to be

a certain color

because of the wayour eyes sense the

various frequencies

of light reflected off of

them. Objects that appear red do not contain the color red. They only contain the molecules that affect

the visible light waves so that only red is reflected. Therefore, coloris the perception of a visible light

wave's frequency.

Lesson Summary

White light is a combination of all the frequencies of visible light. When white light strikes an object,

each individual frequency of light is transmitted, reflected, or absorbed, depending on the properties of

the surface molecules. If all frequencies are absorbed by the object, then it appears black. If all

frequencies are reflected, then it appears white. Selective absorption describes how some frequencies

of light are absorbed while others are reflected. This results in the appearance of color. Pigments are

natural or man-made chemicals that give color to objects through selective absorption. While we often

describe objects as 'being' a certain color, the truth is that color is only our perception of the light

frequencies reflected off of objects.

http://education-portal.com/academy/lesson/color-white-light-reflection-absorption.html#lesson

Chlorophyll is one type of pigment that can determine what color waves are absorbed.

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