Light and ColorChapters 27 – 28

Created by N. Ferreira with the help of A, Kirby

27.1 Early Concepts of LightInitially, light was thought to be made of tiny

particles because light moves in straight lines.

Later, the wave theory of light was developed. The evidence that supported the wave theory was that light will spread out (diffract).

Currently, light is believed to have a dual nature - part particle, part wave

27.2 The Speed of LightThe speed of light is 3 x 108 m/s or

187,000mi/s.

The speed of light is constant when it travels through one type of medium. Light travels faster in air than in water.

Light takes 8 minutes to travel from the sun to the earth.

27.3 Electromagnetic Waves• Light is energy that is emitted by

accelerating charges – often electrons in atoms.

• The energy travels in a wave that is partly electric and partly magnetic. These waves are called electromagnetic waves.

• The range of electromagnetic waves is called the electromagnetic spectrum.

The Electromagnetic Spectrum• All of the electromagnetic waves are the same in

nature, differ only in the wavelength and frequency

• Saying to help you remember the EM spectrum:

Ronald McDonald Is Very Ugly X-tra Gross

Notice: Visible light makes up very small portion of the EM spectrum

• Red light is the lowest frequency of light we can see.

• Violet light is the highest frequency light we can see.

• Infrared waves are lower in frequency than red light. Heat lamps give off infrared radiation.

• Ultraviolet waves are higher in frequency than violet light. UV rays from the sun cause sunburn

Transparent, Opaque and Shiny• Transparent materials allow light to pass

through

• Opaque materials do not allow light to pass through. When light hits opaque materials, the light energy is converted into heat.

• The electrons of shiny materials, like metals, are too far apart to transfer energy so the energy is re-emitted as light.

Shadows

• Shadows are places where light rays do not reach.

• Two types of shadows:

1) Umbra total shadow where all light is blocked

2) Penumbra partial shadow

• Solar eclipses do not occur as often as lunar eclipses because the sun is much larger than the moon.

27.7 Polarization• Polarized light travels in only one direction.

• Unpolarized light travels in many directions.

• A polarizing filter polarizes light.

• Light passes through a pair of polarizing filters when their axes are aligned, but when the filters are crossed at right angles, no light will pass through.

Demo: Polarization

28.1 The Color Spectrum• Sunlight is a mixture of all the colors of the

rainbow.

• Sunlight is an example of white light. White is not a color, but a combination of all colors.

• Black is also not a color, but is the absence of light. An object appears black when all the frequencies of light are absorbed.

28.2 Color by Reflection• Most material absorb light of some

frequencies and reflect the rest. We see the reflected light.

28.3 Color by Transmission• The color of a transparent object depends

on the color of light it transmits.

• A piece of blue glass will appear blue because it absorbs all colors of except blue.

• The material that selectively absorbs colored light is known as pigment.

28.4 Sunlight• Yellow – green light is the brightest part of

sunlight. This is the color human eyes are the most sensitive too.

28.5 Mixing Colored Light• Light of all the visible frequencies mixed

together produces white light.

• White light can also be produced by mixing red, blue, and green light. Red, blue, and green are the 3 primary colors of light.

• Color T.V.s produce all the colors we perceive by combing red, blue, and green light in a variety of ways.

Adding Primary Colors

• Red + Green = Yellow• Red + Blue = Magenta (Purple)• Blue+ Green = Cyan (Aqua)• Red + Blue + Green = White

28.6 Complementary Colors• When two different colors add together to

form white, they are called complementary colors.

Yellow + Blue = White(Red + Green)

Magenta + Green = White(Blue + Red)

Cyan + Red = White(Blue + Green)

Yellow and blue are complementary colorsMagenta and green are complementary colorsCyan and red are complementary colors

28.7 Mixing Color Pigment• When pigments are mixed to produce a

different color this is called color mixing by subtraction.

• The three primary subtractive colors are

1) magenta

2) yellow

3) cyan

28.8 Why the Sky is Blue

• Small particles (N2 & O2) scatter light of high frequency such as blue

28.9 Why Sunsets are Red• When the sun sets, sunlight reaches us

through a long path so light of long wavelengths (red) reach us

Why Clouds are White• Clouds are white because water

molecules in the clouds scatter different colors of light.

28.11 The Atomic Color Code• Every element has its own characteristic

color when made to emit light.

• The glow of each atom is unlike the glow of any other element.

• When light from a glowing element is analyzed through a spectroscope, it is found that the colors are a composite of a variety of different frequencies.