16.1 Properties and Sources of Light

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. 2

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? 4

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.