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Today’s Topics

• Light– What is it?– Visible Spectrum

• Why do we see color?

• Why are objects specific colors?

http://priory.net/~fyamasaki/images/rainbow_sun.jpg

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Why is light important?

• Almost everything in nature depends on light for life:– Plants use light to

make food as a byproduct oxygen is produced.

– Humans use light to see, make X-Rays, as electricity (solar energy), and many other things. http://home.howstuffworks.com/irrigation1.htm

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What is Light?

• Light is a form of energy that sometimes behaves like waves.– All waves have a

wavelength which is the distance between one point on one wave and the same point on the next wave.

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Wavelengths of Light

• Different wavelengths of light include radio waves, ultraviolet rays, and X-rays.

• These wavelengths of light and others make up a spectrum.

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Visible Spectrum• Shorter wavelengths

have more energy than longer ones.

• Sunlight, which is the most important source of light, contains all of the electromagnetic spectrum.– Humans can only see

a small range of light called the visible spectrum.

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Visible Spectrum• The visible spectrum

contains seven colors: violet, indigo, blue, green, yellow, orange, and red.

• The seven colors of the visible spectrum are the seven colors in a rainbow.

http://ldt.stanford.edu/ldt1999/Students/tita/mjrproj/color/wavereview.html

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Colors in a Rainbow

• How can I remember the seven colors of the rainbow?

RROOYY GG BBIIVVED

RANGE

ELLOW

REEN

LUE

NDIGO

IOLET

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Colors in a Rainbow

• Another way to remember: Make a sentence.– Example:

Rainbows Over Your Garden Bring Ideal Vegetables.

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White Light

• White light contains all colors of the rainbow.

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Why do we see color?

• When our eyes see different wavelengths of light in the visible spectrum, we see colors.

• How do humans see color? – What we actually see is the

light an object reflects or in some cases the light an object produces.

– Our eyes absorb the wavelengths of light in the visible spectrum. Then, turn these wavelengths into signals our brains can understand. The brain interrupts these signals as color images.

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Why are objects a specific color?

• The grass appears green, because the leaves reflect green light. The leaves do not absorb (soak up) this wavelength of light and reflect it back (like a mirror), so we see the grass as green.

• On the other hand a green stoplight appears green because it produces green light.

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Sun’s R

ays (W

hite

Light)

Green Wavelength of

light

Absorbs all other colors

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White and Black Color

• White Color– When you see white, all of the

wavelengths of light (all colors) are being reflected back at you.

• Black Color– When you see black, all of the

wavelengths of light are absorbed and no light is reflected back at you.

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Review• What is light?

– Light is a form of energy that sometimes behaves like waves.

• What is a wavelength?– Distance from crest to crest or trough

to trough.

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Review• What are the colors of a rainbow?

RROOYY GG BBIIVVED

RANGE

ELLOW

REEN

LUE

NDIGO

IOLET

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Review• Why do we see color?

– You see different wavelengths of light as color.

http://ldt.stanford.edu/ldt1999/Students/tita/mjrproj/color/wavereview.html

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References• Barkstrom, Bruce R. August 18, 2005. What Wavelength

Goes With a Color?. 2005 August 18. <http://eosweb.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html>.

• Brown, Earle B. “Light.” The World Book Encyclopedia. 1986 ed.

• Gourdeau, Justine. November 3, 2003. Clouds and Particles: Basics – Rainbows. 2005 August 18. <http://www.atmosphere.mpg.de/enid/f962eee1b06705f8f51b4db2901f8668,55a304092d09/3__Sun_and_clouds/-_Rainbow_tj.html>.

• McGrath, Susan. Fun With Physics. Washington, D.C.: The National Geographic Society, 1986.

• Wyszecki, Gunter. “Color.” The World Book Encyclopedia. 1986 ed.