unit 6, chapter 16 cpo science foundations of physics
TRANSCRIPT
Unit 6, Chapter 16
CPO ScienceFoundations of Physics
Unit 6: Light and Optics
16.1 Properties and Sources of Light
16.2 Color and Vision
16.3 Photons and Atoms
Chapter 16 Light and Color
Chapter 16 Objectives1. Describe at least five properties of light.
2. Describe the meaning of the term “intensity.”
3. Use the speed of light to calculate the time or distance traveled by light.
4. Explain how we perceive color in terms of the three primary colors.
5. Explain the difference between the additive and subtractive color processes.
6. Arrange the colors of light in order of increasing energy, starting with red.
7. Describe light in terms of photons, energy, and color.
Chapter 16 Vocabulary Terms reflection refraction black fluorescence intensity color blue light ray CMYK color ultraviolet
infrared photon RBG color photoluminescen
ce additive color white red green spherical pattern cyan
magenta yellow pigment speed of light
(c) incandescence pixel rod cell cone cell subtractive
color photoreceptor
16.1 Properties and Sources of Light
Key Question:
What are some useful properties of light?
*Students read Section 16.1 AFTER Investigation 16.1
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 Properties and Sources of 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.
16.1 Electric Light
The filament heats up and gives off light.
The other common kind of electric light is the fluorescent bulb.
16.1 Electric Light
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.
16.1 Calculate time
Calculate the time it takes light and sound to travel the distance of one mile, which is 1,609 meters.
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.
16.2 Color and Vision
Key Question:
How do we see color?
*Students read Section 16.2 AFTER Investigation 16.2
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.
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 thingsWhen we see an object,
the light that reaches our eyes can come from two different processes:
1. The light can be emitted directly from the object, like a light bulb or glow stick.
2. 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.
16.3 Photons and Atoms
Key Question:
How does light fit into the atomic theory of matter?
*Students read Section 16.3 AFTER Investigation 16.3
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.
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.
Application: Color Printing
