UNIT 6: Light and Optics Chapter 16: Light and Color Chapter 17: Optics Chapter 18: Wave Properties of Light 2

Chapter 16 Light and Color 16.1 Properties and Sources of Light 16.2 Color and Vision 16.3 Photons and Atoms 3

Chapter 16 Objectives Describe at least five properties of light. Describe the meaning of the term “intensity.” Use the speed of light to calculate the time or distance traveled by light. Explain how we perceive color in terms of the three primary colors. Explain the difference between the additive and subtractive color processes. Arrange the colors of light in order of increasing energy, starting with red. Describe light in terms of photons, energy, and color. 4

Chapter 16 Vocabulary Terms additive process black blue CMYK color process color cone cell cyan fluorescence green incandescence infrared intensity inverse square law light ray magenta photoluminescence photon photoreceptor pixel red RGB color model rod cell speed of light spherical pattern subtractive process ultraviolet white light yellow

Inv 16.1 Properties and Sources of Light Investigation Key Question: What are some useful properties of light? 6

16.1 Properties and Sources of Light Light travels almost unimaginably fast and far. Light carries energy and information. Light travels in straight lines. Light bounces and bends when it comes in contact with objects. Light has color. Light has different intensities, it can be bright or dim.

16.1 Electric Light The process of making light with heat is called incandescence. Incandescent bulbs generate light when electricity passes through a thin piece of metal wire called a filament. The filament heats up and gives off light.

16.1 Electric Light The other common kind of electric light is the fluorescent bulb. Fluorescent bulbs convert electricity directly to light without generating a lot of heat. Fluorescent bulbs use high-voltage electricity to energize atoms of gas that fill the bulb.

16.1 Light carries energy and power Light is a form of energy that travels. The intensity of light is the amount of energy per second falling on a surface. Most light sources distribute their light equally in all directions, making a spherical pattern. Because light spreads out in a sphere, the intensity decreases the farther you get from the source.

16.1 Light intensity The intensity of light from a small source follows an inverse square law because its intensity diminishes as the square of the distance.

16.1 Light carries information The fiber-optic networks you read about are pipelines for information carried by light.

16.1 Light carries information In some cities, a fiber-optic cable comes directly into homes and apartments carrying telephone, television, and Internet signals.

16.1 The speed of light The speed at which light travels through air is approximately 300 million meters per second. Light travels almost a million times faster than sound.

16.1 The speed of light The speed of light is so important in physics that it is given its own symbol, a lower case c. The best accepted experimental measurement for the speed of light in air is 299,792,500 m/sec. For most purposes, we do not need to be this accurate and may use a value for c of 3 × 108 m/sec.

Calculating time for light and sound Calculate the time it takes light and sound to travel the distance of 1 mile, which is 1,609 meters. You are asked for time. You are given distance. Use v = d ÷ t , rearrange to solve for t = d ÷ v Solve for sound: t = (1,609 m) ÷ (340 m/s) = 4.73 s Solve for light: t= (1,609 m) ÷ (3 x 108 m/s) = 5.4 x 10-6 s

16.1 Reflection and refraction When light moves through a material it travels in straight lines. When light rays travel from one material to another, the rays may reflect. The light that appears to bounce off the surface of an object is shown by a reflected ray.

16.1 Reflection and refraction Objects that are in front of a mirror appear as if they are behind the mirror. This is because light rays are reflected by the mirror. Your brain perceives the light as if it always traveled in a straight line.

16.1 Reflection and refraction Another example of refraction of light is the twinkling of a star in the night sky As starlight travels from space into the Earth’s atmosphere, the rays are refracted. Since the atmosphere is constantly changing, the amount of refraction also changes.

16.1 Reflection and refraction The light that bends as it crosses a surface into a material refracts and is shown as a refracted ray.

Chapter 16 Light and Color 16.1 Properties and Sources of Light 16.2 Color and Vision 16.3 Photons and Atoms 21

Inv 16.2 Color and Vision Investigation Key Question: How do we see color? 22

16.2 Color and Vision When all the colors of the rainbow are combined, we do not see any particular color. We see light without any color. We call this combination of all the colors of light "white light ".

16.2 Color and Vision We can think of different colors of light like balls with different kinetic energies. Blue light has a higher energy than green light, like the balls that make it into the top window. Red light has the lowest energy, like the balls that can only make it to the lowest window.

How the human eye sees color The retina in the back of the eye contains photoreceptors. These receptors release chemical signals. Chemical signals travel to the brain along the optic nerve.

Photoreceptors in the eye Cones respond to three colors: red, green and blue. Rods detect intensity of light: black, white, shades of gray.

How we see colors Which chemical signal gets sent depends on how much energy the light has. If the brain gets a signal from ONLY green cones, we see green.

16.2 How we see other colors The three color receptors in the eye allow us to see millions of different colors. The additive primary colors are red, green, and blue. We don’t see everything white because the strength of the signal matters. All the different shades of color we can see are made by changing the proportions of red, green, and blue.

16.2 How we see the color of things When we see an object, the light that reaches our eyes can come from two different processes: The light can be emitted directly from the object, like a light bulb or glow stick. The light can come from somewhere else, like the sun, and we see the objects by reflected light.

16.2 How we see the color of things Colored fabrics and paints get color from a subtractive process. Chemicals, known as pigments, in the dyes and paints absorb some colors and allow the color you actually see to be reflected. Magenta, yellow, and cyan are the three subtractive primary colors.

16.2 Why are plants green? Plants absorb energy from light and convert it to chemical energy in the form of sugar (food for the plant). Chlorophyll is an important molecule that absorbs blue and red light.

16.2 How does a color TV work? Televisions give off light. To make color with a TV, you can use red, green, and blue (RGB) directly. The screen is made of tiny red, green, and blue dots. The dots are called pixels and each pixel gives off its own light. TV sets can mix the three colors to get millions of different colors.

Chapter 16 Light and Color 16.1 Properties and Sources of Light 16.2 Color and Vision 16.3 Photons and Atoms 35

Inv 16.3 Photons and Atoms Investigation Key Question: How does light fit into the atomic theory of matter? 36

16.2 Photons and atoms Just like matter is made of tiny particles called atoms, light energy comes in tiny bundles called photons. White light is a mixture of photons with a wide range of colors (energies). For a given temperature, the atoms in a material have a range of energy that goes from zero up to a maximum that depends on the temperature.

15.2 White light White light is a mixture of photons with a wide range of colors or energies. The number of atoms with a given amount of energy depends on the temperature.

16.2 Photons and intensity Intensity measures power per unit area. There are two ways to make light of high intensity. One way is to have high- energy photons. A second way is to have a lot of photons even if they are low-energy. The number and energy of photons determine the intensity of the light.

16.2 Photons and intensity The light from the flashlight cannot energize phosphorus atoms that your hand blocks. These atoms will not glow because they did not receive any energy from photons from the flashlight. The explanation is that each phosphorus atom absorbs (or emits) only one photon at a time.

16.3 Light and atoms Almost all atoms absorb and emit light. For most atoms, the absorption and emission of light happens in less than one-millionth of a second.

Color Printing Modern printing presses use the four-color, CMYK process to produce rich, vivid colors from only four inks. Careful alignment of the printing stations ensures that the dots line up and do not overlap each other.