Light and Color Chapters 27 – 28 Created by N. Ferreira with the help of A, Kirby

27.1 Early Concepts of Light Initially, 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 Light The 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.

Reflection and Light If a material absorbs all the light that shines on it and reflects none it appears black The appearance of a colored object depends on the kind of light used. A candle flame emits light that is deficient in higher frequencies; it emits a yellowish light. An incandescent lamp lamp emits light of all the visible frequences but richer toward the lower enhancing reds. A fluorescent lamp is richer in higher frequencies, so blues are enhanced.

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 light 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. When light enters a new medium its color does not change because its frequency does not change. What changes is the speed and wavelength of the light.

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 additive 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 colors Magenta and green are complementary colors Cyan 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.