Chapter Twenty-Five: Light

• 25.1 Properties of Light• 25.2 Color and Vision• 25.3 Optics

25.1 Properties of light• Light travels fast over long

distances and carries energy and information.

• Light travels in straight lines, but can be bent by lenses or reflected by mirrors heat and warmth.

• Light has color and can be bright or dim.

The electromagnetic spectrum• Light, like sound and heat, is a form of energy. • The visible light we see is part of the

electromagnetic spectrum.

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The Electromagnetic Spectrum

• Visible light is a very small part of the electromagnetic spectrum.

Where does sound fit into the electromagnetic spectrum?

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Properties of light• You see book pages because light in the

room reflects from the page to your eyes. • Your eyes and brain use the information

carried by the light to make a mental picture.

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Light is produced by atoms• Most light is produced by atoms.• When you put some energy into the atom,

it excites the atom’s electrons.• Light is produced when the electron

releases this energy.

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Incandescent light

• Making light with heat is called incandescence.

• Atoms in the filament convert electrical energy to heat and then to light.

• Incandescent bulbs are inefficient, but their waste heat can be useful.

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Fluorescent light

• To make light, fluorescent bulbs use high-voltage electricity to energize atoms of gas in the bulb.

• These atoms release the electrical energy directly as light (not heat), in a process called fluorescence.

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Color and energy• When all the colors of the rainbow are combined,

we see light without any color. • We call the combination of all colors white light.

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Color and energy• Compare the hot, blue flame from a gas stove to

the orange flame of a match.• The light from a gas flame is blue (high energy)

and the light from a match is red-orange (low energy).

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25.1 The speed of light• The speed at which light travels through air is

about 300 million meters per second. • The speed of light is so important in physics

that it is given its own symbol, a lower case “c”.

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25.1 Speed of light• The speed at which electromagnetic waves

travel through air is about 300 million meters per second.

• The speed of light is so fast that when lightning strikes a few miles away, we hear the thunder after we see the lightning.

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Wavelength & Frequency of Light

• Because the wavelength of light is so small, scientists measure it in nanometers.

• One nanometer (nm) is one billionth of a meter (0.000000001 m).

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Wavelength and Frequency of Light

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• One THz is one trillion Hz. • 1,000,000,000,000 Hz

What kind of wave is light?

• A sound wave is a oscillation of air. • A water wave is an oscillation of the surface of

water. • An oscillation of electricity or magnetism

creates electromagnetic waves.

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Electromagnetic waves

• If you could shake the magnet up and down 450 trillion times per second, you would make waves of red light with a frequency of about 450 THz.

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Electromagnetic spectrum• The entire range of electromagnetic waves,

including all possible frequencies, is called the electromagnetic spectrum.

• This spectrum includes visible light and invisible waves:– radio wave– microwaves– infrared light– ultraviolet light– X-rays– gamma rays

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Electromagnetic spectrum

End Section 25.1 Begin Section 25.2

The human eye

• The eye is the sensory organ used for vision.

• The retina contains light-sensitive cells called photoreceptors.

• Photoreceptors convert light into nerve impulses that travel through the optic nerve to the visual cortex of the brain.

Photoreceptors

• The human eye has two types of photoreceptors: cones and rods.

• Cones respond to color and rods respond to the intensity of light.

• Rod cells “see” black, white, and shades of gray.

How we see color

• Our eyes work according to an additive color process – 3 photoreceptors (red,

green, and blue) in the eye operate together so that we see millions of different colors.

Cone Cells

• There are three types of cone cells.

• One type responds best to low-energy (red) light.

• Another type responds best to medium-energy (green) light.

• The third type responds best to higher-energy (blue) light.

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Cones • If the brain gets a signal

from only a green cone, we see green.

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• If there is a strong red signal and a weak green signal, we see orange.

• All of the light is still there. We just see orange.

The Additive Primary Colors • They're called primary because any color can

be made from a suitable combination of red, green, and blue.

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White object

29Green object

Red object

Black object

An object appears the color it reflects!

Reflection• An object is the color of the light reflected.• Red reflects red light.

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Reflection• Blue reflects blue light.

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Reflection• Blue light on a red object.

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White light

• Illuminated by white light (a mixture of all colors) all parts of the girl's clothing show their colors because each reflects its own part of the spectrum.

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Red Light

• Illuminated only by red light, the girl's red top shows up red because it reflects red light. Other clothing absorbs red light, so reflects nothing (and therefore looks black)

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Complementary Colors • Red + Green = Yellow• Red + Blue = Magenta• Green + blue = Cyan • (Red + Green) + Blue = White

• Colors which combine to produce white are said to be complementary.

• Therefore, yellow and blue are complementary colors.

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Additive Color Mixing (mixing of light)

Wavelength & Frequency of Light

25.2 Making an RGB color image

• A television makes different colors by lighting red, green, and blue pixels in different proportions.

• Color images in TVs and computers are based on the RGB color model.

Red Green WhiteBlue

• TV’s• Computer screen• Spot lights

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25.2 Subtractive color process

• A blue shirt looks blue because it reflects blue light into your eyes.

• Chemicals known as pigments in the dyes and paints absorb some colors and reflect other colors.

25.2 The CMYK color process

• The subtractive color process is often called CMYK for the four pigments it uses.

• CMYK stands for cyan, magenta, yellow, and black.

25.2 The CMYK color process

The CMYK color process

• The full color image is a combination of all four images!

Mixing Colored Pigments

1. Magenta: reflects red and blue; absorbs green.

2. Yellow: reflects red and green; absorbs blue.

3. Cyan: reflects green and blue; absorbs red.

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Mixing Colored Pigments

4. If magenta, yellow, cyan are combined, the picture looks like this.

5. Addition of black.

6. Combined finished results.

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25.2 Why plants are green

• Plants absorb energy from light and convert it to chemical energy in process called photosynthesis.

• Chlorophyll is the main pigment of plants absorbs red and blue light and reflects green light.

25.2 Why plants are green

• Plants must reflect some light to avoid absorbing too much energy.

• A plant will die if placed under only green light!

Dustyn McKnight